The rocks composing the greater portion of the Appalachians are stratified marine sediment such as sandstone, shale, limestone, etc., which were laid down one on another until a great depth was attained, corresponding, as we may fancy, to a pile of rugs, the original thickness in Pennsylvania being about 40,000 feet. Lateral pressure resulting, as it is believed, from the cooling and consequent contraction of the earth's highly heated interior, and the movement of the cool and rigid crust in order to keep in contact with the shrinking mass beneath, has led to the folding and occasional breaking of the rocks, which at the same time were elevated above the sea. A crushing together or folding of the rocks similar to that which has taken place along the central part of the Atlantic border of North America, as is well known, has occurred also in many other regions, and the Appalachians may be taken as the type of a class of mountains, sometimes termed corrugated mountains, which includes the Alps and Pyrenees, the Coast Range of California, etc. For convenience we may speak of such mountains as being of the Appalachian type. Had the folding in the Appalachian region gone on without erosion, the surface would to-day be a series of great, elongated arches or upward folds, rising in many instances 5,000 or more feet above the intervening valleys, and where breaks or faults occur their upraised borders would stand as mighty cliffs, in some localities a mile or more high. The central part of the region with this strange topography had there been no erosion would, perhaps, be fully as prominent as the Himalayan Mountains are at present. No sooner, however, were the Appalachian Mountains upraised above the sea than the destructive agencies of the atmosphere began their attacks upon them. The rocks were shattered by changes of temperature, and at times at least crumbled by the freezing of absorbed water and also underwent chemical changes which softened and disintegrated them. The rains beat upon them, and streams flowing to the sea cut channels and carried away the material forming the land. These processes of disintegration and erosion have been in progress since islands and continents first appeared on the earth, and every mountain range now giving diversity to the surface of the land represents the net result of elevation over denudation. The Appalachians are not an exception, but a typical illustration of this general law. The great folds of which they are composed have been truncated by erosion and the surfaces thus produced, etched, as it were, by the action of the air, rain, and by streams, so as to leave the edges of the more resistant layers in relief.