GEOLOGY SUBDIVIDED Geology covers an extensive territory and is usually subdivided into the following more or less generally recognized branches.
Cosmic Geology treats of the origin of the earth, its relations to the other bodies of our solar system, and its general relations in space, thus encroaching on the field of astronomy. See COSMOGONY.
Geognosy treats of the materials of which the earth is composed, air, water, and solid crust, known as the atmosphere, the hydro sphere and the lithosphere.
Mineralogy includes a study of the chemical composition, crystal form, origin, and occur rence of the large number of definite chemical compounds (minerals) of which the earth is made up. Optical mineralogy deals with the study of the optical properties of minerals with the polarizing microscope.
Petrology treats of the origin, occurrence, constituent minerals, and texture of the rocks of the earth.
Petrography, used loosely as a synonym for petrology, is in reality a more restricted term applying to a study of the structure, texture, and composition of rocks either macroscopically or microscopically, but not concerned with origin or occurrence. In later usage there is even a tendency to restrict petrography to microscopic study.
Lithology, formerly used in the same sense as petrology, is beginning to be confined by many to the macroscopic study of rocks as contrasted with petrography. See MINERALOGY; METAMORPHIC ROCKS ; PETROLOGY ; ROCKS ; SEDI MENTARY ETC.
Dynamical Geology treats of the forces which tend to change or modify earth structure or earth form. It details the agencies at work and the processes by which they operate. These forces are hypogene (internal) and epigene (surfitiall. Three great processes are usually recognized, gradation, diastrophism and volcan ism. Gradation is surficial and due directly to the action of rain, wind, running water, glaciers, wave work, tides, plants and animals. Volcan ism is internal and deals with great movements of fluid rock, the most striking exhibition of which is to be seen in volcanic phenomena.
Diastrophism, also internal, treats of the move ment and distortion of rock masses (deforma tion) which the earth's crust undergoes, and which are best known through the manifesta tions of earthquakes.
Structural Geology, also known as geotec tonics, deals with the arrangement of the ma terials of which the earth is made up. Its province is to investigate the origin of these structures and their practical importance in applied geology. It has to do with the causes of layering or stratification in rocks, the origin of folds and dislocations, and other problems of similar nature.
Physiography (geomorphology), now gen erally recognized as a science distinct from geology, deals with the origin and development of land forms, traces out the topographic ex pression of structure, and embodies a logical history of oceanic basins, and continental ele vations; of mountains, plateaus and plains; of hills and valleys. Physical geography is used loosely as a synonym, but the term is more properly applied to the borderland between geography and physiography; dealing, as it does, largely with the human element as influ enced by its physiographic surroundings. See GEOMORPHOLOGY; PHYSIOGRAPHY.
Paleontology treats of the life of the geo logic past. It outlines the methods by which the evidences of that life are preserved in the rocks, traces in detail the development of vari ous life forms, and attempts to correlate extinct with living genera. It overlaps the fields of botany and zoology, throws enormous light on the problems of evolution, and constitutes the real basis of all efforts to determine the relative ages and relations of strata in widely separated regions (Stratigraphic Correlation). Palao botany is a sub-branch which deals exclusively with the palaeontology of the plant world. See FOSSILS ; PALEONTOLOGY ; PALEOBOTANY ; PETRI FACTION ; etc.