Part 15

Rhyolite.—This rock corresponds in composition with granite and gneiss, but is less frequently micaceous. The orthoclase in rhyolite, and generally in volcanic rocks, is the clear, pellucid variety—sanidine. It is more difficult to separate from quartz than ordinary orthoclase, the chief distinguishing feature being its cleavage. Plagioclase and hornblende are common, but not abundant, constituents. The mica, when present, is usually biotite. The texture of rhyolite is often more or less distinctly porphyritic, having a finely crystalline or granular matrix, with interspersed crystals of sanidine and quartz. The rock has usually a rough, harsh feel; and while the coarser varieties have the aspect of granite, the finer approach petrosilex; but all are somewhat porous, which is seen in the lower specific gravity of rhyolite as compared with granite and gneiss.

Trachyte.—In texture and general aspect rhyolite and trachyte are nearly identical. Trachyte, however, is darker, contains little or no quartz, and more hornblende and plagioclase. In fact, it agrees in composition with syenite. This is one of the most important of the volcanic rocks.

Obsidian.—Obsidian is sharply distinguished from all other rocks by its perfect vitreous texture; it is a true volcanic glass. Its surface (specimen 47) is smooth and glassy, and its fracture eminently conchoidal. To the naked eye, and usually under the microscope, the typical variety is perfectly homogeneous; chemical analysis, however, shows that it has the composition, commonly of rhyolite, but sometimes of trachyte. Obsidian is, in fact, simply rhyolite or trachyte which, cooling quickly, has not had time to crystallize, but has remained permanently in the amorphous or glassy state. The composition is sometimes partially revealed where a portion of the sanidine comes out in distinct crystals porphyritically interspersed through the glass. The homogeneity of the texture is sometimes disturbed: by numerous minute concentric cracks, forming what is known as perlitic structure and the variety perlite; by numerous small spherical concretions, forming the spherulitic structure and the variety spherulite; and also by the banding, which is the result of flowing while in a plastic state, whereby portions of the glass of slightly different colors are drawn out into layers and interlaminated. The bands are rarely continuous for any distance, being usually merely elongated lenticular streaks. The glassy state is generally one of inferior density, and hence we find that obsidian is lighter than the crystalline rocks of the same composition. Obsidian is a good illustration of a non-essential color, for its capacity and jet-black color are due entirely to impurities. In very thin flakes it is transparent and white. It also forms a white powder when crushed, i.e., it has a white streak.

Obsidian is often vesicular, from the expansion of the steam and other gases which it contained when liquid. The most thoroughly vesicular varieties are known as pumice (specimen 48). The vesicular texture, by rendering the rock impervious to light, conceals the impurities, and thus we get a snow-white pumice from black obsidian. The vesicles are frequently elongated, sometimes in a definite direction, though often forming an irregular net-work of glassy fibres. Pumice is often light enough to float on water, and it is transported thousands of miles by the oceanic currents. It is employed in the arts, and good specimens can be obtained at almost any drug-store.

Petrosilex and Felsite.—Sharply defined groups are unknown in lithology, but all is gradation; and between rhyolite and trachyte, which are always more or less distinctly crystalline, and obsidian, which is a true glass and perfectly amorphous, there is no break. It is impossible to draw a sharp line and say, Here the vitreous texture ends and the crystalline begins; for the transition is not abrupt, but gradual. We recognize, really, in these feldspathic rocks, an intermediate state, which is neither crystalline nor colloid, but both; and this lithologists have designated the felsitic texture. Felsitic matter cannot, even with the highest powers of the microscope, be resolved into separate grains or particles; and it does not exhibit, except perhaps very indistinctly, the phenomenon of double refraction. In other words, it is not truly crystalline or stony, and yet it is just as clearly not amorphous or glassy.

Feldspathic rocks exhibiting the felsitic texture in whole or in part are known as felsites. Many high authorities hold that true felsites are found only among the eruptive rocks; while others claim that they are in part, or wholly, of sedimentary origin. The writer accepts the former view. The felsites are in part acid lavas which have cooled too slowly to form a true glass, like obsidian, and yet too quickly to become truly crystalline, like rhyolite and trachyte. But they are also in large part simply devitrified obsidian. Glass is an unstable form of mineral matter; and every species of glass, including obsidian, tends with the lapse of time to become crystalline or stony, the amorphous changing to the felsitic structure. Thus, in many cases or usually, what we now call felsites were originally true glassy obsidian. Being perfectly intimate mixtures of the component minerals, the composition of felsites can usually be determined with certainty only by means of chemical analysis. By this means chiefly, it has been proved that there are felsites agreeing in composition with both rhyolite and trachyte. There is this general difference in composition, however, between these crystalline rocks and the felsites; viz.: mica, hornblende, and augite are generally wanting in the latter. From this it follows that the felsites are, with unimportant exceptions, composed either of quartz and feldspar or of feldspar alone.

The physical differences between the felsites of unlike composition are not great; but they are sufficient to warrant the division of the felsites into two species: a basic species, to which the term felsites may properly be restricted; and an acidic species, for which petrosilex is a very appropriate name. According to this arrangement, felsite is composed chiefly of orthoclase, and, as the table shows, agrees in composition with trachyte; while petrosilex consists mainly of orthoclase and quartz, agreeing in composition with rhyolite. We find here nothing new in composition; but petrosilex and felsite are simply the crystalline rocks which we have already studied, repeated under a different texture.

The typical felsite or petrosilex is composed entirely of felsitic matter, and is perfectly homogeneous, like flint or jasper, which it closely resembles in hardness and other physical characteristics. As a rule, however, the rock is not entirely homogeneous, but there is a manifest tendency in the component minerals, and especially in the feldspar, to separate out, usually in the form of crystals. In the banded variety (specimen 42) the rock is built up of thin layers, which are often alternately quartzose and feldspathic. There is not a perfect separation of the minerals; but that the quartz is chiefly in the dark layers, and the feldspar in the light, is shown by the way in which the layers are affected by the weather.

One of the most common varieties is where a portion, frequently a large portion, of the feldspar comes out in the form of distinct, separate crystals, producing a porphyritic texture. Specimens 5, 6, and 7 are examples of porphyritic felsite; and after examining these we can no longer doubt that feldspar is an important constituent of the rock. Petrosilex and felsite are more generally porphyritic than any other rocks; and they are commonly called porphyry. It is better, however, since almost any rock may be porphyritic, and since this texture cannot be correlated with any particular composition, not to use porphyry as a rock-name, but simply as the name of a very important rock-texture. The banded and porphyritic textures are about equally characteristic of petrosilex and felsite. In petrosilex, quartz, as well as feldspar, is sometimes porphyritically developed, forming the variety known as quartz-porphyry. There is no limit to the proportion of the quartz and feldspar which may crystallize out in this way, and thus we find a perfectly gradual passage from normal petrosilex or felsite to thoroughly crystalline granite and syenite.

Andesite.—This rock has nearly the texture of rhyolite and trachyte, but is darker and heavier, and corresponds in composition to diorite, consisting of plagioclase and hornblende, with usually more or less sanidine, quartz, augite, biotite, and magnetite.

Basalt.—The rock bearing this familiar name represents diabase among the dike rocks. It is the most basic of the volcanic rocks, and consists of the more basic varieties of plagioclase, especially labradorite, with augite, magnetite, and titanic iron. Olivine is a very common and characteristic constituent, and the plagioclase is often replaced in part by leucite and nephelite. The basalts are usually black, and of high specific gravity; and vary in texture from compact to coarsely crystalline. The contraction due to cooling frequently results in the development of a columnar structure of remarkable regularity, the columns being normally hexagonal and standing perpendicularly to the cooling surfaces of the mass. This structure occurs in other eruptive rocks, but is most characteristic of basalt.

Tachylite.—Tachylite is a highly basic volcanic glass, standing in the same relation to basalt and andesite that obsidian does to trachyte and rhyolite. It is much heavier than obsidian, and is perfectly black and opaque, except in the finest fibres. It is a comparatively rare rock, for the reason that basalt and andesite crystallize more readily than the acidic rocks on passing from the liquid to the solid state. On the surface of the basic lava, however, where it is in contact with the air, and congeals almost instantly, a film of glass is formed; but this may not be more than a small fraction of an inch in thickness. Like obsidian, tachylite is often vesicular; but the vesicular basic rocks, as well as the solid, are usually stony. They occur in vast abundance in many volcanic regions, and may be considered the typical lava (specimen 49).

In the more ancient lavas, the vesicles are frequently filled by various minerals—chlorite, epidote, quartz, calcite, etc.—deposited by infiltrating waters, and derived in most cases from the decomposition of the original constituents of the rock. Thus the vesicular is changed to the amygdaloidal texture, and the lava becomes an amygdaloid (specimen 50). The amygdaloidal texture is common in the basic lavas and rare in pumice, simply because the former are more readily decomposed and contain a greater variety of bases from which secondary minerals can be formed.

Porphyrite and Melaphyr.—These two rocks hold essentially the same relation as regards origin and structure to the basic lavas that petrosilex and felsite do to the acidic lavas. Porphyrite agrees in composition with andesite, and melaphyr with basalt. They are usually dark-colored rocks having a compact or felsitic texture. Porphyrite is, as the name implies, very commonly porphyritic; while melaphyr is often vesicular or brecciated, exhibiting all the structural features of tachylite and basalt, and being in its older forms very generally amygdaloidal.

Volcanic Tuff and Agglomerate.—Besides the crystalline, glassy, and felsitic lavas, already described, and due chiefly to the rate of cooling of the liquid rock, we may recognize a fourth class to include the very abundant lavas which, during explosive eruptions, are ejected in the solid state, being violently blown out of the crater in the form of dust and fragments. Falling on the slopes of the volcano or over the surrounding country, as in the case of the buried city of Pompeii, the fragmental lavas remain largely unstratified. But when, as frequently happens, they fall into the sea, they are assorted by the waves and currents and arranged in layers after the manner of ordinary sediments, with which they are often more or less mixed. Before they become consolidated the finer fragmental lava, of whatever composition, is called volcanic dust, and the coarser lapilli or volcanic sand; while the consolidated materials are known as tuff and agglomerate respectively.

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SUPPLEMENT TO LITHOLOGY.

VEIN ROCKS.

All rocks are not embraced in the sedimentary and eruptive divisions, but there is a third grand division, which, although rarely mentioned or recognized in the more comprehensive works on geology, it is deemed best not to leave entirely unnoticed here. These are the vein rocks. They present an immense number of varieties, and yet, taken altogether, form but a small fraction of the earth’s crust. They are, however, the great repositories of the precious and other metals, and hence are objects of far greater interest to the miner and practical man than the eruptive rocks, or, in some parts of the world, even than the sedimentary rocks.

The vein rocks, like the eruptive rocks, occupy fissures in the earth’s crust intersecting the stratified formations; but the fissures filled with vein rocks are called veins, and not dikes. We will first notice the mode of formation of a typical vein, and then examine its contents. Geologists are agreed that water penetrates to a very great depth in the earth’s crust. All minerals are more or less soluble in water; and we may consider the water circulating through the rocks, especially at considerable depths, as, in most cases, a saturated solution of the various minerals of which they are composed. Very slight changes in the conditions will cause saturated solutions to deposit part of their mineral load. The water at great depths has a high temperature, and is subjected to an enormous pressure; and both of these circumstances favor solution. Suppose, now, that these hot subterranean waters enter a fissure in the crust and flow upwards, perhaps issuing on the surface as a warm mineral spring; as they approach the surface, the temperature and pressure, and consequently their solvent power, are diminished; and a portion of the dissolved minerals must be deposited on the walls of the fissure, which thus becomes narrower, and in the course of time is gradually filled up. The vein is then complete; and the mineral waters are forced to seek a new outlet.

Veins have the same general forms as dikes, since the fissures are formed in the same way for both; but the vein is of slow growth, and may require ages for its completion, while the dike is formed in an hour or a day. It is now generally believed that water is an important agent in the formation of eruptive rocks; since they all contain water at the time of their eruption; and since it has been demonstrated that, while ordinary rocks require a temperature of 2000° to 3000° for their fusion in the absence of water, they are liquified at temperatures below 1000° in the presence of water. In other words, common rocks are very infusible and insoluble bodies, and heat and water acting independently have little effect upon them; but when fire and water are combined in what is now known as aqueo-igneous fusion, they prove very efficient agents of liquefaction.

If we adopt these views, then it can be shown that, in origin, veins and dikes differ in degree only, and are not fundamentally unlike; and the formation and relations of the three great classes of rocks may be summarized as follows:—

The ocean and atmosphere, operating on the earth’s surface, have worked over and stratified the crust, until the sedimentary rocks have now an average thickness variously estimated at from ten to thirty or forty miles. This entire thickness of stratified rocks, and a considerable depth of the underlying unstratified crust, must be saturated with water; and all but the more superficial portions of this water-soaked crust must be very hot, the temperature increasing steadily downwards from the surface. Both eruptive and vein rocks originate in this highly heated, hydrated crust. Eruptive rocks are formed when the heat, aided by more or less water, softens the rocks, either stratified or unstratified, by aqueo-igneous fusion, and the plastic materials are forced up through fissures to or toward the surface. Vein rocks are formed when the water, aided by more or less heat, dissolves the rocks, either stratified or unstratified, by what may be called igneo-aqueous solution, and subsequently deposits the mineral matter in, i.e., on the walls of, fissures leading up to or toward the surface. In the case of the dike rocks, heat is the chief agent, and water merely an auxiliary; while with the vein rocks it is just the reverse. But between the two it is probably impossible to draw any sharp line.

The water circulating through the crust, and saturated with its various mineral constituents, has been called the “juice” of the crust; and veins are formed by the concentration of this earth-juice in fissures. One of the most important characteristics of the vein rocks, as a class, is the immense variety which they present; for nearly every known mineral is embraced among their constituents; and these are combined in all possible ways and proportions, so that the number of combinations is almost endless. The solvent power of the subterranean waters varies for different minerals; and appears often to be greatest for the rarer species. In other words, there is a sort of selective action, whereby many minerals which exist in stratified and eruptive rocks, so thinly diffused as to entirely escape the most refined observation, are concentrated in veins in masses of sensible size; and our lists of known minerals and chemical elements are undoubtedly much longer than they would be if these wonderful storehouses of fine minerals which we call veins had never been explored. As a rule, the minerals in veins form larger and more perfect crystals than we find in either of the other great classes of rocks. This is simply because the conditions are more favorable for crystallization in veins than in dikes or sedimentary strata. In both dike and stratified rocks, the growing crystals are surrounded on all sides by solid or semi-solid matter; and, being thus hampered, it is simply impossible that they should become either large or perfect. In the vein, on the other hand, there are usually no such obstacles to be overcome; but the crystals, starting from the walls of the fissure, grow toward its centre, their growing ends projecting into a free space, where they have freedom to develop their normal forms and to attain a size limited only, in many cases, by the breadth of the fissure. With, possibly, some rare exceptions, all the large and perfect crystals of quartz, feldspar, mica, beryl, apatite, fluorite, and of minerals generally, which we see in mineralogical cabinets, have originated in veins. Those fissures which become the seats of mineral veins are really Nature’s laboratories for the production of rare and beautiful mineral specimens; and hence the vein rocks are the chief resort of the mineralogist, to whom they are of far greater interest than all the eruptive and stratified rocks combined.

The leading characteristics, then, of the vein rocks may be summarized as follows: (1) They contain nearly all known minerals, including many rare species and elements which are unknown outside of this class of rocks. (2) These mineral constituents, occurring singly and combined, give rise to a number of varieties of rocks so vast as to baffle detailed description. (3) They exceed all other rocks in the coarseness of their crystallization, and in the perfection and beauty of the single crystals which they afford.

PETROLOGY.

In lithology we investigate the nature of the materials composing the earth’s crust—the various minerals and aggregates of minerals, or rocks; while in petrology we consider the forms and modes of arrangement of the rock-masses,—in other words, the architecture of the earth.

Petrology is the complement of lithology, and in many respects it is the most fascinating division of geology, since in no other direction in this science are we brought constantly into such intimate relations with the beautiful and sublime in nature. The structures of rocks are the basis of nearly all natural scenery; for what we call scenery is usually merely the external expression, as developed by the powerful but delicate sculpture of the agents of erosion—rain and frost, rivers and glaciers, etc.—of the geological structure of the country. And to the practised eye of the geologist, a fine landscape is not simply a pleasantly or grandly diversified surface, but it has depth; for he reads in the superficial lineaments the structure of the rocks out of which they are carved.