Part 14

Amphibolite (Hornblende Rock).—This is the name applied to a rock having hornblende as its sole essential constituent. Hornblende schist sometimes passes into amphibolite, through the absence of quartz; and so does diorite, when the feldspar is deficient or wanting. Specimen 20, though small, is a typical example of this rock. The physical and chemical characteristics are essentially the same as for the mineral hornblende. The texture varies from coarsely to finely crystalline. The crystals are usually short and thick, and lie in all directions in the rock, which is thus very massive, the schistose texture being entirely wanting, and the stratification rarely showing in small masses. Biotite is a common accessory in amphibolite, and garnet and magnetite frequently occur.

By the substitution of augite for hornblende, in the description of amphibolite, we get the much rarer, but otherwise very similar, rock, pyroxenite.

Talc Schist (Steatite or Soapstone).—Although not abundant, this is a useful and familiar rock. The composition is implied in the name; and by comparing it with the specimen of talc (No. 58) we can readily see that they are essentially identical. Typical talc schist is pure talc; but the talc is often mixed with more or less quartz or feldspar; and mica, chlorite, hornblende, garnet, and other minerals are of common occurrence.

This rock embraces two distinct varieties, the massive and the schistose, or foliated. The former is the common soapstone (specimen 71), which is a confused mass of crystals lying in all directions, and with no visible stratification in the small mass. In the latter, as in specimen , the talc scales lie in parallel planes, giving the rock a micaceous structure, and causing it to split easily in the direction of the stratification. The cleavage surfaces are often wavy or corrugated; and the same is true of all schistose rocks. Talc schist is easily distinguished from all other rocks by its light-grayish or greenish color, combined with its extreme softness, and its smooth, slippery feel.

Chlorite Schist.—The one essential constituent of this rock is chlorite, and the mineral specimen (No. 26) answers equally well as an example of the rock. As with talc schist, quartz, feldspar, and hydromica are rarely entirely absent. Besides these, the principal accessories are hornblende, magnetite, garnet, and epidote. This rock also agrees with talc schist in presenting two principal varieties, the massive and the schistose. It is easily distinguished from talc schist by its darker color and streak, which are very characteristic; while its green color, softness, and unctuous feel separate it from all other rocks.

This is the most basic of all the silicate rocks; but, in consequence of containing a large proportion of water, it is not the heaviest. It is, in fact, interesting and important to observe that all these hydrous silicate rocks—talc schist, chlorite schist, greensand, and serpentine—are distinctly lighter in each case than anhydrous rocks containing the same proportion of silica. They are also notable, as a class, for their softness, smooth feel, and green color.

Serpentine.—As the name implies, this rock is simply the mineral serpentine occurring in large masses, and its characteristics are precisely the same. It is fine-grained, massive, compact, rather soft, but very tough, and varies in color from very dark green to light greenish-yellow. The dark colors predominate, and specimen 25 is a typical example.

Serpentine is often intimately associated with limestone and dolomite. The white veins running irregularly through the variety known as Verd Antique Marble, however, are not calcite, as commonly supposed, but magnesite. They do not effervesce freely with cold, dilute acid, for the entire rock is magnesian, and it is probable have been at one time simply cracks along which water holding carbon dioxide has penetrated, changing the magnesia from a silicate to a carbonate.

Geologists were, at one time, almost unanimous in the opinion that all serpentine is of eruptive origin; but now it is conceded by the great majority to be in some cases a sedimentary rock. It is found interstratified with gneiss, limestone, all the schists, and many other stratified rocks. When occupying the position of an eruptive it is never an original rock; but has been formed by the alteration, in situ, of some basic anhydrous rock, most commonly olivine basalt.

Greensand.—This rock (specimen 27) consists chiefly of the mineral glauconite, mingled usually with more or less sand, clay, or calcareous matter. It is usually very friable, or in an entirely unconsolidated state. It is most abundant in the newer geological formations, especially the Cretaceous and Tertiary; and is, perhaps, the only one of the stratified silicate rocks now forming on an extensive scale in the ocean. Its value as a fertilizer, for which purpose it is extensively employed, is due to the potash that it contains.

Following is a systematic summary of the mineralogical composition of the rocks of this great division of silicates; and this, combined with the classification on page 69, presents in a condensed form all the more important facts contained in the preceding descriptions. Only the more constant and normal constituents of the species are enumerated in each case:—

====================+==================================== Names of Species. | Constituent Minerals. --------------------+------------------------------------ ⎧| Orthoclase and Quartz. Gneiss ⎨| Orthoclase, Quartz, and Mica. ⎩| Orthoclase, Quartz, and Hornblende. --------------------+------------------------------------ ⎧| Orthoclase. Syenite ⎨| Orthoclase and Hornblende. ⎩| Orthoclase and Mica. --------------------+------------------------------------ ⎧| Plagioclase (chiefly Oligoclase). Diorite ⎨| Plagioclase and Hornblende. ⎩| Plagioclase and Mica. --------------------+------------------------------------ ⎧| Plagioclase (chiefly Labradorite). Norite ⎨| Plagioclase and Augite (Diallage). ⎩| Plagioclase and Mica. --------------------+------------------------------------ ⎧| Mica. Mica Schist ⎨| Mica and Quartz. ⎩| Mica and Kaolin. --------------------+------------------------------------ Hornblende Schist | Hornblende and Quartz. --------------------+------------------------------------ Amphibolite | Hornblende. Pyroxenite | Pyroxene. --------------------+------------------------------------ Talc Schist | Talc. --------------------+------------------------------------ Chlorite Schist | Chlorite. --------------------+------------------------------------ Serpentine | Serpentine. --------------------+------------------------------------ Greensand | Glauconite. ====================+====================================

2. Eruptive or Unstratified Rocks.

The rocks of this great class are formed by the cooling and solidification of materials that have come up from a great depth in the earth’s crust in a melted and highly heated condition. When the fissures in the earth’s crust reach down to the great reservoirs of liquid rock, and the latter wells up and overflows on the surface, forming a volcano, then we may, as was pointed out on page 33, divide the eruptive mass into two parts: first, that which has actually flowed out on the surface, and cooled and solidified in contact with the air, forming a lava flow; second, that which has failed to reach the surface, but cooled and solidified in the fissure, forming a dike.

Lava flows or volcanic rocks and dikes or plutonic rocks are identical in composition; but there is a vast difference in texture, due to the widely different conditions under which the rocks have solidified. The dike or fissure rocks solidify under enormous pressure, and this makes them heavy and solid—free from pores. They are surrounded on all sides by warm rocks: this causes them to cool very slowly, and allows the various minerals time to crystallize. Other things being equal, the slower the cooling the coarser the crystallization; and hence, the greater the depth below the surface at which the cooling takes place, the coarser the crystallization.

The volcanic rock, on the other hand, cools under very slight pressure; and the steam, which exists abundantly in nearly all igneous rocks at the time of their eruption, is able to expand, forming innumerable small vesicles or bubbles in the liquid lava; and these remain when it has become solid. Cooling in contact with the air, the lava cools quickly, and has but little chance for crystallization. Hence, to sum up the matter, we say: plutonic rocks are solid and crystalline; and volcanic rocks are usually porous or vesicular, and uncrystalline.

As we descend into the earth’s crust, it is perfectly manifest that the volcanic must shade off insensibly into the dike rocks, and we find it impossible to draw any but an arbitrary plane of division between them; but this is no argument against this classification, for, as already stated, all is gradation in geology, and we experience just the same difficulty in drawing a line between conglomerate and sandstone, or between gneiss and mica schist, as between the dike rocks and volcanic rocks.

We will now observe to what extent the distinctions between these two great classes of eruptives can be traced in the rocks themselves, beginning with the dike rocks. But first it is important to notice the general fact, clearly expressed in the classification, that, with perhaps some trifling exceptions which need not be mentioned here, all eruptive rocks are silicates, and nearly all are feldspathic silicates. They are of definite mineralogical composition, and, like the chemically and organically formed stratified rocks, can be classified chemically. But, although there are eruptives corresponding closely in composition to the feldspathic silicates, which we have just studied, we find among them little to represent the non-feldspathic silicates, and nothing corresponding in composition to the limestones, dolomites, gypsum, flint, tripolite, siliceous tufa, iron-ores, bitumens or coals.

1. PLUTONIC (DIKE) ROCKS.—These are also known as the ancient eruptive rocks, and for this reason: It is impossible, of course, for us to observe them except where they occur on or near the earth’s surface. But, since they are formed wholly below the surface, and usually at great depths in the earth, it is evident that they can appear on the surface only as the result of enormous erosion; and erosion is a slow process, demanding, in these cases, many thousands or millions of years. Therefore, the more ancient dike rocks alone are within our reach; those of recent formation being still deeply buried in the earth’s crust. It follows, as a corollary to this explanation, that the coarseness of the crystallization of any dike rock must be a rough measure of its age and of the amount of erosion which the region has suffered since its eruption.

As regards composition, the dike rocks present, as already stated, essentially the same combinations of minerals as the feldspathic silicates of the stratified series, but occurring under different physical conditions and having a widely different origin. The only important difference in texture between the two classes of rocks is that the sedimentary rocks are stratified and the dike rocks are not; and when we consider that the dike rocks sometimes present a laminated structure that resembles stratification, while the sedimentary rocks frequently appear unstratified, it is easy to understand why, in the absence of any marked difference in composition, geologists have often found it difficult to distinguish the two classes of rocks. We also find here the explanation and the justification of the fact that the names of the dike rocks are in most cases the same as those of the sedimentary rocks of similar composition.

Granite.—Granite (from the Latin granum, a grain) is a crystalline-granular rock, agreeing in composition with gneiss. The essential constituents are quartz and orthoclase; and when they alone are present we have the variety binary granite. Mica, however (commonly muscovite, sometimes biotite, and frequently both) is usually added to these, forming micaceous granite (specimen 44); and often hornblende, forming hornblendic granite (specimen 45). The orthoclase is sometimes replaced in part by triclinic species, especially albite and oligoclase. Accessory minerals are not so abundant in granite as in gneiss; but, besides those named, garnet, tourmaline, pyrite, apatite, and chlorite are most common. Orthoclase is always the predominant ingredient; and, except when there is much hornblende present, usually determines the color of the granite. Thus, specimens 44 and 45 are gray because they contain gray orthoclase; while all red granites contain red or pink orthoclase. The quartz has usually been the last of the constituents to crystallize or solidify; and, having been thus obliged to adapt itself to the contours of the orthoclase and mica, it is rarely observed in distinct crystals.

In texture, the granites vary from perfectly compact varieties, approaching petrosilex, to those which are so coarsely crystalline that single crystals of orthoclase measure several inches in length. Of course one of the most important things to be observed about granite, especially in comparing it with gneiss, is the complete absence of anything like stratification; that, as before stated, being the only important distinction between the two rocks. Gneiss is the most abundant of all stratified rocks, and granite stands in the same relation to the eruptive series.

Syenite.—This is an instance where stratified and eruptive rocks, agreeing in composition, have the same name. That rocks consisting of orthoclase, of orthoclase and hornblende, or of orthoclase and mica, i.e., having the composition of syenite, do occur in both the eruptive and stratified series there can be no doubt. They should, however, have distinct names on account of their unlike origins; and would have but for the practical difficulty in determining, in many cases, whether the rock is stratified or not. The best that we can do now, when we desire to be specific, and have the necessary information, is to say stratified syenite or eruptive syenite, as the case may be.

Diorite.—Here, again, we find identity of names, as well as of composition, between the two great series. Eruptive diorite is an abundant and well known rock, and consists of the same minerals as stratified diorite combined in the same proportions. Diorite includes a large part of the dike rocks commonly known as “trap” and “greenstone.” The principal accessories are chlorite, epidote, pyrite, magnetite, apatite, and quartz. The texture varies from perfectly compact or felsitic to coarsely crystalline; averaging, however, less coarse than syenite and granite.

Diabase.—By referring to the classification it will be seen that diabase occupies the same position among the dike rocks as norite among the stratified rocks. Like norite it consists usually of the more basic varieties of plagioclase with or without augite, diallage, or hypersthene. Augite, or one of its representatives, is usually present, and is often the principal constituent. Specimen 1 shows a somewhat equal development of the feldspar and augite. The name gabbro is sometimes applied to the coarser and more feldspathic diabases, and especially to those containing diallage or hypersthene in the place of common augite. In the opinion of some high authorities, however, it is unnecessary to recognize two species here; and it makes the classification more simple and symmetrical not to do it. The principal accessories in diabase are biotite, chlorite, magnetite, pyrite, calcite, and olivine. Chlorite is often an important constituent, giving the rock a greenish aspect; but here, as well as in diorite, the chlorite is due chiefly or entirely to the alteration of the augite and feldspar; and the chloritic varieties of diorite and diabase together make up the old species “greenstone.” Similarly, the more compact and darker varieties of these two rocks, forming regular, wall-like dikes, are known as “trap.” Specimen 46.

In consequence of their more basic composition, diabase and diorite are usually strongly contrasted with granite and syenite in color and specific gravity, being darker and heavier. The basic rocks, too, decay much more readily than the acidic.

2. VOLCANIC ROCKS.—As regards composition, we shall find nothing new in the volcanic series; for the rocks of this group present essentially the same combination of minerals as the dike rocks. In composition, the dike and volcanic rocks are identical; but in texture, as already explained, there is a vast difference. The volcanic rocks differ so widely in texture from both the dike and stratified species, that there is rarely any difficulty in distinguishing them; and hence they have in every instance distinct names.

Volcanic rocks are rarely found in this part of the world; and specimens of most of them are difficult to obtain. For this reason they can only be noticed briefly here, since it is the plan of this Guide to give especial attention only to those portions of the subject which can be illustrated by material within easy reach of teachers.