Physiography
the science of the earth's surface features and conditions and how they came to be what they are.
The term physiography is sometimes used in a wider sense to denote a general introduction to the physical and biological sciences, but in America it is commonly used as interchangeable with physical geography. In this sense, however, it is not restricted to a mere description of existing features of the earth or to the features of the land alone, but includes the study of the ocean and the ocean basins, together with the his tory of the factors and forces which have produced and are still producing geographical changes. In this latter respect it will readily be seen how physiography verges on one hand into geology and, on the other hand, how it is related to certain branches of meteorology. Two sets of agents have given the earth its present relief forms: -(1) subterranean forces and changes, producing crustal movements of elevation, depression, and compression of the surface; and (2) socalled agents of erosion or denudation, such as water, wind, glaciers, frost, etc., etc., which have been wearing away sur- PLASTIC SURGERY face features and leveling hills and mountains for countless ages. The continental platforms and the great ocean basins constitute the first order of relief features of the earth. The second order is composed of the topographical features of the continents and the ocean floors themselves. Land features are familiar as mountain ranges, plateaus, and plains. But the ocean floor also has a somewhat similar relief, although far less sharp and pronounced, owing chiefly to the nature of the element beneath which it lies, the comparative absence of erosion, and the tendency of sediment to fill up depressions. It is not, however, either flat or uniform. Of depressions there are so-called basins, troughs, and known as "deeps." Of elevations there trenches, the deepest depressions being Telegraph Plateau of the North Atlantic are rises, ridges, and plateaus, like the Ocean, whose position made possible the laying of the first Atlantic cables. Volcanoes also are numerous in parts of the ocean, and volcanic cones have been built ing the basis of islands and groups of up to the surface and far above it, formislands which are often of considerable extent and importance.