America, southern Mexico, and southwestern Alaska, proclaim the recency of the birth of the frequently magnificent craters built of rocks that were once molten, from which they emerge.
THE ROCKS OF WHICH THE CONTINENT IS COMPOSED
The rocks of which North America is built belong to three classes, which are world-wide in their distribution. These are: First, rocks produced by the cooling and crystallizing of formerly molten magmas; second, those deposited by water; and third, those which previously belonged to either of the two classes just referred to, but have been recrystallized and so greatly changed that their preceding condition is no longer clearly recognisable.
These three classes or subkingdoms, as perhaps they might be termed from analogy with systems of biological classifications, are in technical language:
1. Igneous rocks, such as the lava of Vesuvius.
2. Sedimentary rocks, such as sandstone, shale, limestone, coal, etc.
3. Metamorphic rocks, such as gneiss, schist, some granites, etc.
These major divisions are based principally on mode of origin, but do not indicate relative age. While theoretically at least, and in a general way, the rocks of these three great classes came into existence on the earth in the order named, it is convenient to consider first those of sedimentary origin.
The Sedimentary Rocks (Plate IV).—Whenever land exists or the waves and currents of the ocean come in contact with the rocks denudation occurs. That is, the rocks are broken through the action of mechanical or chemical agencies, such as the friction of the gravel and sand swept along by streams, the solvent power of water, etc., and the fragments thus produced are removed principally through the action of flowing water and deposited. Resulting from this general process of rock decay and disintegration, combined with transportation and deposition,
there result mechanically formed sedimentary beds, such as shale, sandstone, conglomerate, etc.; chemically formed sedimentary beds, such as the deposits of springs, the saline precipitates from inclosed lakes, etc.; and organically formed sedimentary beds, as, for example, peat, coal, and limestone.
Since the first appearance of land in the region now occupied by North America, sedimentary rocks have been in process of formation, and in this way the growth of the continent, with the aid of movements in the earth's crust, has been produced.