LITHOGRAPHIC GEOLOGY.
The ultimate particles of material bodies, of which we know but little, exert such force or influence on each other as to decide the character of the mass; even if the atoms are identically the same in substance they may come together in a way to secure different results. The bulk of the solid part of the earth is rock, but all rock is not the same. We find several species of granite, of limestone, and sandstone, a long list. But the whole may be divided into two classes, stratified and unstratified. Whatever the two classes seem to have in common, they are not of the same origin. The first occur in layers or strata, others are crystalline and massive. The loose materials, such as sand, clay and gravel, that have accumulated at the bottom of the pond or lake, are found arranged in beds or parallel layers. The streams carry the materials from the highlands, and they are at length deposited in the basin, and when hardened become stratified rocks. As this process is still going on, and recently formed strata are found approaching the consistency of stone, it is but reasonable to conclude that all rocks of this class, being formed in like manner under the water, are of aqueous origin. They are further classed according to certain peculiarities, either of material or formation.
Gneiss, abundant in all parts of New England, is a kind of stratified granite, of about the same materials, but splits readily into slabs that are used both for building purposes and flagging stones.
Mica slate resembles gneiss, has the same minerals, but more mica, and is of a more slaty structure, and the glistening particles of mica abound in it.
There are several other kinds of slate, named from the minerals that predominate in them, or the purposes for which they are mostly used. Roofing slate of excellent quality is extensively quarried in Maine, Vermont and Massachusetts.
Quartz rock consists mainly of quartz, but often has more or less mica. Sandstone is of kindred formation, the principal part of which is quartz, reduced to sand, and the grains more or less firmly united. In both the colors are various.
Conglomerate consists of water-worn pebbles of various kinds and sizes cemented together, and sometimes making a strong, compact rock.
The limestone formations are extensive in nearly all countries. In their structure some are very compact and break with a smooth surface. Those capable of a fine polish are called marble, the more common uses of which are well known. The purest crystalline limestone is used in sculpture; the best quality being obtained from Carrara, Italy, and that called Parian from the island Paros.
Chalk, a useful formation, is a carbonate of lime. In some caves the dropping of calcareous water forms stalactites, which hang from the roof like immense icicles, and are often extended till they meet the accumulations below, called stalagmites, and form beautiful columns. Of the more than seven hundred crystals from this source alone, and of the many other varieties of minerals having much in common, and yet enough that is peculiar to distinguish them, no mention can be made. A careful reader and close observer will gather from familiar objects a fund of information of great value.
The parallel strata mentioned are not always horizontal, but sometimes nearly, if not quite perpendicular. Occasionally a ledge broken quite through separates, and the rock on one side of the fissure is either elevated or depressed, making what is called a fault.
The fissures crossing a bed of rock are often filled with a mineral entirely different from the rock itself. In some cases where the vein is small the foreign substance may have come in from above or laterally, deposited from water as in the case of stalactites. The larger fissures were evidently filled with the melted material thrust up from beneath.
The unstratified rocks are in masses, without fossils of animals or plants, and of igneous origin. Some of this class were probably formed later, and by the melting of secondary rocks, but most of them by the gradual cooling of the central mass containing the melted minerals embodied in them.