{ Permian. { Coal Measures or Pennsylvanian. { Subcarboniferous, or Mississippian. +Paleozoic+ { Devonian. { Silurian. { Ordovician. { Cambrian. Great interval. { Keweenawan. { Interval. +Proterozoic+ { Animikean or Penokean. { (Upper Huronian of some authors). { Interval. { Huronian. Great interval. { { Great Granitoid Series. { { (Intrusive in the main, { { Laurentian.) +Archeozoic+ { Archean Complex. { Great Schist Series. { { (Mona, Kitchi, Lower { { Keewatin, Coutchiching, { { Lower Huronian of some { { authors.) The purpose of this general survey is to bring the salient features of the earth’s structure into view preparatory to entering in more detail into the study of particular processes and special formations and to lay a foundation for the fuller apprehension of the successive stages of the history of the earth, which constitutes the chief purpose of geological study. It is now advisable to turn to the detailed consideration of individual processes and specific structures. The complexity of the actions involved in the history of the earth is so great that such separate consideration at the outset is helpful. CHAPTER II. THE ATMOSPHERE AS A GEOLOGICAL AGENT. While it is convenient to regard the lithosphere as the earth proper, and the atmosphere as its envelope, the latter is as truly a part of the planet as the former, and its activities and its history are as truly subjects of geological study as the formation of the rocks. This view is in no way vitiated by the fact that the special study of the atmosphere is set apart under the name _Meteorology_, for in the same way the special study of rocks is set apart under the name _Petrology_, that of ancient life under the name _Paleontology_, and that of other phases of the subject under other names. The atmosphere is one of the three great formations of the earth, and as a geological factor takes its place beside the hydrosphere and the lithosphere. It has played a part in the history of the earth comparable to that of the water, though its mass is less and its record more elusive. Unsubstantial as the atmosphere seems when contrasted with the liquid and solid portions of the earth, its extreme mobility and its chemical activity compensate for its lightness and tenuity, and give it a function of the first order of importance.