Stratigraphy is not infrequently called his torical geology though the latter term also properly includes paleontology. It is concerned chiefly in the working out of the history of past geologic ages. One of its problems, as has just been stated, is the correlation of strata in widely separated regions. Without this phase of geology little systematic advance in the science would have been possible.
Economic Soil, water supply, mineral fuels and oils, building stone, base and precious metals; all these and many other constituents of the earth have been widely ex ploited by man for his use. Economic geology deals with the application of geologic principles to this exploitation. It is concerned with the distribution, mode of occurrence, mineralogic content, and origin of these economically valu able substances. See COAL; Economic GEouocv; GOLD ; SILVER ; IRON.
Mining Geology.— There has grown up a treatment of applied geology particularly adapted to the mining engineer and known as mining geology. As one of its fundamental phases this obviously embraces economic geol ogy. It also is concerned largely with structural geology, since the miner is interested likewise m the structure of the rocks in which the eco nomically valuable minerals occur.
Glaciology.— Glaciers have been responsi ble for much modification of topographic form over large areas, and as detailed study of exist ing glaciers and glaciation and glacial phenom ena in the past has increased, there has grown up a separate treatment of the subject under the name of glaciology. See Gi.scus; GLACIAL. PERIOD; PLEISTOCENE EPOCH.
The publication of a vast mass of information resulting from several im portant deep-sea dredging expeditions has re sulted in the development of this branch of the science to such proportions that many writers give it the rank of a separate branch of geology. See OCEANS; DkaP SEA EXPLORATION; CHAL LENGER EXPEDITION ; etc.
Metamorphic Geology.— Under the influ ence of favorable conditions in nature many minerals break down and their elements re combine to form new compounds. This process is very extensive and frequently results in the formation of entirely new rock types. The process by which one rock is altered into an other is known as metamorphism, and the branch of geology which treats of the cause and nature of this change is known as metamor phic geology. See Rock CLEAVAGE; METARIOR PH IS ; METAMORPHIC Rocks; ROCKS.