CHAPTER I.
PRELIMINARY OUTLINE.
Geology treats of the structure of the earth, of the various stages through which it has passed, and of the living beings that have dwelt upon it, together with the agencies and processes involved in the changes it has undergone. Geology is essentially a history of the earth and its inhabitants. It is one of the broadest of the sciences, and brings under consideration certain phases of nearly all the other sciences, particularly those of astronomy, physics, chemistry, zoology, and botany. It also embraces the earlier expressions of mental development and of life-relationships, chiefly as found in the lower animals.
Subdivisions.—Naturally so broad a science has many special aspects which constitute subdivisions, in a sense, though they are rather dominant phases than independent sections. That phase which treats of the outer relations of the earth is Cosmic or Astronomic geology; that which treats of the constituent parts of the earth and its material is Geognosy, of which the most important branch is Petrology, the science of rocks. That branch which investigates the structural arrangement of the material, or “the architecture of the earth,” is Geotectonic, or Structural geology; while that which deals with the surface changes and topographic forms, that is, with the face of the earth, is Physiographic geology. The study of the fossils that have been preserved in the rocks, and of the faunas and floras that these imply, constitutes Paleontologic geology, or Paleontology. The treatment of the succession of events forms Historical geology. This is chiefly worked out by the succession of beds laid down in the progress of the ages, which constitutes Stratigraphic geology. The treatment of causes, agencies, and processes is the function of Dynamic or Philosophic geology.
Besides these there are special applications which give occasion for other terms, as Economic geology, which is concerned with the industrial applications of geologic knowledge; Mining geology, which is a sub-section of economic geology, relating to the application of geologic facts and principles to mining operations; Atmospheric geology, Glacial geology, and others that define themselves, and are for the greater part but limited aspects of the broad science.
Dominant processes.—Three sets of processes, now in operation on the surface of the lithosphere, have given rise to most of the details of its configuration, and even many of its larger features. These processes have been designated diastrophism, vulcanism, and gradation. Diastrophism includes all crustal movements, whether slow or rapid, gentle or violent, slight or extensive. Many parts of the land, especially along coasts, are known to be slowly sinking relative to the sea-level, while other parts are known to be rising. The fact that rocks originally formed beneath the sea now exist at great elevations, and the further fact that areas which were once land are now beneath the sea, are sufficient evidence that similar changes have taken place in the past. Vulcanism includes all processes connected with the extrusion of lava and other volcanic products, and with the rise of lava from lower to higher levels, even if not extruded. Vulcanism and diastrophism may be closely associated, for local movements at least are often associated with volcanic eruptions, and more considerable movements may be connected with the movements of subsurface lavas, even when the connection is not demonstrable. Gradation includes all those processes which tend to bring the surface of the lithosphere to a common level. Gradational processes belong to two categories—those which level down, degradation, and those which level up, aggradation. The transportation of material from the land, whether by rain, rivers, glaciers, waves, or winds, is degradation and the deposition of material, whether on the land or in the sea, is aggradation. Degradation affects primarily the protuberances of the lithosphere, while aggradation affects primarily its depressions.