Chemical Briefly enumer ated, these are salt and gypsum, found in alter nating beds, and considered to have formed by the evaporation of salt lakes, or by the isolation and evaporation of bodies of sea-water. Cal careous sinter or tufa consists of carbonate of lime, which has been brought up in solution in the waters of hot springs and deposited as por ous or cellular material about their vents. It is also deposited from solution in the waters of rivers and lakes, and occasionally forms de posits of considerable extent. Travertine is a more compact form of the same material, as is also Mexican onyx.
Eolian In regions unprotected by vegetation, as long sea-beaches, the shores of lakes, or in the region of deserts, .the finer sands are picked up by winds and drifted like snow into oval or rounded hillocks called dunes. Often these drifting sands are a menace to agri culture, since they gradually encroach upon and bury under them, the arable land. Sands thus deposited by winds are only rudely strati fied. In the same manner fine impalpable dust is transported long distances, and gradually ac cumulates to form loess. The loess deposits of China are locally over 1,000 feet thick. Similar accumulations occur in the United States, Europe, and in Argentina. The Mississippi Valley loesses are believed by some to have been laid down in water under unusual conditions. True loesses are unstratified, and have a char acteristic verticle cleavage.
Glacial Deposits.— The great continental glaciers that, geologically speaking, existed only yesterday over a large portion of the continents of the northern hemisphere, left behind them.
vast amounts of characteristic material, which was in part deposited by the glaciers directly and in part by the waters resulting from the melting of the ice. These latter deposits are, therefore, partly of aqueous origin, and are . called fluvio-glacial. Glacial drift is the gen eral term applied to all the materials resulting from glacial action direct or indirect. Glacial till is the dense bluish clay, with numerous im bedded bowlders, characteristically striated. formed underneath the (sub-glacial material). This till-sheet (or ground-moraine) is found over a large portion of the area cov ered originally by the glaciers, and varies in thickness from zero to hundreds of feet. Other sub-glacial accumulations are called drumlins, till-billows, till-tumuli, crag-and-tail, etc. Ma terials dumped down at the margins of the glaciers in confused heaps are termed moraines (terminal, lateral, or medial). Streams flowing within or underneath the glaciers deposited along their courses material (sand, gravel, clay), which remained behind after the melting of the glaciers in elongated ridges called eskers or osars. Where these streams issued at the margin of the ice-sheets they left irregular mounds called kames. Streams flowing on the surface of the ice would likewise accu mulate materials of the same sort, which would remain after the melting of the ice as long serpentine ridges known as super-glacial kames or eskers. These floods of water escaping from the glaciers carried the gravels, sands and silts far beyond the margins of the glaciers, and de posited them as deltas, fans, overwash aprons, and valley trains.