Section Iii.--the Mineral Masses Which Form the Crust of the Earth.

That portion of the structure of the earth which is accessible to man is called the crust of the earth.

The mineral masses which compose it, whether in a solid state, like granite and limestone, or in a yielding state, like beds of sand and clay, are called rocks.

The unstratified rocks are Granite, Hypersthene rock, Limestone and Serpentine, and the Trappean and Volcanic rocks.

Granite is a rock of a light gray color, and is composed of quartz, felspar and mica, in variable proportions, confusedly crystallized together. The felspar is generally the predominant mineral. It is sometimes of a very coarse texture, the separate minerals occurring in masses of a foot or more in diameter. At other times it is so fine-grained that the constituent minerals can scarcely be recognized by the naked eye; and between these extremes there is every variety. The term granite is not, however, confined to an aggregate of these three minerals. In some instances the felspar so predominates as almost to exclude the other minerals, when it is called felspathic granite. When the quartz appears in the form of irregular and broken lines, somewhat resembling written characters, in a base of felspar, it is called graphic granite. When talc takes the place of mica, it is talcose granite. When hornblende takes the place of mica, it is syenite. Granite or any rock becomes porphyritic when it contains imbedded crystals of felspar.

There is a rock of crystalline structure, like granite, but of a darker color, which is called hypersthene rock. It is composed of Labrador felspar and hypersthene. The mineral species serpentine and limestone often occur unstratified in considerable quantities.

Volcanic rocks consist of the materials ejected from the craters of volcanoes. They are composed of essentially the same minerals as trap rocks. When the material has been thrown out in a melted state, it is called lava. Lava, at the time of its ejection, contains a large amount of watery vapor at a high temperature. Under the immense pressure to which it is subjected in the volcanic foci, it may exist in the form of water; but when the lava is thrown out at the crater, the pressure cannot much exceed that of the atmosphere. The particles of water at once assume the gaseous form. As lava possesses considerable viscidity, the steam does not escape, but renders the upper portion of the mass vesicular. This vesicular lava is called scoriæ. By the movement of the stream of lava, these vesicles become drawn out into fine capillary tubes, converting the scoriæ into pumice-stone.

A large part of the materials ejected from volcanoes is in the form of dust, cinders and angular fragments of rock. These soon become solidified, forming volcanic tuff, or volcanic breccia. In submarine eruptions these fragments are spread out by the water into strata, upon which other materials, not volcanic, are afterwards deposited. These interposed strata are called volcanic grits.

The trappean rocks are composed of felspar, mingled intimately and in small particles with augite or hornblende. They also contain iron and potassa. They are often porphyritic. When they contain spherical cavities, filled with some other mineral, such as chlorite, carbonate of lime or agate, they are called amygdaloidal trap.

The principal varieties of trappean rock are basalt, green stone, and trachyte. In basalt, augite, or, in some cases, hornblende, is the predominant mineral. It is a heavy, close-grained rock, of a black or dark brown color. Greenstone differs from basalt in containing a much larger proportion of felspar. Its structure is more granular, and frequently it assumes so much of the crystalline form as to pass insensibly into syenite or granite. It is a dark colored rock, with a slight tinge of green. Both green stone and basalt are disposed to assume the columnar form, the columns being arranged at right angles to the faces of the fissure into which the trap is injected. When it is spread out into broad horizontal masses, the columns are vertical. (Fig. 1.) Trachyte is composed principally of felspar, is of a grayish color, and rough to the touch.

Of the stratified rocks the following are the most important:

Gneiss is a rock closely resembling granite. It is an aggregate of the same minerals, but the proportion of mica is somewhat greater. The only distinction between them is that the gneiss is stratified, but the stratification is often so indistinct that it passes insensibly into granite. Generally, however, the stratification is so distinct as to present a marked difference.

Mica slate is such a modification of gneiss that the mica becomes the predominant mineral, with a small intermixture of quartz and felspar. Consequently the stratification becomes very distinct, so as sometimes to render the mass divisible into thin sheets. The stratification is often wavy, and sometimes much contorted.

Sandstone consists of grains or fragments of any other rock, but more frequently of siliceous rocks. The fragments are consolidated, sometimes without any visible cement, but often by a paste of argillaceous or calcareous substance. The color varies with that of the rock from which it was derived. Generally, however, it is either drab or is colored red by oxide of iron. The fragments are sometimes so minute as scarcely to give the rock the appearance of sandstone. When they are of considerable size and rounded, the rock is called conglomerate. When they are angular, it is called breccia. Greensand is a friable mixture of siliceous and calcareous particles, colored by a slight intermixture of green earth or chlorite.

Limestone is a very abundant rock, and occurs in many different forms. In transparent crystals it is Iceland spar. When white and crystalline, it is primary limestone, saccharine limestone, or statuary marble. When sub-crystalline it is generally more or less colored. It is often clouded with bands or patches of white in a ground of some dark color. When its texture is close, and the crystallization scarcely apparent, it is compact limestone. The white, earthy variety is chalk. A variety of limestone composed of small spheres is called oölite. Lias is the name given to an impure argillaceous variety of a brown or blue color. Any rock which contains a considerable proportion of carbonate of lime, and which rapidly disintegrates on exposure to the atmosphere, is called marl. Limestone sometimes contains carbonate of magnesia. It is then magnesian limestone, or dolomite.

Clay consists of a mixture of siliceous and aluminous earth. It is tough, highly plastic, and generally of a lead blue color. It is always stratified, and often divided into very thin laminæ, which are separated by sprinklings of sand only sufficient to keep them distinct.

Clay slate, or argillaceous schist, is composed of the same materials as clay, and differs from it only in having become solidified. Its color is gray, dark brown or black. In some beds it is purple. Shale is the same material in a state of partial solidification. On exposure to the weather, it soon disintegrates, and is finally reconverted into clay. All the varieties of argillaceous rock are easily distinguished by a peculiar odor which they emit when breathed upon.

Argillaceous slate sometimes takes into its composition portions of some other mineral, such as talc, mica, or hornblende. When any of these minerals becomes so abundant as to constitute a considerable part of the mass, the rock becomes talcose, micaceous, or hornblende slate. Sometimes this last variety loses all appearance of a fissile structure, and is composed almost wholly of hornblende. It is then called hornblende rock.

Diluvium is the name applied to masses of sand, gravel, and large rocks, called boulders, heaped confusedly together on the surface of the earth. It is also called drift.