=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.