CHAPTER VII.. The Origin and Descent of Rocks.

THE ORIGIN AND DESCENT OF ROCKS.

It has been the current opinion that the earth was once in a molten state, and thence cooled to a solid condition, and hence that all the primitive rocks were igneous. Even those who think that the earth may never have passed through a molten state agree that the oldest known rocks are either true igneous rocks, or rocks of very similar nature. A molten magma may, therefore, be taken as the mother state of the rocks. Starting with this conception, the natural order of events suggests the inquiries (1) how rocks are formed from molten magmas, (2) what natures they assume, (3) how other rocks are derived from them, (4) how still other rocks are derived from these derivatives, and so on. To answer these inquiries is to trace out the generations of rocks and learn the general history of rock-formation.

(1) The process by which igneous rocks are formed from lavas is actually taking place in existing volcanoes. As these are widely scattered over the face of the earth, the material poured out by them represents different parts of the interior and varies in nature accordingly. This affords the means of studying the differences that arise from differences of material. This is a radical consideration, for variations in composition give rise to the most fundamental distinctions between rocks, though by no means the only ones. Rocks which have the same composition often differ greatly in texture or structure, owing to the varying conditions under which they were formed. In the solidification of rocks from the molten state, the rate of cooling causes many differences. A means of studying this is afforded by the various lava flows that are now being poured out on the surface under different conditions; but a more important means is afforded by extinct volcanoes, especially by those which have been deeply cut open by erosion. In certain very ancient volcanoes, not only have the solidified lava streams of the surface been cut across by erosion, but the lava that remained in the crater, or in the neck that led up from below, is laid bare for inspection. Exposures of even more profound nature have been made by the great disruptions which the outer part of the crust has suffered. In certain tracts there have been profound fractures, and the formations on one side of these have settled down and on the other side have been pushed up (faulted), so as to expose parts that were once much below the surface. Sometimes also the crust has been folded and crumpled, and the wrinkles thus formed have afterwards been worn away or cut open by deep valleys, and rocks that were once deeply buried have been laid bare. By the revelations made in these and other ways, it has been learned that at various times in the history of the earth molten matter has been thrust into fissures or intruded between layers of the crust and cooled there, without coming to the surface. Sometimes the lava appears to have forced its way into the rocks, and sometimes to have lifted the upper beds and formed great subterranean layers or tumor-like aggregates, called bathyliths and laccoliths (Fig. 334). Such intruded bodies of molten rock, solidifying under the varying conditions of such subterranean situations, are a fruitful source of instruction respecting the influence of varying rates and modes of cooling, as well as of other attendant conditions.

It will thus be readily seen that the rate of cooling of the various molten rocks must have differed very greatly. In the portions poured out upon the surface there were sometimes narrow streams and thin sheets, giving large exposure in proportion to the mass (Fig. 335), and sometimes thick flows and deep pondings in basins and choked valleys, giving massive bodies with relatively small surface exposure. There were explosions of the lava into minute particles with almost instantaneous cooling, and there were eruptions beneath the sea the peculiar effects of which are rather matters of inference than of positive knowledge. In the portions underground there were insinuations into thin fissures, on the one hand, and in-thrustings of thick bodies, on the other. Some intrusions entered the upper part of the crust where the rocks were cold and wet, and some were thrust into the deeper portions where the rocks were warmer and less penetrated by water. Sometimes the lava rose rapidly and was little cooled in passage, sometimes slowly with more cooling en route, and sometimes there were long halts between eruptions, with much opportunity to cool. An almost infinite variety of conditions is thus presented, and with it a rich field for the study of the modes of solidification.

In the underground intrusions the additional factor of high pressure was also present, and this is the third important condition in determining the nature of igneous rocks.

The three factors, composition, rate of cooling, and degree of pressure, require special consideration.

Composition of Igneous Rocks.

All or nearly all the chemical elements known on the earth are found in greater or less amounts in igneous rocks, and in a broad sense are constituents of them. If there are any exceptions, they are most likely to be found in the rarer elements in the atmosphere. Oxygen, nitrogen, hydrogen, aqueous vapor, and carbonic acid, which make up the mass of the present atmosphere, are all found in lavas and in their cooled products. Probably all the rarer elements also occur in igneous rocks. Helium is known to be given forth by springs.

=Leading elements.=—But although nearly or quite all the known chemical elements enter into the igneous rocks, only a few of them are abundant. These are regarded as normal or essential constituents, while the rarer substances are regarded as incidental. By combining a large number of the most trustworthy analyses of rocks of all sorts, F. W. Clarke has estimated the relative amounts of the more abundant elements in the crust of the earth with the following result:

Percent. in Element. Symbol. the Solid Crust. Oxygen (O) 47.02 Silicon (Si) 28.06 Aluminum (Al) 8.16 Iron (Fe) 4.64 Calcium (Ca) 3.50 Magnesium (Mg) 2.62 Sodium (Na) 2.63 Potassium (K) 2.32 Titanium (Ti) .41 Hydrogen (H) .17 Carbon (C) .12 Phosphorus (P) .09 Manganese (Mn) .07 Sulphur (S) .07 Barium (Ba) .05 Strontium (Sr) .02 Chromium (Cr) .01 Nickel (Ni) .01 Lithium (Li) .01 Chlorine (Cl) .01 Fluorine (Fl) .01 ------ 100.00

It will be seen that only eight of the elements hold a high rank in quantity. Many that are of the utmost importance in the history of the earth and the affairs of men are low in the list, or do not even appear in it at all, because their quantity is too small to be estimated in percentages. The precious metals, and even some of the more common metals, as lead, zinc, and copper, are too scarce to form an appreciable percentage.

=Union of elements.=—In a general study of the igneous rocks we may for the present neglect all but the first eight of these elements. Out of these elements spring various chemical combinations, and out of these combinations spring the various minerals, and out of the combinations of minerals come the various rocks. The union of oxygen with the other seven elements may be taken as a fundamental step in this series of combinations. The result is the following oxides: Silica (SiO₂), alumina (Al₂O₃), ferrous, ferric, and magnetic oxide (FeO, Fe₂O₃, and Fe₃O₄), magnesia (MgO), calcium oxide or lime (CaO), soda (Na₂O), and potash (K₂O). The oxygen sometimes unites in proportions different from those here given, but such exceptions may be neglected in a general study. We thus have nine leading oxides. Of these, silica acts as an acid, or more strictly according to the newer chemical view, as an acid anhydride. All the rest, except the magnetic oxide of iron, and sometimes the oxide of aluminum, act as basic oxides.

In the older chemical philosophy these oxides were supposed to combine by the simple union of an acid oxide with a basic oxide, and to remain as oxide joined to oxide; thus silica (SiO₂) and lime (CaO) formed silicate of lime (CaO,Si₂). The symbols express the idea better than the words. This method is used in the older geological works and in some of the later. But in the newer chemical doctrine, the oxides are not believed to remain so distinct after their union, and the symbols are written CaSiO₃, and the compound is named calcium silicate. According to the modern doctrine of solution, some of the calcium, silicon, and oxygen may exist as free ions in molten rock. The precise way in which the elements are related to each other in these compounds can scarcely be said to be known. For the general purposes of geology it is most convenient to think of these oxides as uniting in the simple fashion first named, and this involves no apparent geological error in general studies, since they are oxides when they enter the compound, and if the compound is decomposed they usually come forth again as oxides; but in closer studies more complex unions, attended by dissociations (ionization), must be recognized.

=Formation of minerals.=—As but one of the leading oxides that abound in an average magma plays the part of an acid, the silica, a very simple conception of the general nature of igneous rocks may be reached by noting that they are mostly silicates of the seven leading basic oxides—alumina, potash, soda, lime, magnesia, and the iron oxides. This general idea is a very useful one and represents a most important truth; but in its use we must not forget that there are many exceptions. Sulphur, phosphorus, chlorine, and other elements unite with the bases to form sulphates, sulphides, phosphates, phosphides, chlorides, etc. So also there are many minor bases that form silicates; and these minor bases unite with the minor acids to form many more or less rare minerals. Again, there are native metals in some igneous rocks. But altogether these hardly reach more than one or two percent. of the whole.

There are, however, two exceptions of more importance. In the molten magma the acid and basic elements are not always evenly matched. When there is an excess of silica, a portion remains free and takes the form of quartz (SiO₂). If there is an excess of the basic oxides, the weakest one is usually left out of the combination. This is commonly the iron oxide, which then usually takes the form of magnetite (Fe₃O₄). It is a singular fact that quartz often forms when there is no excess of silica, and magnetite when there is no excess of base. Quartz (free acid anhydride) and magnetite (free basic oxide) sometimes occur in the same rock. The explanation for this is yet to be found. These form rather important exceptions to the generalization that the igneous rocks are mostly made up of silicates, but, thus qualified, it expresses the essential truth, and has the merit of embodying the central chemical fact relative to these rocks.

=Sources of complexity.=—But here simplicity ends. As we pass on to the specific silicates that are formed, we encounter several sources of complexity. In the first place, the silica unites with the bases in different ratios and thus gives rise to unisilicates or orthosilicates (ratio of oxygen of bases to oxygen of silica, 1:1), subsilicates (ratio more than 1), bisilicates (ratio 1:2), trisilicates or polysilicates (ratio 1:3 or higher), and combinations of these. All the bases are not known to combine in all these ways, but many do in more than one of them. Still, if the silica were content to unite with each of the bases by itself alone, the results would remain comparatively simple; but instead of this it unites with two or more at the same time; and, more than that, it unites with them in varying amounts. The case would still remain measurably simple if these chemical compounds always crystallized out by themselves, each compound forming one mineral, and but one; but the different silicates have the confusing habit of crystallizing together in the same mineral. A crystal may thus sometimes be seen, under the microscope, to be made up of alternating layers of different silicates; e.g., a microscopic layer of an aluminum-calcium silicate may be overlain by a microscopic layer of an aluminum-sodium silicate, and the alternation may be repeated throughout the crystal, giving it a banded structure. There is reason to believe that this is true in many cases where the microscope fails to detect it, and that less symmetrical comminglings of silicates may take place. As such alternations or mixtures are not governed by any known mathematical law, as is the case in chemical compounds, there is no determinate limit to the number of combinations that may arise. As a matter of fact, new ones are still being discovered in the progress of research, and the total number that may ultimately be found can scarcely be prophesied.

As a result of all this fertility of combination, the total number of silicious minerals in igneous rocks is large. It is the function of the mineralogist to treat of these minerals as such. The geologist deals with them as constituents of the earth and as factors in its history. Only a few of them are so abundant as to require special individual notice in a general study of the earth. It may be remarked also that only a few of them can be identified by simple inspection as they occur in the rocks, partly because of the delicacy of the distinctions between many of them, and partly because of their minuteness and intricate intermixture. The resources of the polarizing microscope are necessary for safe determination in most cases. The student need not feel embarrassment or discouragement if he is often unable to recognize the constituents of the intimately crystalline rocks. Their determination has grown to be a profession by itself.

=The leading minerals of igneous rocks.=—Fortunately for the simplicity of geological study, a few minerals make up the great mass of the igneous rocks. These few are quartz, the feldspathic minerals, the ferromagnesian minerals, and the iron oxides. Quartz (silica, SiO₂) is the free acid already mentioned. The feldspathic and ferromagnesian minerals are the leading silicates of the earth’s crust, and vastly surpass all others in abundance. The feldspathic group embraces minerals formed by silica in union with alumina, together with either potash, soda, or lime, or two or more of these together. The ferromagnesian group embraces minerals formed by the union of silica with iron, magnesia, and lime, together with more or less of the other basic oxides. These statements are only true in a very general sense. Admixtures, replacements, and impurities are so frequent as to break down all sharp, simple definitions. The feldspathic minerals are normally light in color, ranging from white to red or gray. The ferromagnesian minerals are normally dark (commonly greenish) from the presence of iron, the great coloring element of rocks. But these color distinctions do not hold good in detail and cannot be much trusted as a means of identification.

=The feldspathic minerals= (p. 462) embrace the potash feldspars, orthoclase and microcline; the soda feldspar, albite; the lime feldspar, anorthite; and the mixed feldspars intermediate between albite and anorthite, viz., the soda-lime feldspar, oligoclase, the lime-soda feldspar, andesine, in which lime and soda are nearly equal, and the lime-soda feldspar, labradorite, in which the lime predominates; together with leucite, a potash silicate higher in alkali than orthoclase, and nephelite, a soda silicate higher in soda than albite. Leucite and nephelite are usually classified as feldspathoids, not as feldspars. It is to be understood that alumina is normally present in all these. Additional details respecting these minerals may be found in the reference list, p. 460.

Among the =ferromagnesian minerals= the most important are the pyroxenes, the amphiboles, and the biotite type of mica. Olivine is of subordinate importance. The pyroxenes (p. 465) and amphiboles (p. 460) have nearly the same chemical composition, but differ in crystallization and physical properties. Hornblende (an amphibole) has been melted, and on cooling under proper conditions found to take on the form of augite (a pyroxene). Pyroxene is sometimes altered into uralite, one of the amphiboles. The pyroxenes and amphiboles are the most abundant of the dark minerals in crystalline rocks. The leading members of the pyroxene group are augite, diallage, hypersthene, enstatite, and soda pyroxene. The chief members of the amphibole group are hornblende and the soda amphiboles. All are essentially silicates of magnesia and iron oxide, with or without the addition of lime, soda, and alumina. Details respecting these may be found in the reference list.

The two leading =micas= are the iron-magnesia mica, biotite, and the potash mica, muscovite, the familiar “isinglass” of the stove-door. Chemically, muscovite should go with the potash feldspars, but it is distinguished from them by its crystalline habit and physical properties. The biotite should go chemically with the pyroxenes and amphiboles, which it closely resembles except in its crystalline properties. Details respecting the micas may be found in the reference list, p. 464.

Two =iron oxides=, magnetite (Fe₃O₄) and hematite (Fe₂O₃) are widely disseminated in igneous rocks. They constitute the free bases already mentioned.

=Summary of salient facts.=—The salient facts are, therefore, (1) that out of the seventy-odd chemical elements in the earth, eight form the chief part of it; (2) that one of these elements uniting with the rest forms nine leading oxides; (3) that one of these oxides acts as an acid and the rest as bases; (4) that by their combination they form a series of silicates of which a few are easily chief; (5) that these silicates crystallize into a multitude of minerals of which again a few are chief; and (6) that these minerals are aggregated in various ways to form rocks. Possessed of these leading ideas, we are prepared to turn to the consideration of some of the conditions under which these combinations take place in the formation of rocks from molten magmas.

THE NATURE OF MOLTEN MAGMAS.

We easily fall into the habit of thinking of molten rock as we think of a molten metal, merely as a substance which has passed from the solid to the liquid condition because of high temperature. With the return of low temperature a molten metal returns to the solid state usually in the same molecular condition which it possessed before. The point of fusion and the point of solidification are the same and are rigidly fixed. If this were true of the constituents of a rock, a definite order for the solidification of the several minerals might be anticipated. As a matter of fact, the order is not the same under all conditions, and, what is especially significant, the order is far from being that in which the constituents would fuse or would solidify separately. For instance, in a granite composed chiefly of quartz, feldspar, and mica, the quartz is often the last to take form, although it is more infusible than the feldspar or the mica. This and other phenomena show that a molten magma is not to be viewed simply as a fused substance, but rather as a solution of one silicate in another, or as a solution of several silicates in one another mutually. The high temperature is to be regarded merely as a condition prerequisite to solution, or as the condition of fusion of some one constituent which then dissolves the others. If crystals of snow, sugar, and salt be mixed at a low temperature and compacted, the mass may be regarded as an artificial rock. On raising the temperature, all will pass into solution while the temperature is still somewhat below the melting-point of the snow, the most fusible, and while it is much below that of either the sugar or the salt. This particular case is instructive because the ice is not simply fused by temperature; the affinity of the salt plays a part. If the temperature were again lowered, the sugar and salt would not crystallize out at their fusing-points, but would remain in solution down to and even below the normal freezing-point of water; in other words, they would remain in solution until the water crystallized out and forced them to take the solid state. This holds good when the amounts of the sugar and salt are small relative to the water. If, on the contrary, their quantity is large relatively, crystallization will take place at higher temperatures and before the water crystallizes to ice. From this it appears that the salt and sugar might crystallize either before the water or after it, according to the degree of concentration. The behavior of mixtures of minerals in passing into and out of the molten condition appears to be quite analogous to this, and hence a great variety of results attend the process, dependent upon the number, the nature, and the relative quantities of the ingredients. The approved conception of the genesis of a rock from a molten magma (when ample time is given) is that one compound after another crystallizes out as the temperature falls and its point of saturation for each is reached, until the whole has been solidified. The modes of combination of the elements in the molten magma are not necessarily the same as those in the derivative crystals; indeed, the combinations doubtless change as the process proceeds; certain constituents being taken out, the remaining ones probably rearrange themselves.

=Time required in crystallization.=—The liquid magma of igneous rocks is essentially a fluid glass or slag. It is analogous to common glass, which is a silicate of potash, soda, or other base, except that usually common glass is relatively free from iron and other coloring substances, while these abound in the natural magmas and render them dark and more or less opaque; but the fundamental nature is the same, except that the natural lavas are usually mixtures of several silicates, while the artificial glasses consist of only one, or at most a few. Furnace slag is essentially an artificial lava.

When a lava is cooled quickly, the commingled silicates solidify in the diffused condition essentially as they were in the liquid; for there is no time for the silicate molecules of a like kind to come together, particle by particle, in regular systematic order, as required in crystallization. The essential feature of crystallization is this systematic arrangement of the molecules according to a definite plan, giving a specific crystal form, as a cube, a hexagonal prism, etc.

There are six (sometimes made seven) fundamental systems of crystallization, and a multitude of variations of special form in each system. The treatment of these forms belongs to mineralogy.

In a thick viscid liquid, this systematic arrangement of molecules into definite crystal forms takes place slowly, for the crystalline force in the silicates is far less energetic than that in water, which crystallizes into ice with much rapidity and with great force. Because of this slowness, the solidification of the lava may catch the process of crystallization at any stage. If the lava is cooled quickly, the result is a glass; if less quickly, part glass and part crystals; if slowly enough, all becomes crystalline. In general the slower the growth the larger the crystals. The solidification product may, therefore, range from a glass to a mass of crystals; i.e., it may be (1) wholly glass, (2) a glassy matrix with a few small crystals scattered through it, (3) a less abundant glassy matrix with more and larger crystals, (4) a mere remnant of glass in a mass of crystals, or (5) a mass of crystals with no glass.

=Successive stages of crystallization.=—Since eruptions take place intermittently, it is obvious that cooling of the lava may be in progress in its hidden reservoir during the quiescent intervals between eruptions. After a certain stage of partial crystallization has been reached during such time of quiet, a renewal of eruption may take place and the whole mass of lava be shifted into quite new conditions, and a second phase of solidification may be superposed on the one already started. The rock will then show two phases of crystallization: (1) large crystals of the kind or kinds most prone to develop in the given lava may have grown during the first long stage of slow subterranean cooling, while the greater part of the lava still remained liquid; and (2) small crystals or glass may have developed when the more rapid cooling under the new conditions took place. The result would be large crystals set in a matrix of small crystals or of glass, a combination styled porphyritic. In such cases the lava, in its later stages, carries the large crystals floating throughout its mass, and is not a simple liquid.

THE FRAGMENTAL PRODUCTS OF SUDDEN COOLING.

=Pyroclastic rocks.=—The extreme example of sudden cooling is presented when lavas are violently exploded into the air and solidify almost instantly. The resulting glassy particles or filaments, if small, constitute volcanic ash. The explosion appears to be due to steam and other gases which are held in the deeper lava under great pressure, but which, as they rise toward the surface of the lava where the pressure is relieved, expand with explosive violence. It is probably also due in part to progressive crystallization, which forces the gases out from the part that crystallizes and overcharges the rest. Sometimes the projected particles draw after themselves long filaments like the threads of spun glass, and sometimes while in the air they divide and draw apart, spinning a filament of viscid lava between them. A variety of this kind at the volcano of Kilauea in Hawaii is known as “Pele’s hair.” These light filaments drift with the wind and lodge on the lee side of the volcano, covering the surface “like mown grass” (Dana).

When the exploded fragments are coarser they fall about the volcanic vent and form the tuffs (tufa) of which most steep volcanic cones are chiefly built. In these larger fragments, crystals are not infrequently found, and the same is even true of the volcanic ash. These crystals are undoubtedly such as had already been formed in the lava before it exploded, and their formation, as suggested above, may have contributed to the explosion.

Fragments too large to be borne far away by the air, but still small, are known as lapilli, especially if they are somewhat rounded and gravel-like. A finer variety, of the nature of sand, much used in making Portland cement, is locally known as puzzolana.

The rougher, irregular fragments of a clinker-like nature ejected by volcanoes are known as scoriæ or cinders. They are more or less distended by gas-bubbles and are hence light and pumiceous.

The larger masses of lava ejected into the air are often caused to rotate by the unequal force of the projection, or by the unequal friction of the air, and to assume spheroidal forms, the internal gases at the same time often expanding and rendering the mass vesicular. These rounded projectiles are known as volcanic bombs (Figs. 336 and 337). Balls of lava that have originated in rolling movements of the seething mass, or in other ways, are also styled bombs. Usage is not altogether harmonious or consistent in the application of the term.

The larger masses that are projected into the air are more or less vesicular from the expansion of included gases, as already noted, and so the fragmental products of volcanic action grade into the vesicular. The type of this class is pumice, in which the gas cavities make up by far the larger part of the volume of the whole mass, and the whole is reduced to the condition of a solidified froth or foam. So thin are the dividing films of glassy material in some cases that the whole is pure white, though the same material in solid mass would be dark. This solidified glassy froth is often lighter than water and floats freely on the sea until it becomes “water-logged” and sinks. Dredgings of the deep sea show that much pumice has accumulated there, and being far from the land has escaped burial by the sediments borne in by the rivers.

All of these fragmental rocks produced by volcanic action are known as pyroclastic (fire-fragmented) rocks, a general term of much convenience in distinguishing them from lava-flows, on the one hand, and from the fragmental rocks produced by air and water (ordinary clastics), on the other.

THE GLASSY ROCKS.

=The solid glasses.=—The quick cooling of lava-flows into solid glasses is chiefly dependent on their exposure at the surface. Hence it is often the case that the exterior of a lava-flow is glassy in greater or lesser degree, while the interior is more or less crystalline. Quick cooling is sometimes also due to the intrusion of the lava in thin sheets into fissures in cold rocks. When massive bodies of lavas penetrate solid rocks, the lava does not usually cool so fast as to prevent some degree of crystallization, and the crystallization may even become complete; but if the intruded lava sheet be very thin, the lava is liable to be cooled to a nearly perfect glass. The glassy condition is, therefore, subject to indefinite gradations. As a rule, the acid lavas are stiffer at the same temperature than the basic ones, and crystallize more slowly, so that acid glasses are more common than basic ones. The basic rocks usually crystallize pretty thoroughly, except on the immediate surface of the flows.

=The first stages of crystallization.=—The microscopic study of the volcanic glasses reveals great numbers of minute forms known as crystallites, microlites, globulites, etc., that appear to be first steps in crystallization, though many of them do not take definite geometrical shapes and some do not show the optical characters of crystals. There are minute globules (globulites), needles, and hair-like bodies (trichites) of more or less indeterminate nature, together with other forms that can be seen to be certainly the initial forms of well-known minerals.

=The obsidians.=—Of the compact glassy rocks, obsidian is the best type. It is essentially a natural glass, formed usually of acid silicates. It has the close texture, conchoidal fracture, and other qualities of glass. It is usually black, but sometimes red, brown, purple, bluish, or gray. While chiefly of glass, it usually contains more or less of the incipient crystals above described, showing that even here the first step in the gradation to the next or the crystalline stage has been taken. These incipient crystals sometimes become so abundant as to change the texture from the vitreous to the stony order. In some cases, the stony texture seems to have been developed in the obsidian after it was formed, the change being a part of a subsequent process of devitrification, but in other cases the crystals seem to be original. Besides these, there are often small globular bodies known as spherulites.

Varieties of glassy rock in which the embryo crystals are more numerous and the glassy texture less perfect, are known as pitchstones. The fresh surfaces of these have rather the aspect of pitch or resin than that of true glass; hence their name. Like the obsidians, they are usually dark, but they take on greenish, brownish, yellowish, and light-colored hues as well. Sometimes glassy rock fractures in small spheroidal forms like pearls, and is known as perlite. Basic glasses are relatively rare, and while usually included under the term obsidian, are sometimes given special names.

SPECIAL STRUCTURES.

=Flow structure.=—Lavas that cool into glassy rocks frequently contain gas cavities, colored spots and variations of texture which, together with the hair-like embryo crystals, are drawn out into lines, streaks, and parallel belts by the flow of the viscous mass, giving rise to rhyolitic or flow structure (Figs. 338 and 339). Rocks in which this is the most pronounced feature were formerly known as rhyolites, though the term has drifted away from this original meaning and has been applied to a class of acidic rocks. The obsidians and pitchstones may be more or less rhyolitic under the microscope, though to the naked eye they may appear only as a glassy or resinous mass. The rhyolites generally have but an imperfect glassy texture, since the crystals and the cavities sometimes make up a notable part of the mass, the glassy portion being scarcely more than a matrix in which the crystals, spherulites, and cavities are carried. By an increase of the crystals in number and size, the rock passes by gradations into porphyry or phanerite.

=Amygdaloids.=—In lava-flows the included steam often collects in bubbles near the surface as the lava cools and forms a vesicular portion with a scoriaceous texture (Fig. 341). In its upper part, the vapor bubbles may be numerous, while below they become more and more scattered until they disappear. Similar bubbles are also often found near the bottom of a sheet of lava. This is perhaps due to the rolling under of the frontal surface of the lava-stream as it flows. Later, these cavities often become filled with minerals deposited from solution and the rock then becomes an amygdaloid, but this filling is a secondary action.

THE PORPHYRITIC ROCKS.

When the conditions are such that after a part of the magma has formed distinct crystals floating in the remaining liquid lava, there is a change which causes the rest to solidify as a glass or as a mass of small crystals, the structure is known as porphyritic, and the rocks possessing it are called porphyries. This differentiation into distinct crystals set in a ground-mass of minute crystals or of glass often gives a mottled or variegated aspect to the rock, especially if the matrix of glass or minute crystals differs in color from the distinct crystals. This structure is much oftener developed in acidic rocks than in basic ones, because the latter crystallize more readily. The most common porphyritic crystals are feldspar and quartz, though they are by no means the only ones. The matrix is also usually felsitic or quartzose, but not necessarily so. The character is a structural one, and is not dependent upon any special chemical or mineralogical constitution. The distinct crystals are known as phenocrysts, and the varieties of porphyries are named from the characteristic phenocryst, e.g., quartzophyre (quartz-porphyry) if the conspicuous crystals are quartz, orthophyre if orthoclase, augitophyre if augite, etc. A convenient classification has recently been proposed into (1) leucophyre (white porphyries), which have a light-colored ground-mass set with phenocrysts of any kind, and (2) melaphyres (black porphyries), which have a dark-colored ground-mass, with phenocrysts of any kind. While it is to be hoped this usage will prevail, it is to be noted that these terms, especially the latter, have been used in a different sense. (See reference list of rocks, p. 445.)

In many cases the ground-mass itself becomes minutely crystalline and the porphyritic aspect is due simply to large distinct crystals set in a mass of minute obscure ones. The rock is then really holocrystalline, but the term porphyry is applied to it. In other rocks the crystals of the ground-mass become more and more distinct, the porphyritic aspect gradually disappears, and there is a graduation into the next class.

THE PHANEROCRYSTALLINE ROCKS.

=The phanerites.=—When time enough is given for the cooling process the molten magma becomes completely crystalline. The holocrystalline rocks hence include a large series, ranging from the most acid to the most basic. In this class the differentiation of the rock material and the formation of distinct minerals reach a high stage, and as a natural result the varieties of rock are numerous. Taken as a group they are phanerites. If they are to be more particularly characterized, it is usually done on the basis of the minerals of which they are composed. The following are the leading types, beginning with those which are rich in silica and poor in basic oxides, and ending with those which are rich in basic oxides and poor in silica.

=The granites.=—The term granite was originally used to designate a granular, i.e., a distinctly crystalline, rock, and it is still popularly and properly so used. In scientific treatises it has usually been confined to a special aggregate of crystals of quartz, feldspar, and mica. It has recently been proposed to give it again a more general application, though not quite its original one, by including under it all holocrystalline rocks composed of dominant quartz and feldspar of any kind, with mica, hornblende, or other minerals in subordinate amount. In scientific literature as it now stands, granite consists of quartz, feldspar, and mica, the feldspar being of the alkali-potash or soda variety (orthoclase, microcline, or albite), and the mica, either muscovite or biotite. In the type form the crystals are distinct and sometimes large (Fig. 344). They are intimately mingled with one another, and in growing, interfered more or less with each other and so became interlocked. The granites are among the most common and easily recognized of the holocrystalline rocks. Their color is mainly dependent upon the feldspar, the red and pink varieties of the mineral giving rise to red granite, and the white varieties to gray granite.

Very few granites conform strictly to the type. They vary by the addition and substitution of other minerals, and these sometimes become as prominent as the type minerals. The soda-lime feldspars sometimes take the place of the orthoclase, or accompany it; hornblende and other minerals take the place of the biotite, or occur with it; and so on. Whenever one of these replacing or accessory minerals is notable in quantity, its name is often prefixed, as hornblende-granite, oligoclase-granite, zircon-granite, etc. In this way the rock grades almost insensibly into the syenites, diorites, etc. Variations also arise from the absence of one of the three leading minerals. If mica is absent, the rock is termed an aplite (quartz and feldspar). If the feldspar is absent, it is called a greisen (quartz and mica). If quartz is absent, it is termed a minette (feldspar and mica). These varietal terms are neither universally nor always consistently used, and it is to be hoped they will be replaced by the systematic nomenclature recently proposed and outlined later (p. 451).

The granites were formed from a magma rich in silica, alumina, potash, and soda, but generally poor in lime, iron, and magnesia. Incidentally other substances were present. The alumina, potash, and other bases united with so much of the silica as was required to form the feldspars and micas, and the remaining silica crystallized into quartz.

Granite is normally a massive rock without foliation or banding. If it takes on these characters, it becomes a gneiss, and passes into the foliated or schistose class of rocks, to be discussed later. The texture of graphic granite (see pegmatite) is notably peculiar, due to the simultaneous crystallization of the quartz and feldspar (Fig. 345).

=The syenites.=—When the mica of a granite is replaced by hornblende, the rock is now commonly known as a hornblende-granite, but it was formerly called syenite, because found at Syene on the Nile. The term syenite is now applied to a rock consisting essentially of feldspar and hornblende or mica, but there is a complete gradation from the granites to the syenites. The magma of the syenites was richer in iron and magnesium than the typical granitic magma. The syenites also grade into other classes, as do the granites, and are named by similar prefixes, as augite-syenite, etc., and some of these varieties have special names. The syenites are red or gray, according to the color of the feldspar, and are usually darker than the granites. The texture of syenite is like that of granite. In the scheme of field names recently proposed, syenite is made to include all holocrystalline rocks composed mainly of feldspar of any kind, with subordinate amounts of mica, hornblende, pyroxene, and other minerals, but without a noticeable amount of quartz.

=The diorites.=—These embrace rocks which were crystallized from a magma still poorer in silica and the alkalies, and richer in the earthy bases. In composition they closely approach the ideal average rock, but usually fall a little below it in silica and the alkalies, and rise a little above it in the earthy bases. In current usage, diorite is defined as an intimate mixture of crystals of hornblende and a plagioclase feldspar. It differs from the syenite in having plagioclase feldspar instead of orthoclase. By substitutions and the addition of accessory minerals, the diorites graduate toward the granites and syenites on the one hand, as already noted, and into gabbros on the other.

In the scheme recently proposed, all holocrystalline rocks in which hornblende is dominant and feldspar subordinate are classed as diorites.

=The gabbros.=—The name gabbro was formerly applied to a coarse-grained basic rock consisting of labradorite and diallage, but the name has been gradually extended until it embraces a large group of rocks that have essentially the same composition as the dolerites mentioned below, but are coarser in crystallization, and the crystals do not embrace one another (i.e., are not ophitic). The principal minerals are plagioclase (normally labradorite) and pyroxene (normally diallage) with magnetite or ilmenite. They are usually dark, heavy rocks. The pearly luster of the cleavage faces of the diallage, when present, gives a peculiar sheen to a fresh surface of the rock. In the recently proposed field names, gabbro is made to include all phanerocrystalline rocks in which pyroxene predominates, attended by feldspar of any kind in subordinate quantity, with or without hornblende or mica.

=The peridotites.=—These stand at the basic end of the series, having been formed from a magma in which the silica was low (39–45 per cent.), as were also the alumina, lime, and alkalies, but in which the magnesia was relatively very high, ranging from 35 to 48 per cent. The rock consists very largely of olivine associated with pyroxene, magnetite, and other very basic minerals. Little or no feldspar is present. The peridotites are much less abundant than the preceding classes and represent a very distinctive phase of the magma in which the magnesia was greatly concentrated.

Closely allied to the peridotites are rocks which are made up largely of a single basic mineral, as augitite, pyroxenite, hornblendite, rocks essentially formed of the minerals augite, pyroxene, and hornblende respectively. It will be noted that in these rocks the magma became quite simple in nature, just as at the acid end of the series certain rocks become comparatively simple from the concentration of the acid element, as in certain acidic granites, felsites, etc. (See pp. 523–524.)

=The basalts.=—The term basalt is used in a somewhat comprehensive way to embrace dark, compact, igneous rocks that appear to be nearly homogeneous, owing to the minuteness of the crystals, which are usually so small as to be identifiable only under the microscope. In some cases the crystals are scattered throughout a ground-mass after the porphyritic fashion. In some of these cases there is a true glassy base, and in such cases the rock does not strictly belong in the holocrystalline group. In the more typical cases the constituent minerals are very minutely crystallized and intimately intermixed. The leading minerals are plagioclase (usually labradorite or anorthite) and pyroxene (usually augite), with olivine and magnetite or ilmenite usually present. There is a considerable range in chemical nature, but the basalts are relatively poor in silica, usually also low in potash and soda, but rich in lime, magnesia, and the iron oxides. They are classed as basic and are sometimes highly so. The magmas of the basalts are especially fluid, and when poured forth upon the surface easily spread out in thin sheets. In cooling they are prone to take on a columnar or basaltic structure, the columns standing at right angles to the surfaces exposed to cooling. The columns are sometimes curved, owing to the peculiar attitude of the cooling surface. The columns of Giant’s Causeway and Fingal’s Cave are familiar examples.

=The dolerites.=—The basalts graduate insensibly into the dolerites; indeed the dolerites may be regarded simply as basalts of coarser crystallization. The minerals are evident to the eye and range up to medium size. The more abundant minerals are plagioclase feldspar (labradorite or anorthite), with one or more of the ferromagnesian minerals (augite, olivine, or biotite), and magnetite or ilmenite. In the growth of the minerals one crystal frequently embraces others, giving an ophitic structure. The dolerites have many varieties, due either to accessory minerals or to the development of some of the constituents more amply than the rest. The type may be said to consist of plagioclase and augite, the other minerals being regarded as accessories. Magnetite or ilmenite is almost universally present. The varieties are usually designated by prefixes, as olivine-dolerite, enstatite-dolerite, etc., but special names are also used for some of these.

The ancient dolerites have usually undergone internal changes and such rocks are often called diabases. While the use of the term has not been uniform, it accords with the better practice to regard the diabases simply as partially altered dolerites and basalts. In general, therefore, the diabases are but ancient dolerites.

General names.

The difficulty of distinguishing many of the foregoing rocks from each other by any means available in the field, owing to the minuteness of the crystals, and to the gradation of one type of rock into another, makes it desirable to employ certain general names which will correctly express the leading character of the rock without implying a knowledge of the precise mineral composition. A convenient term of this kind is greenstone, which merely indicates that the ferromagnesian minerals are prominent and usually give a greenish or dark cast to the rock. The greenstones embrace the diorites, dolerites, some of the gabbros and the basalts, and may even extend to the peridotites and some of the more hornblendic of the granitoid rocks. Another convenient name is trap, which may be used for any dark, heavy igneous rock. The name (from trappe, stairs) refers to the step-like arrangement which the edges of the superimposed sheets of lava often take, especially when the lava is of the free-flowing, basaltic kind.

The term basalt is sometimes used to embrace any of the very fine-grained dark igneous rocks. In such cases, it covers the very fine-grained dolerites, diorites, peridotites, etc. The term granite was used originally for any coarse-grained crystalline rock, and there is a tendency to revive this early use. In general descriptions, some of our best petrographers call any coarsely crystalline rock (e.g., coarse-grained syenites, diorites, gabbros, etc.) granite. The term granitoids may be used with strict propriety to cover all rocks of this class.

DERIVATION OF SECONDARY ROCKS.

Rocks, though commonly made the symbol of the abiding, are subject to constant slow changes. Through these changes newer rocks have been derived from older ones, and still others in turn from these derivatives, and so on in an endless chain. All derived rocks are conveniently termed secondary, though they may be several generations removed from the primitive rocks, and even the primitive rocks, as we now understand them, may be themselves derived. The ordinary changes of rock are most active at or near the surface, and the processes of such change have already been discussed in part under the titles “Weathering” (pp. 54 and 110), “Erosion” (pp. 119–123 and 342–349), “Transportation” (pp. 115–119 and 354–355), and “Deposition” (pp. 177–204 and 355–363).

=Regolith.=—The first great product of the surface changes is mantle-rock (regolith), which comprehends all the loose matter that springs from rock decay, wear, fracture, and other forms of disintegration. It lies in an unconsolidated sheet on the face of the land, whether as soil, sand, clay, earth, gravel, or loose rock.

=Disrupted products: arkose and wacke.=—In dry regions, in cold regions, on mountain heights and precipitous slopes, and under other conditions where sudden changes of temperature and frost action work efficiently, rocks are broken down into fine fragments without much chemical decomposition. Such disaggregated rather than decomposed matter, if derived from granitic and similar crystalline rocks, is termed arkose, or arkose sand, and consists of fragments of quartz, feldspar, mica, etc. Common sand consists essentially of quartz grains. If the fine fragments are derived from the darker igneous rocks, and consist mainly of grains of plagioclase feldspar, and ferromagnesian minerals, it is sometimes called wacke. This term is not widely used in just this sense, but there seems to be an important place for it, and it will be so employed in this work. These disaggregated sands are but special phases of the mantle-rock.

=Disintegrated products.=—When the surface-rock is chemically decomposed, the residual material is confined mainly to the insoluble portions, i.e., the silicious and clayey parts; while the lime, magnesia, soda, potash, and similar substances are largely dissolved and borne to the ocean. The potash is somewhat more disposed to remain with the clays than the soda, lime, or magnesia; but residues of all are usually present.

Classes of Sedimentary Rocks.

=Shales, sandstones, and conglomerates.=—As already shown in the discussion of the atmosphere and surface-waters, the mantle-rock is constantly being borne away and redeposited in lodgment spots on the land or in the basins of the sea, while it is constantly being renewed below. It is an evanescent but ever-renewed derivative mantle. In this process of renewal, removal, and redeposition, the mantle material is usually assorted into mud, sand, and gravel; and these several classes of material are laid down more or less separately, and usually take the stratified form, because their deposition depends on different degrees of motion of the transporting waters or wind. When these several classes of material become cemented or otherwise hardened, they give rise to shales (cemented muds), sandstones (cemented sands), and conglomerates (or pudding-stones, cemented gravel, Fig. 346). If the coarse material remains angular, they form breccia instead of conglomerates (Figs. 347 and 348). For the most part, the deposits of mud, sand, and gravel are made under the sea or in lakes and estuaries, but they are also formed on the land in lodgment basins, in low-gradient valleys, and on base-plains. The deposits of sediment on land have received less recognition than they deserve. When formed under the sea or in other life-sustaining waters, shells and other organic material are liable to be entrapped and to form a part of the rock. These organic remains, or fossils, greatly aid in interpreting the deposits in which they occur. Fossils are less liable to be preserved in sedimentary deposits formed on land. There is, therefore, some ground to suspect that great series of sandstones and shales which do not contain marine or fresh-water fossils were formed in lodgment basins on land, though the absence of fossils cannot be regarded as proof of such origin.

=Limestones and dolomites.=—Of the lime, magnesia, soda, and potash leached out of the surface-rocks and carried to the ocean in solution, the lime is largely extracted to form the shells, skeletons, teeth, armor, and other hard parts of sea-animals and sea-plants. These limy parts are at length left on the floor of the ocean and become more or less disintegrated and help to form beds of lime-mud and lime-sand which in time are cemented into limestone (Figs. 349, 350, 351, and 352). A larger proportion of the magnesia remains in solution in the sea-water, but in ways not yet well understood, the magnesia sometimes unites with the lime to form dolomite, a double carbonate of lime and magnesia (Ca,Mg)CO₃. This change is sometimes local, and sometimes affects great series of beds, more commonly the ancient ones than the modern. Sometimes the dolomization appears to have taken place long after the original limestone was formed and probably sometimes after it was lifted out of the sea, while in other cases it seems to have taken place while the sediment was accumulating, or at least before the next overlying beds were laid down. The potash in solution is to some large extent taken up by the land- and sea-plants or is retained in the clays, and through them becomes again incorporated in the sediments. The soda largely remains in solution in the sea-water.

=Precipitates.=—When a portion of the ocean-water is isolated in a region where evaporation from the surface of the water is greater than the rainfall on it, and the inflow from the tributary basin, the lime, magnesia, soda, potash, and other dissolved substances (solutes) are concentrated until the water becomes saturated. The solutes are then precipitated in the order in which they reach the point of saturation. This order, when taken in strict and full detail, gives a very complex series, but the leading deposits are calcium carbonate (limestone), calcium sulphate (gypsum), and sodium chloride (halite or rock salt) (see p. 375). Isolated lakes in arid regions may give rise to similar deposits. It has sometimes been thought that the ancient limestones were produced largely by precipitation from concentrated sea-water. While this is probably the case in some instances and to some degree, it has not been demonstrated that the great limestone formations were made to any large extent in this way. The more accepted view is that the limestones in the main were made from organic remains. The lime in solution in the ocean is chiefly in the form of the sulphate.

=Iron Ore-beds.=—In a somewhat different way iron ore-deposits are formed by the precipitation of iron oxide or iron carbonate from solutions of ferrous compounds. The ferrous compounds in solution were leached from iron-bearing rocks by percolating waters. The most familiar case is that of iron-bearing springs. On exposure to the air, the iron compounds in solution undergo change, and ferric oxide is thrown down, usually forming limonite (Fe₂O₃,3H₂O), but sometimes hematite (Fe₂O₃). This change is common in marshes and gives origin to “bog-ore.” Similar deposits take place in certain shallow lakes, and hence are known as “lake ore.” Iron ore sometimes also forms at the bottom of a peaty bed or in muddy soil. In connection with the great coal formations, beds of iron carbonate (siderite) occur. Organic matter seems to play a great part both in the original solution and the later deposition of these ores. From certain soils and clay-beds on which the ancient coal-producing forests grew, the iron has been almost completely removed, either by the action of the roots, or more probably by organic acids arising from their decay and from the decaying vegetation on the surface. On flowing into shallow bodies of water or into marshes, the waters containing such dissolved iron compounds usually throw down their iron content either as a carbonate (siderite), or as a hydrous ferric oxide (limonite). The siderite is formed where decaying vegetation is present to furnish abundant carbon dioxide and to partially protect the iron solution from oxidation, and the limonite where free oxidation takes place. Sand, silt, clay, or calcium carbonate often accumulates with the iron precipitate, and the result is an impure deposit which becomes an ironstone. Such deposits often become segregated into nodules, as will be explained later. It is thought that diatoms sometimes aid in the deposit of iron ore in shallow waters.

=Silicious deposits.=—In the decomposition of igneous rocks, a certain portion of the silica, as well as of the bases, is dissolved and carried away in solution. Certain organisms extract this from solution for their skeletons, just as others extract calcium carbonate. The accumulation of these silicious skeletons often forms silicious rocks. The diatom, radiolarian, and other oozes (Fig. 353) of the deep sea are the great examples. Sometimes layers of infusorial earth, tripolite, arise from the shells of diatoms and other aquatic organisms secreting silica. The waters in which such earths accumulate are rather shallow, and either fresh or salt. The most familiar examples of indurated rocks formed in this general method are the flints and cherts (impure flints) that occur in limestone and chalk, chiefly as nodules, but sometimes in distinct beds.

=Organic rocks.=—While most limestones, chalks, flints, cherts, and the silicious and calcareous oozes are formed through the agency of organisms, they are not themselves strictly organic. There is, however, a small but important group of rocks formed directly from organic matter. In favorable situations the woody parts of plants, falling into water, are so far preserved from decay that they accumulate in beds, and by slow changes pass into peat, lignite, bituminous coal, anthracite, and graphite. The first of these is composed essentially of carbohydrates and hydrocarbons much as plants are, while the last two are mainly carbon, and the intermediate members represent stages of passage from the first to the last. They are all derived from the strictly organic part of the plants, and spring essentially from the atmosphere and hydrosphere. They are only indirectly associated with the evolutions of the inorganic series.

INTERNAL ALTERATIONS OF ROCKS.

Besides the extreme alterations of rocks at the surface of the earth by which they pass into solution and into residual mantle-rock, and at length by transportation and re-sedimentation become stratified rocks, as just described, those rocks which are not at the surface are subject to changes that give rise to several varieties of altered rocks. These changes are taking place constantly under ordinary conditions, though usually very slowly. Under great pressure and heat the changes are relatively rapid and intense, and lead to results not reached under other conditions. These more profound changes are termed metamorphism, and will be considered later. It is, however, important to recognize the great fact that the outer part of the earth, for perhaps 20,000 or 30,000 feet, is more or less fractured and permeated by water containing in solution various substances dissolved from the atmosphere, the soil, and the rocks through which it has already passed, and that this permeating and circulating water is now, and for long ages has been, working changes in the rocks, partly by dissolving matter out of them, partly by depositing matter in them, and partly by furnishing a medium through which new combinations of their constituents may take place. This outer fractured portion of the lithosphere has been called the zone of fracture.

=Oxidation and deoxidation.=—At and above the surface of the underground water, where the rocks are easily reached by atmospheric waters carrying much free oxygen, and by the air itself, oxidation prevails. Through oxidation the ferrous oxides are changed to ferric oxides, a change which is usually manifested by a transition from a gray, green, or blue color, to buff, brown, yellow, or red. The partial progress of such oxidation is often shown in a fractured block or bowlder whose exterior shows the latter colors, while the interior shows the former. The sulphides, of which common pyrites (FeS₂) is the most familiar, are oxidized into sulphates, and then sometimes pass on into the higher oxides and other compounds. Thus copperas (FeSO₄) arises from pyrites (FeS₂) by direct oxidation of both Fe and S. The sulphuric acid of this compound, uniting with some base stronger than the ferrous oxide, gives rise to further oxidation and results in hematite (Fe₂O₃) and limonite (Fe₂O₃,3H₂O). In general, the mineral constituents of the rocks in this upper zone take on their maximum states of oxidation. This oxidation affects more or less profoundly the character of the rock as a whole. Deeper in the earth oxidation is less prevalent, and the action is sometimes reversed and deoxidation takes place. So also wherever organic matter is undergoing decomposition deoxidation is likely to occur.

=Solution and deposition.=—Solution preponderates in the upper part of the zone of fracture, but deposition is prevalent in its deeper parts. The calcium carbonate and silica dissolved near the surface are often deposited below as calcite and quartz. The sulphates and other sulphur compounds that are formed and dissolved near the surface are apt to be changed into sulphides lower down by deoxidation. The soluble oxides and other compounds formed near the surface are often likewise precipitated below. This is particularly true where the descending waters encounter decomposing organic matter, and where they mingle with waters that have followed other routes and have become charged with different solutes. On coming together, reaction between the constituents takes place, resulting sometimes in new solutions and sometimes in precipitation.

If these lower deposits of calcite, quartz, sulphides, etc., are made in the pores of the rock, they change its texture and composition. If they are made in fissures they constitute veins, and if a sufficient percentage of the vein matter consists of valuable metallic compounds, they constitute ores.

As the waters descend they suffer greater and greater pressure and some increase of temperature, and these changes modify their power to hold substances in solution. In general, the waters increase in solvent power, but the effect is different for different mineral substances, and hence as a rule the waters are taking up some substances and laying down others as they proceed. After penetrating to greater or less depths, the waters may come again to the surface, either because they are pushed up by the higher head of the waters behind, or because they become warmer and thus lighter, and are forced up by the heavier cold waters above, or else they pass up by diffusion through the descending waters. In any case, the deep, warm waters, usually rather highly charged with material dissolved in their previous courses, are apt to deposit some of their burden as they ascend to horizons of lower pressures and temperatures. They are particularly liable to make deposits where they commingle with other waters differently charged with solutes. Thus internal changes in the body of the rocks are, and for ages have been, taking place. In the upper part of the depositing zone, calcite is the dominant mineral deposited, while in the lower, quartz is more common; but much depends on local conditions and other influences, and no rigid rule holds good.

=Hydration and dehydration.=—Water sometimes unites directly with some of the constituents of a rock and produces hydrated minerals, i.e., minerals that have water as an element of their constitution, not simply water absorbed into their pores. A large class of minerals known as zeolites, because they swell up and undergo life-like contortions when their basic water is driven off by heat, are examples of hydrous products. A more familiar example is limonite (Fe₂O₃,3H₂O), of which yellow ocher is a variety, which on heating sufficiently gives off its water and becomes hematite (Fe₂O₃) or red ocher. The turning of yellow clay to red brick on burning is a familiar example of dehydration. The general tendency in the upper zones penetrated by water is toward hydration. In the lower zones, where the pressure is great, Van Hise holds that there is a tendency toward dehydration, if the rocks have been previously hydrated. This may be the case if rocks have once been near the surface and later deeply buried by the accumulation of sediments on them. If the principle holds, rocks subjected to intense lateral pressure may be dehydrated.

=Carbonation and decarbonation.=—The igneous rocks are largely silicates. The carbonic acid of the surface-waters and of the air acting upon them, converts them, in part, into carbonates. In this way has arisen most of the original supply of calcium and magnesium carbonates. Original carbonates formed in this way are precipitated and redissolved again and again. The carbonates in river-waters are much more largely solutions of previously solid carbonates than original carbonates formed from the silicates. The potassium and sodium of the silicates also form carbonates, but by preference they unite with the sulphur and chlorine, and hence appear more largely as sulphates and chlorides.

Carbonation is usually accompanied by oxidation and hydration. These several processes break up the complex and relatively unstable silicates into simpler and more stable silicates, carbonates, and oxides. This is illustrated by the following formulas illustrative of the changes undergone by augite and labradorite, two common rock-forming minerals.

The composition of augite may be represented by the formula

{ CaO.(Mg,Fe)O.2SiO₂ { (Mg,Fe)O.(Al,Fe)₂O₃.SiO₂.

Assuming Mg and Fe to be equal in amount in the first half of the above formula, and Mg and Fe to be equal in the first part of the second half, and Al and Fe to be equal in the last part of the second half, doubling the whole and allowing it to be acted on by CO₂ and H₂O, we have

2CaO.2MgO.2FeO.Al₂O₃.Fe₂O₃.6SiO₂ + 6CO₂ + 2H₂O = 2CaCO₃ + 2MgCO₃ + 2H₂O.Al₂O₃.2SiO₂ + 2FeCO₃ + Fe₂O₃ + 4SiO₂.

The hydrous silicate of the last part of the equation is kaolin.

The composition of labradorite is represented by the formula

{ CaO.Al₂O₃.2SiO₂ { Na₂O.Al₂O₃.6SiO₂.

Assuming the two molecules represented by this formula to be equally abundant, and allowing the whole to be acted on by H₂O and CO₂, we have

CaO.Na₂O.2Al₂O₃.8SiO₂ + 4H₂O + 2CO₂ = CaCO₃ + Na₂CO₃ + 2(2H₂O.Al₂O₃.2SiO₂) + 4SiO₂.

When waters charged with carbonates descend into the earth they are likely to precipitate a portion of their burden, forming calcite and other crystalline carbonates, and hence these are among the most common minerals found in veins and rock cavities. Carbonates are also deposited when carbonate-charged waters come to the surface and evaporate or lose a part of their carbon dioxide.

Decarbonation also takes place, but it is, at least at the surface, a much less common process, and its conditions are less well understood. Sufficiently high heat will drive off the carbon dioxide, as in the artificial process of burning lime, but this is rarely observed in nature. Even lava intrusions do not usually reduce limestone to caustic lime at any appreciable distance from the contact. It is believed, however, that in the deeper zones, where high pressure and heat prevail, carbonates are changed into silicates, thus in a way reversing the process that prevails at the surface, and setting free again a portion of the carbon dioxide that had become locked up in the formation of the carbonates. To this action some of the carbon dioxide of deep-seated thermal springs is assigned.

The carbonation of the silicates takes place at the expense of the carbon dioxide of the atmosphere and hydrosphere, and hence in proportion as the igneous rocks are changed into carbonates, the atmosphere and hydrosphere are depleted of carbon dioxide, new supplies being neglected. As plants are dependent on carbon dioxide for their principal food, and as animals are dependent on plants for their food, directly or indirectly, the process of carbonation has a profound bearing on the life-history of the earth, and will often invite attention in the historical chapters. It is sufficient here to note that carbonation is one of the chief processes in the alteration of igneous rocks and furnishes, directly and indirectly, a larger percentage of the mineral substances dissolved in the waters that flow from the land, than any other single process.

=Molecular rearrangements.=—Besides these and similar changes that involve additions and subtractions through the agency of percolating water, the molecules of some of the rock constituents rearrange themselves, or the elements enter into new chemical relations; thus, pyroxene may pass into hornblende by a change of the crystalline arrangement of the molecules. The change may sometimes be caught in progress, the outer part of the crystal being hornblende (which when thus formed is called uralite), while the heart of the crystal remains pyroxene. So aragonite may pass into calcite.

By changes of the foregoing kinds, many crystalline rocks are much altered. Some become chloritic from the development of the soft, green hydrated mineral, chlorite, derived from the pyroxene, amphibole, biotite, and perhaps other silicates of the original rock. Others become talcose from the development of talc, a very soft, unctuous, hydrous magnesian silicate developed from the magnesian minerals of the original rock. Soapstone or steatite is a rock composed essentially of such secondary material. Serpentine is a rock made up of a similar secondary mineral (serpentine) apparently derived from chrysolite (olivine) and other magnesian minerals. Epidote, a complex lime-iron-alumina silicate, often recognizable by its peculiar pistachio-green color, is derived from other silicates, and is rather common in many varieties of crystalline rocks. Melaphyre is a name applied rather loosely and variously to certain altered basic rocks of the basalt family. Diabase is essentially an altered dolerite. Nearly all the very ancient basaltic rocks show notable degrees of alteration, even though they appear to have escaped unusual dynamic conditions since their original formation, and hence their alteration seems to have resulted chiefly from the operation of unobtrusive agencies, chief among which is the circulation of water.

The Salient Features of Rock Descent.

The foregoing processes by which primitive or igneous rocks are disintegrated and their constituents converted into fragmental material may be said to constitute the descent of rocks in its fuller sense. Viewed chemically, the great features of the process are (1) the breaking down of the complex silicates, and (2) the gathering of the resultant simpler silicates (mainly aluminum silicates) into the silt and clay beds, (3) the assembling of a large part of the free acidic element (the quartz) into the sand and gravel beds, and (4) the concentration of a large part of the earthy basic element (the calcium, magnesium, and iron oxides) into the calcareous, magnesian, and iron deposits, while (5) a large part of the alkaline basic remainder (the sodium, and potassium oxides) is dissolved and held in the sea-water. Physically, the great features are (1) the disaggregation of the antecedent rock, and (2) the separation from one another of products which are physically unlike, that is, the coarser from the finer, and the heavier from the lighter, and (3) the aggregation of these diverse materials in more or less distinct beds. It is to be noted that while the rearrangement of the sediments is made on the basis of their physical characters, it results in chemical differentiation as well, for the products of rock decay, which are physically diverse, are often chemically diverse as well. The physical assortment and the stratification are to be looked upon as a step in the direction of a simpler grouping of the material. On the whole, the process is descensional in character.

THE REASCENSIONAL PROCESS.

Running hand in hand with this descensional process, there has always been a reascensional process by which the coherence, the crystallization, and in some measure the complex composition of the rocks are restored. This is partially due to external mechanical agencies, but chiefly to internal chemical and molecular forces.

Two general phases of this reconstructional work are recognized. The first, simplest and most universal, is that by which the incoherent materials produced by the descensional processes, i.e., the muds, sands, and clastic materials generally, are hardened into firm, coherent shales, sandstones, and limestones, and incidentally more or less changed in composition and molecular arrangement. The second is that by which more profound changes of induration and of composition are wrought, bringing the rock back to a state resembling its original crystalline character. This is known as metamorphism. Often, however, it is but an extension and intensification of the more common processes of the first class. Metamorphism is essentially reconstruction.

=Induration under ordinary pressures and temperatures.=—All kinds of loose fragmental material, whether soils, earths, clays, sands, gravels, volcanic ashes, cinders, or other forms of clastic or pyroclastic material, may become hardened into firm rock either by pressure, or by cementation, or by both. Pressure and cementation commonly act together and aid each other. The ordinary pressures arise from the weight of the overlying material, and these of course increase with depth. Extraordinary pressures arise from the shrinkage of the earth and perhaps from other sources. The fragments of the clastic material, on being pressed together for long periods, weld more or less at the points of contact. If they are irregular, angular, or elongate, they come to interlock more or less like the fragments of macadam, and this coöperates with the welding. The process is greatly aided by water-bearing solutions of lime, silica, etc. which are deposited at the points where the fragments press upon each other. It is here that the capillary spaces are most minute and deposition is most liable to take place. Sometimes a film of mineral matter is laid down over the surfaces of the fragments and serves to bind them together. This process goes on wherever the ground-waters are in a depositing condition, just as the opposite process of disintegration takes place wherever the waters are in a solvent state. At and near the surface of the land, the waters are usually in the latter condition and disintegration is in progress, as already noted, but this is not always so. At times and places, the water from within the rock-mass may come to the surface and evaporate, and in so doing leave all its dissolved material on the surface, or within the outer pores of the mass, as cementing material. The exterior thus becomes firmly bound together, “case-hardened,” as it is termed. This may be seen in the drying of a lump of mud, the exterior of which often becomes quite firm. It is seen in quarry-rock, especially sandstone, which is sometimes soft and easily worked when taken wet from the earth, but which hardens as the water—the “sap” of the quarrymen—dries out and deposits its solutes in the capillary spaces of the grains of the surface. It is obvious that it is the very last of the “sap” which contains the most concentrated solutes, and that this last remnant is held in the minute capillary spaces where the grains touch each other, and hence the last stage of drying leaves the cement at the points where it is most effective. In natural exposures of sandstone, the pores of the outer shell sometimes become almost completely filled in this way with silicious deposits, and the sandstone is changed into a quartzite.

In the sea, and in the deep water underground, the common habit of the water is to deposit more than to dissolve, though it is doing more or less of both. As a rule, therefore, loose material in these situations becomes bound more or less firmly into rock, and hence what were originally loose sand beds become sandstones; what were soft muds become shales or limestone, according to composition; what was gravel becomes conglomerate; what was chipstone becomes breccia; what were volcanic ashes, cinders, and lapilli become tuffs; and what were masses of volcanic blocks and coarse fragments become agglomerates.

+Fig.+ 354.—Quartz crystal enlarged by secondary growth. The shaded outline represents the outline of the sand grain; the solid lines, the outline after secondary growth. Magnified 67 diameters. (Van Hise.)

+Fig.+ 355.—Sandstone and quartzite texture. The shaded outlines represent the surfaces of the sand grains before growth, the intervening white portions, the added quartz, and the black portions, unfilled spaces. Open spaces characterize sandstone. When the spaces are filled with quartz, the rock becomes quartzite. Magnified 35 diameters. (Van Hise.)]

The cementing process works at times in specially interesting ways. In quartz sandstones, the grains are worn fragments of quartz crystals, formed originally in quartz-bearing rock. The crystalline force in these remnants controls the arrangement of the new molecules of silica deposited about them. The result is that the new deposits tend to build up the original forms of the crystals from which the sand grains were derived (Fig. 354). Sometimes a film of iron oxide has formed about the grain of sand before the addition of the new silica. This, or some difference of color, may clearly distinguish the original grain from subsequent additions. Sometimes the adjacent grains of sandstone are rebuilt in this way until the interstices are completely filled. When this has been accomplished, the sandstone becomes a quartzite (Fig. 355). Most quartzites indeed appear to have been formed in this way, but mainly under special conditions that promote the deposition of silica. Grains of other minerals, such as feldspar, are subject to similar secondary enlargement (Fig. 356).

Sometimes the new material is deposited in the form of concentric shells about the particles of sediment, building them up into little spheres. Rock formed of such spherules is known as oolite, from the resemblance of the grains to the roe of fish (Fig. 357). Sometimes the nuclei of the concretions are grains of quartz sand, and the added concentric layers are of calcium carbonate. In this case the structure is quite obvious; but perhaps more frequently the nuclei are minute and difficult to identify, and the concentric shells make up the main mass of the grains. Certain formations, as the oolitic limestone of Indiana and elsewhere, and the Upper and Lower Oolites of England, are characterized by this structure. In most cases these accretions probably grew in depositing waters that gently rolled the grains while layers were being added. They thus do not fall under the head of cementation after the beds were formed; but concentric additions to the grains appear sometimes to have taken place after they were formed into beds.

=Cavity filling.=—When cavities of some size occur in rocks and the percolating waters are in a depositing state, the interiors of the cavities are sometimes lined with concentric layers of deposit. Here, instead of building out from a nucleus, the waters build in from the walls of the cavity. The agate structure (Fig. 358) is a case of this kind, in which the successive layers are commonly silica in the form of chalcedony and differ from each other in color and texture. Often before the cavity is entirely filled, the deposit changes from chalcedony, to crystals of quartz, which grow with their bases on the walls and their pyramidal points toward the center of the cavity. Geodes are examples of a similar process in which the cavity is but partially filled with crystals which have their bases set on the walls of the cavity and their points directed inwards (Fig. 359). The crystals of geodes are most commonly quartz or calcite, but they may be any other mineral that the waters are capable of depositing. Very large cavities lined in this way are known to miners as vuggs, and these grade on into caves lined with crystals and with stalactite and stalagmite. These are the largest expression of the solidifying process by means of internal deposition.

=Fissure-filling; veins.=—Cracks, crevices, and fissures filled by deposition in a similar way give rise to veins (Fig. 360). Here the filling grows from the walls toward the center, and hence often has a banded appearance. By this filling of cracks and crevices, the circulating water heals the breaks in the rocks. Frequently a crushed zone is thus restored to a solid state. When the fissures are deep and wide and traverse different formations, conditions are afforded for very complex deposits, and for the concentration of rare and valuable material originally dispersed through a great mass of rock. Ore deposition in such veins is usually treated as a theme by itself, but it is really but a declared expression of the work which the percolating waters are doing throughout all the rocks which they penetrate. Most of the fine crystals that grace mineralogical collections were formed in cavities and fissures by deposition from circulating mineralized waters.

=Solution as well as deposition.=—A further phase of the process needs attention. The percolating waters are constantly taking up matter as well as throwing it down, and so, while they are cementing fragments together and healing fractures, they are also removing material, and a rock may be growing porous and cavernous at the same time that its fragments are being united. Cavities may be formed at one stage and filled at another; matter may be taken up at one point and put down at another, and so an internal reconstruction is in slow progress.

=Concretions.=—A notable phase of this internal reconstruction is the assembling together of like kinds of matter. For instance, silica that was probably deposited in the form of the silicious shells and spicules of plants and animals, and was disseminated through the sediments as originally formed, is aggregated into nodules of chert or flint (Fig. 361); similarly, concretions of ferrous carbonate or calcium carbonate grow in sands, silts, or muds; clusters of crystals of pyrite (FeS₂), of sphalerite (ZnS), and galenite (PbS) are formed in clayey layers, pressing the clay back as they grow; and in many other cases, kind comes to kind. Some concretions probably form during the accumulation of the beds in which they lie.

=Replacements and pseudomorphs.=—So also there are replacements, sometimes resulting in imitative or false forms. Frequently the calcium carbonate of corals, molluscan shells, etc., is replaced by silica, and this substitution is brought about so gradually, particle by particle, that the minutest details of structure are sometimes fully preserved. This is often of great service in their study, since the limestone in which they are imbedded may often be dissolved away, while the silicified fossil is unaffected. So woody matter is sometimes replaced by silica, forming silicified wood. Similarly, the molecules of one crystal are sometimes replaced by different material, as the molecules of calcite by zinc carbonate, giving a pseudomorph of zinc carbonate after calcite.

=Incipient crystallization.=—A more general change is incipient crystallization. Some common limestones and dolomites are now largely made up of small crystals, though the mass was originally a calcareous mud or ooze. Incipient crystals are formed in shales and other sediments. This process, like the preceding, is a kind of incipient metamorphism or reconstruction, but it is a pervasive process, taking place under ordinary conditions of heat and pressure, and through the agency of circulating ground-waters.

By these and similar processes the fragmental deposits are solidified into firm rock and undergo internal changes which more or less reorganize the matter of which they are composed. The process is a very slow one usually. Some of the sands and muds of very early geologic ages are yet imperfectly solidified; e.g., much of the St. Peter’s sandstone, a very ancient formation, is yet so incoherent as to break down into sand in being dug out, and is used for mortar sand much more than for building stone. Some of the Hudson River shales of scarcely less age are more nearly clay than hard rock. But these are examples of excessive slowness and slightness of change. In general, all but the most recent deposits show notable progress in reconstruction.

Reconstruction under Exceptional Conditions.

Two special conditions greatly influence changes in rocks, viz., pressure and heat. Their action gives rise to three general cases, but these blend indefinitely: (1) exceptional pressure without great heat, (2) great heat without exceptional pressure, and (3) great heat and great pressure conjoined. Exceptional pressure may arise from the weight of overlying rocks, or from lateral thrust due to the shrinkage of the globe, and occasionally from other causes. Exceptional heat may arise from pressure, from the intrusion of hot lavas, and occasionally from other sources. In the case of intruded lavas there may or may not be exceptional pressure. Thrust usually gives heat as well as pressure, but if lateral thrust acts on rocks near the surface, they may be mashed into new forms without becoming very exceptionally heated, though some rise of temperature is inevitable.

(1) =Slaty structure.=—When rocks made up of clastic particles are compressed in a given direction and are relatively free to expand at right angles to the direction of pressure, the particles that are already elongated tend to take positions with their longer axes at right angles to the direction of pressure, and all particles, whether elongate or not, are more or less flattened in a plane transverse to the direction of pressure. This may be readily seen where the particles are large (Fig. 362). As a result of the orientation and flattening of their particles, rocks so affected split more readily between the elongate and flattened particles than across them. In other words, the rocks cleave along planes normal to the direction of compression, and break with difficulty and with rough fracture across the planes of cleavage. The condition thus induced is known as slaty structure (Fig. 363), and is best illustrated by roofing-slate, which was originally a mud, later a shale, and finally assumed the slaty condition under strong compression. Sometimes the original bedding may still be seen running across the induced cleavage planes (Fig. 364). As the original mud beds were horizontal or nearly so, and as the thrust is usually horizontal or nearly so, the induced cleavage commonly crosses the bedding planes at a high angle (Fig. 364); but after the beds are tilted or bent, the lines of pressure take new directions relative to the bedding planes, and the angles between the original bedding and the slaty cleavages usually become smaller, and may even disappear in exceptional cases. Limestones, sandstones, and conglomerates are not so easily compressed as mudstones, and they usually take on only an imperfect cleavage normal to the direction of pressure. Often they merely show some little compacting, while the shaly strata between them are converted into slate. Obviously the direction of slaty cleavage may be used to determine the direction of the compressing force, and is thus serviceable in dynamic studies.

=Foliation, schistosity.=—A more intense application of pressure in a given direction is capable of breaking down and deforming the most resistant rock. This must necessarily be attended with the evolution of much heat, and thermal effects are mingled with pressure effects, but the thermal effects may be neglected for the moment. The first stage of the mechanical effect of the compression may be to crush the rock more or less. It thus becomes granular or fragmental, and is really a peculiar species of clastic rock (autoclastic). At a further stage, the fragmented material may be pressed into layers or leaves, much as in the development of slaty cleavage, but as a result of the nature of the material, the cleavage is less perfect. This is often attended by more or less shearing of the material upon itself, and thus a rude fissility and foliation is developed. The result, including the attendant metamorphism about to be described, is a foliated or schistose structure (Figs. 365 and 366). Even the most massive rocks may be reduced to the foliated form by this process; thus, a granite may be mashed into a gneiss—which is a granite in composition, but has a foliated structure—or a basalt may be converted into a schist, a common term for foliated crystalline rocks. Porphyritic rock rendered schistose by pressure is shown in Fig. 366. When massive rocks like granite or basalt are thus crushed down into the foliated form, the process is in a sense degradational. It is a kind of katamorphism or downward change. It is often difficult to differentiate the schists thus derived by degrading massive rocks, from those developed by ascensional processes from clastic formations (anamorphism). The action of heat is important in the evolution of schists of both classes, but the effects of heat may best be taken up where it acts measurably alone.

=Metamorphism by heat.=—When a mass of lava is poured out upon the surface, it bakes the mantle-rock which it overruns, in greater or less degree, depending on the mass and temperature. The nature of the effect is much the same as in the process of brick-making, a dehydration of the material, a hardening of the loose matter by the partial welding of the particles, and sometimes the partial fusion of the surface and the development of new compounds, usually glassy, but sometimes partially crystalline. In both the natural and the artificial process, the time element is short, the pressure trivial, and the water action limited. If the heat were to become sufficiently intense, the result would be fusion, i.e., a lava which would solidify into a glass. In such a case, the rock cycle would be carried back to the initial molten state and a new cycle instituted, but this does not usually take place when lava merely overflows the surface.

If lavas, instead of rising to the surface, wedge in between layers of rock and form sills, or interstratified sheets, the surface above as well as that below is baked, and as the excess of heat of the lava can only escape through the neighboring rock, the effects for a given mass of lava are more considerable, and as the time element and the water action (and sometimes the pressure) are usually greater than in the case of extruded lavas, the effects tend rather toward chemical and crystalline change than to simple baking. This tendency increases with increase in the mass of the lava and in its temperature. Sometimes enormous masses of very hot lava are thrust in between or among the strata that lie beneath the surface, and bring to bear upon them intense heat for a long period. So also, when a vent or fissure is the passageway for lavas that continue to come to the surface for long periods, as in the case of persistent volcanoes, the rocks which form the walls of the vent or fissure are heated for a long time, and this gives rise to metamorphism through heat, without very unusual pressure, but usually with the free aid of water. In these cases the chief effect is chemical recombination and crystallization. In the limestones and sandstones it is simple; in the shales more complex. In pure limestones and dolomites little chemical change takes place, but the molecules are rearranged into larger and more perfect crystals, and marble is the result. The coarseness of the crystals is, in a general way, a measure of the length of time during which the heat acts, and of its intensity, but much depends on the freedom of the attendant water circulation. Crystals an inch or two across are sometimes formed in the contact zone, where the attendant water action is important. If impurities, as silica, alumina, iron, etc., are present, various minerals, such as tremolite and actinolite, may be formed in the marble. In pure quartzose sandstones, the effect is to cause the building up of the quartz grains until the interspaces are essentially filled and the whole becomes a massive quartzite. Here, as in the marbles, impurities form adventitious crystals, a very common one being hematite, formed from the segregation of the ferric oxide of the sandstone.

In the shales, the material to be acted upon is more complex, for, while the main mass is an aluminum silicate, there is usually much free quartz, not a little potash and iron, and more or less of lime, magnesia, soda, and other ingredients, for the muds from which the shales arose contained not only the fully decomposed matter of the original crystalline rocks, but the fine matter worn from them by wind and water without decomposition. When this mixed matter is acted upon by high heat and moisture, it tends to return to its original crystalline state, so far as its changed constitution permits. The potash chiefly unites with alumina and silica, and forms potash feldspar (orthoclase chiefly) and potash mica (muscovite). The iron often unites with magnesia, alumina, and silica to form biotite or one of the ferromagnesian minerals, chiefly an amphibole. The lime usually aids in the formation of other silicates of either the feldspar or the ferromagnesian group, while the surplus silica crystallizes into quartz. There is usually a predisposition to form mica in preference to other silicates if the proper constituents are present, and the result is that mica schists and gneisses, in which mica abounds, are common products of the metamorphism of shales by contact with bodies of lava. Mica schists and micaceous gneisses are also formed in other ways, and other schists, dependent on the composition of the shales, are formed about intrusions of igneous rock. In all such cases pressure probably attends the heat and is a factor in the development of the schists. When the change induced by the heat is less considerable, the shale is baked, with incipient recrystallization, and often takes the form of argillite, a compact, massive sort of shale.

Beds of hydrous iron oxide (limonite) or of iron carbonate (siderite) are usually converted by heat into hematite or magnetite. Beds of peat, lignite, and bituminous coal are converted into anthracite by the driving off of the volatile hydrocarbons. If the process goes to the extreme, graphite is the result.

=Metamorphism by heat and lateral pressure.=—As already indicated, the more common intense pressures experienced by rocks at and near the surface are those that come from lateral thrusts arising from the shrinkage of the earth. These affect one dimension of the rock-mass, while they permit it to expand in one or both of the other dimensions. This produces a strain in all the constituent particles of the rock, and under such strain they pass more readily into solution than when free from strain, and more readily rearrange their molecules internally into positions of less strain. The crystals grow most freely along the planes of least stress, i.e., at right angles to the pressure. As a consequence, where unidimensional pressure and high heat resulting from the compression unite their influence, the metamorphic changes are not only facilitated, but the rearrangement is controlled by the pressure and results in a parallel arrangement of the constituent crystals, giving a foliated or schistose character to the new rock. The changes themselves are much the same as those produced by heat and water without exceptional pressure, though some distinctions may be noted. It is to be observed, however, that two kinds of work are embraced here: the metamorphism of clastic rocks into crystalline schists, which may be regarded as an upbuilding process, anamorphism, and the mashing down of massive crystalline rocks into schists, which may be regarded as a degradational process, katamorphism. In both cases, however, there is solution and rearrangement of the molecules. The katamorphism of basalts and other basic rocks gives basic schists; that of granitic and similar rocks gives gneisses. The anamorphism of basic pyroclastic tuffs and wackes gives basic schists, while that of acid pyroclastics and most shales gives gneisses, mica schists, or similar acidic schists. It is obvious that ordinary shales cannot usually become basic schists, because in producing the original muds, the bases were generally removed; but when shales are highly calcareous and magnesian, as when they grade toward the limestones and dolomites, they may become basic schists by metamorphism, e.g., certain hornblendic schists. It is even more obvious that the limestone and sandstone formations must largely retain their distinct composition. It is thus seen that, in general, a sedimentary series anamorphosed must differ from a crystalline series katamorphosed, though both give rise to foliated or schistose rocks.

=Deep-seated metamorphism.=—When the exceptional pressure arises from the weight of rocks felt at great depth, it is practically equal in all directions and the crystallization probably develops normally and is not forced into the parallel or foliated form. Rocks metamorphosed under these conditions probably tend to take the massive form rather than the schistose form, but this conclusion is theoretical rather than observational, for little or nothing is known of the history of such rocks.

=Completion of the rock cycle.=—The crystallizing processes of metamorphism are fundamentally similar to the processes by which rocks crystallize out of magmas, only in the first case the work is done chiefly by the aid of an aqueous solution, while in the second it is done through a mutual solution of the constituents in themselves, where water was but an incident. If the heat factor in metamorphism be sufficiently increased, aqueous solution may actually grade into magmatic solution through various degrees of softening and melting, and the cycle of changes be closed in upon itself.

VARIOUS CLASSIFICATIONS AND NOMENCLATURES.

From the foregoing sketch of the processes of rock-making it may easily be inferred that the varieties of rocks may be almost unlimited, and that they may be defined, named, and classified on many different bases; for example.

(1) If the mode of origin is chiefly in mind, rocks may be classed as igneous (lavas, tuffs, etc.); metamorphic (schists, gneisses, anthracite, magnetite, etc.); aqueous (water-laid sediments, stalactites, travertine, etc.); eolian (dunes, loess in part); glacial (till, moraines); clastic (mantle-rock, sandstone, conglomerate, etc.); organic (peat, lignite, coal, etc., and indirectly, limestone, chalk, infusorial earth, etc.); and so on.

(2) If the textural or structural characters are in mind, rocks are designated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure rocks); glassy (obsidian, tachylite); porphyritic (distinct crystals in obscure matrix); granitic (well-grained); compact, porous, earthy, arenaceous (sandy), schistose, etc.

(3) If the chemical composition is chiefly regarded, they may be classed as silicious, calcareous, carbonaceous, ferruginous, etc.; or, if the chemical nature is considered, they are grouped as acidic, basic, or neutral.

(4) If the crystalline character is made the basis, they are designated phanerocrystalline (distinctly crystallized), microcrystalline (minutely crystallized), cryptocrystalline (hiddenly crystallized), and amorphous (non-crystalline).

(5) If attention is fastened on certain ingredients, rocks are characterized as quartzose, micaceous, chloritic, talcose, pyritiferous, garnetiferous, etc.

(6) When rocks are regarded as mineral aggregates, if (a) the aggregates are simple, they are named from the dominant minerals, as dolomite, hornblendite, garnetite, anorthite, etc.; and if (b) the aggregates are complex they take special names, as syenite (orthoclase and hornblende), gabbro (plagioclase feldspar and pyroxene), etc.

(7) When the point of view is structure of the mass, they are classed as massive, stratified, shaly, laminated, slaty, foliated, schistose, etc.

(8) When physical state or genesis is considered, they are grouped as clastic, fragmental, or detrital (conglomeratic, brecciated, arenaceous, argillaceous, etc.); or pyroclastic (tufaceous, scoriaceous, agglomeratic); or massive, in a sense slightly different from that above (7).

As sometimes one of these characteristics and sometimes another is most important in a given rock, or in a given study, no one classification is satisfactory in all cases, yet each has its advantages in particular cases.

New System of Classification and Nomenclature.

The present systems of classifying and naming rocks have grown up gradually out of earlier and cruder methods, many of which were inherited from popular usage. Most of the names and definitions came into use before microscopical and other modern means of study were adopted. These systems, therefore, retain many inherited crudities and inconsistencies, and lack adaptation to present needs. They are too complex and difficult for field use and for general discussions, while not sufficiently exact and systematic for the more rigorous petrological discussions. A more adaptive and consistent practice has been earnestly sought by petrologists, and a new system of classification of igneous rocks has been offered by a group of leading American petrologists, an outline of which is here given. To some extent this may be extended to the metamorphic crystalline rocks with necessary modifications and additions. The classification and nomenclature of the secondary rocks must probably always remain variable and plastic to express the various points of view which it is desirable to take. During the transition to this or some other new system, which seems inevitable, the appended alphabetical reference lists of the most common minerals and rocks, with brief definitions in accordance with current usage, will be found serviceable. The proposed system includes two parts, a field system and a quantitative system, the one applicable to rocks on casual inspection, and the other, only after detailed study.

The proposed field system.

The proposed field names are based largely on texture and color. The mineral constituents are used for subdivisions when they can be determined; otherwise they are neglected.

Classifying chiefly on the basis of texture and crystalline state, there are three groups: Phanerites, in which all the leading mineral constituents can be seen megascopically; aphanites, in which all, or at least an appreciable part, of the constituent minerals cannot be distinguished megascopically; and glasses, in which the material is wholly or largely vitreous.

I. The =Phanerites= may be further classified by their chief mineral constituents as follows:

1. Granites (f.n.), consisting largely of quartz and feldspar of any kind, with or without mica, hornblende, pyroxene, or other minerals. This differs from the present common use in not regarding mica as an essential constituent, and in not distinguishing between alkali feldspars and calcic feldspars, thus broadening the class.

2. Syenites (f.n.), consisting predominantly of feldspar of any kind, with subordinate amounts of hornblende, mica, or pyroxene, but with little or no quartz. This differs from the common use in giving hornblende a subordinate place, and in embracing rocks with calcic feldspars, thus broadening the class.

3. Diorites (f.n.), consisting predominantly of hornblende and subordinately of feldspar of any kind, with which there may be mica, pyroxene, or other minerals. This is nearly the present use except that any kind of feldspar may form the subordinate element.

4. Gabbros (f.n.), consisting predominantly of pyroxene and subordinately of feldspar of any kind, with or without other minerals. This nearly coincides with one of the various present uses of the term except that the range of the feldspar is increased.

5. Dolerites (f.n.), consisting predominantly of any ferromagnesian mineral not distinguishable as hornblende or pyroxene, with subordinate elements of feldspar of any kind, and with or without other accessory minerals. A name to be used when the dominant mineral is clearly ferromagnesian, but cannot be satisfactorily identified as either hornblende or pyroxene, although it may probably be one of these. In other words, the dolerites (deceptive) embrace the whole diorite-gabbro group when too obscure for separation.

6. Peridotites, consisting predominantly of olivine and ferromagnesian minerals, without feldspar, or with very little.

7. Pyroxenite, consisting essentially of pyroxene without feldspar or olivine.

8. Hornblendite, consisting essentially of hornblende without feldspar or olivine.

II. The =Aphanites= may be non-porphyritic or porphyritic.

(a) Non-porphyritic aphanites when light-colored may be classed as felsites; when dark-colored, as basalts.

(b) The porphyritic aphanites or porphyries, when light-colored, are leucophyres; when dark-colored, are melaphyres (f.n.). They may be classified further, according to the kind of phenocryst imbedded in the aphanitic ground-mass, as

Quartz-porphyries, or quartzophyres; Feldspar-porphyries, or feldspaphyres (not felsophyres); Hornblende-porphyries, or hornblendophyres; and so on. These may be subclassed by color, as Quartz-leucophyres, light-colored quartz-porphyries; Quartz-melaphyres, dark-colored quartz-porphyries; Feldspar-leucophyres; Feldspar-melaphyres; and so on.

III. The glasses are classified, according to color and luster, into obsidians or pitchstones when dark and lustrous; perlites, when a spheroidal fracture gives them a pearly appearance; and pumice when greatly inflated by included gases.

In general discussions, it is regarded as serviceable to use the term granitoids in a broad generic sense, to include all crystalline rocks of the general granitoid type, including the granites, syenites, gneisses, etc. In a similar broad way, the term gabbroids may be used to include the dark crystalline rocks in which the ferromagnesian minerals predominate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and comprehensive way, two contrasted groups of igneous rocks may be designated. As the granitoids are usually acidic and the gabbroids usually basic, the grouping represents a broad fact of importance.

THE PROPOSED QUANTITATIVE SYSTEM.

The distinguishing characteristic of the more rigorous system designed to meet the needs of scientific petrology is its quantitative chemical character. All igneous rocks are classified primarily according to their chemical composition and only secondarily according to their mineral constituents, texture, and other characters. The rigorous application of the system requires chemical analyses of the rocks, but as these are not available in many cases, the authors of the system have devised a method of optical mineral analysis by which the nearly exact proportions of all the constituent minerals can be determined, and by knowledge of their chemical nature the results may be converted, by computation, into chemical terms. This can only be done for holocrystalline rocks whose crystals are large enough to be measured under the microscope, but aphanitic rocks may often be approximately classified by comparison with similar rocks already accurately determined. To facilitate this method of chemical analysis by measuring the minerals, the chemical composition of certain common rock-making minerals is expressed in proportional parts and tabulated, and is used somewhat as molecular weight is in ordinary chemical analysis. Certain of these are selected as standard minerals, the selection being such that the standard minerals embrace all the essential elements that enter into the composition of rocks. All other minerals are converted into their chemical equivalents in terms of these standard minerals by the use of the tables. All the mineral constituents being thus reduced to standard minerals, the classification is built up systematically on these standard (or standardized) minerals.

A new system of names is required, and these have been very skillfully formed by selecting significant letters from the names of the leading minerals or from words signifying their preponderance, so that short terms which carry their meaning in their forms, are secured, and this has been done so that these are usually euphonious, however strange they may seem to our preoccupied senses. For example, minerals composed chiefly of silica and alumina are called salic; those of ferromagnesian minerals, femic; those of aluminous ferromagnesian minerals, alferric, etc. When in a combination of salic and femic minerals, the salic are extremely abundant, the rock is persalic; if notably dominant, dosalic; if the salic and femic minerals are nearly equal, salfemic; if the femic are dominant, dofemic; if extremely abundant, perfemic, and so on, the system being mnemonic. This method of deriving names is applicable only to a portion of the necessary divisions. For the rest, a series of roots derived from geographic names, with a system of terminations, has been employed.

All standard minerals are divided into two groups of primary importance: one of minerals characterized by alumina, as the feldspars,—orthoclase, albite, anorthite,—leucite, nephelite, sodalite, noselite, and corundum, to which are added the closely associated minerals, quartz and zircon. This is called the salic group. The second group contains minerals characterized by iron and magnesia with no alumina, as hypersthene (enstatite), acmite, olivine, magnetite, hematite, and ilmenite, to which are added the closely associated minerals, titanite, perofskite, rutile, apatite, and all other rock-making minerals except those containing alumina together with iron and magnesia. The second group is called femic.

Aluminous ferromagnesian minerals, such as hornblende, augite, mica, etc.; are called alferric, and are not classed as standard minerals, because their complexity of composition makes it better to treat them as though made up of the simpler minerals of the standard list.

The composition of all igneous rocks can be expressed in terms of the relative proportions of the two groups of the standard minerals, salic and femic. By subdividing these groups successively on a mineral and chemical basis, a series of classificatory divisions of greater and greater precision has been formed. In each stage of the series, two factors only are compared, and a simple set of ratios has been selected to limit the divisions. Assuming the possibility of a continuous range of variable mixtures of the two factors (A and B) from an extreme composed wholly of one (A), and an extreme composed wholly of the other (B), five ideal cases have been chosen as types or centerpoints about which variation in mixture may take place. These are:

A = 1 A = 3 A = 1 A = 1 A = 0 ——— —, ——— —, ——— —, ——— —, ——— —. B = 0 B = 1 B = 1 B = 3 B = 1

Division lines half-way between these points occur as the following ratios:

A 7 A 7 > 5 A 5 3 A 3 1 A 1 (1) ——— > —, (2) ——— < — > —, (3) ——— < — > —, (4) ——— < — > —, (5) ——— < —. B > 1 B < 1 > 3 B 3 5 B 5 7 B 7

These ratios are used throughout the system. In (1) A is extreme; in (5) B is extreme; in (2) A dominates over B; in (4) B dominates over A; in (3) A and B are equal or nearly equal.

All igneous rocks are grouped in five (5) primary divisions called Classes on a basis of the proportions of the salic and femic minerals, thus:

Class I. Sal 7 ——— > —, extremely rich in salic minerals, called persalane. Fem 7

II. Sal 7 5 ——— < — > —, with dominant salic minerals, called dosalane. Fem 1 3

III. Sal 5 > 3 ——— < — > —, salic and femic minerals, equal or nearly equal, Fem 3 5 called salfemane.

IV. Sal 3 1 ——— < — > —, with dominant femic minerals, called dofemane. Fem 5 7

V. Sal 1 ——— < —, extremely rich in femic minerals, called perfemane. Fem 7

Each of these classes is divided into two subclasses according to the proportions of two subgroups of the preponderant group of standard minerals. Of salic minerals one subgroup includes quartz, feldspars, and the feldspathoids; the other includes corundum and zircon. Of femic minerals one subgroup includes the silicates with magnetite, ilmenite, hematite, and rutile; the other contains apatite and the remaining minerals of this group. Most known igneous rocks fall into the first subclass of each class.

The classes are further divided into orders according to the proportions of certain minerals in the preponderant subgroups. Thus Classes I, II, and III are each divided into nine orders on a basis of the proportions of quartz and the feldspars, and of the feldspars and the feldspathoids, quartz and feldspathoids not occurring together. The orders may be described in the same terms for each of the first three classes as follows:

Order I. Q 7 — > —, extremely rich in quartz, perquaric. F 1

II. Q 7 5 — < — > —, quartz dominant over feldspar, doquaric. F 1 3

III. Q 5 3 — < — > —, quartz and feldspar equal or nearly equal, quarfelic. F 3 5

IV. Q 3 1 — < — > —, feldspar dominant over quartz, quardofelic. F 5 7

V. Q or L 1 —————— < —, extremely rich in feldspar, perfelic. F 7

VI. L 3 1 — < — > —, feldspar dominant over feldspathoids (lenads), lendofelic. F 5 7

VII. L 5 3 — < — > —, feldspar and lenads equal or nearly equal, lenfelic. F 3 5

VIII. L 7 5 — < — > —, lenads dominant over feldspars, dolenic. F 1 3

IX. L 7 — > —, extremely rich in lenads, perlenic. F 1

In classes IV and V the preponderant minerals are femic, and in subclass 1 they are silicates, titanates, and ferrates, with hematite and rutile. These are subdivided as follows:

Silicates—pyroxenes and olivine with akermanite in one subgroup; the other minerals, magnetite, hematite, ilmenite, titanite, perofskite, rutile, in the second subgroup. This first group is called polic, mnemonic of pyroxene and olivine; the second group is called mitic, mnemonic of magnetite, ilmenite, titanite.

There are five orders in each of these classes, as follows:

Order I. PO 7 —— > —, extremely rich in pyroxene or olivine, perpolic. M 1

II. PO 7 5 —— < — > —, dominant pyroxene or olivine, dopolic. M 1 3

III. PO 5 3 —— < — > —, pyroxene or olivine, equal or nearly equal to the mitic M 3 5 minerals, polmitic.

IV. PO 3 1 —— < — > —, dominant mitic minerals, domitic. M 5 7

V. PO 1 —— < —, extremely rich in mitic minerals, permitic. M 7

In the first three orders a distinction between pyroxene and olivine is recognized by sections, five in number: