So far as they are known to us, the materials of which the earth is composed are minerals grouped into various characteristic aggregates known as rocks. Here the science is founded upon mineralogy as well as chemistry, and a study of the rock materials of the earth is designated petrographical geology or petrography. The various rocks which enter into the composition of the earth’s outer shell—the only portion known to us from direct observation—are built into it in an architecture which, when carefully studied, discloses important events in the earth’s history. The division of the science which is concerned with earth architecture is geotectonic or structural geology.
=The study of earth features and their significance.=—The features upon the surface of the earth have all their deep significance, and if properly understood, a flood of light is thrown, not only upon present conditions, but upon many chapters of the earth’s earlier history. Here the relation of our study to topography and geography is very close, so that the lines of separation are but ill defined. The terms “physiographical geology”, “physiography”, and “geomorphology” are concerned with the configuration of the earth’s surface—its physiognomy—and with the genesis of its individual surface features. It is this genetical side of physiography which separates it from topography and lends it an absorbing interest, though it causes it to largely overlap the division of dynamical geology or the study of geological processes. In fact, the difference between dynamical geology and physiography is largely one of emphasis, the stress being laid upon the processes in the former and upon the resultant features in the latter.
Under dynamical geology are included important subdivisions, such as seismic geology, or the study of earthquakes, and vulcanology, or the study of volcanoes. Another large subject, glacial geology, belongs within the broad frontier common to both dynamical geology and physiography. A relatively new subdivision of geological science is orientational geology, which is concerned with the trend of earth features, and is closely related both to physiography and to dynamical and structural geology.