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. |