Part 8

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.

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.