PHYSIOGRAPHIC GEOLOGY
Treats of the earth’s exterior physical features; of its form—an oblate spheroid—of its surface, oceans, continents, seas, lakes and rivers, hills, mountains, valleys and plains; of soils made from previously existing organic or inorganic substances, the detritus of rocks containing various minerals and small particles of decomposed vegetable matter. The materials of this outer covering of the earth are from many different sources, and variously constituted. From the finest grains of sand, clay, and loam, to pebbles, boulders, and fragments of enormous dimensions, they are mingled apparently without any fixed order or proportions; sometimes but slightly covering the solid rock, at others piling it up in ridges and hills of considerable height. In this surface formation are included ancient sea-beaches, lake and river terraces, deltas, deposits of sand and clay, with vast beds of marls, peat and calcareous tufa,[1] all the progressive accumulations since the present order of things began. In some of these deposits, more recent than the Drift[2] period, fossils are abundant and very full of interest. In New Zealand the bones of a bird[3] were found which exceed in bulk those of the largest horse, and are now in the museum of the College of Surgeons, London. The bird when alive was eleven or twelve feet high.
Less than a century ago what might have been a fossil elephant was found imbedded in ice on the coast of Siberia, and in such a perfect state of preservation that the people fed their dogs on its flesh. The animal was well covered with hair, and adapted to a cool climate, a representative of an extinct race. How it was imbedded, or how long it had been preserved in that condition, no one knows.
In Great Britain are found fossils of the rhinoceros and hippopotamus, of elephants, tigers, hyenas and giant elks, all of which are extinct species. The United States is especially prolific in the remains of huge mammals. The mastodon and megatherium were doubtless indigenous to this country. The latter had a thigh bone three times as large as the largest elephant, and the cavity through which it passed, indicates a spinal cord an inch in diameter. These largest skeletons were found in Georgia and South Carolina. Those of the mastodon are numerous, and found in many different places. Physiographic geology is a study intensely interesting, and of great practical importance, as it bears directly on many of the industries of life; but this general notice is sufficient.
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.