Four great sedimentary eras.—The water-laid series represents four great eras in the history of the earth, as shown by the relics of life imbedded in them. Beginning with the latest, these are the Cenozoic (recent life), during which the life took on its modern aspect; the Mesozoic (middle life), during which the life bore a mediæval aspect; the Paleozoic (ancient life), during which the life belonged to older types; and the Proterozoic (earlier life), during which it is inferred that much life prevailed, though its record is very imperfect. It may safely be assumed to have been more primitive than that of the Paleozoic, as it was earlier. Each of these great divisions embraced several lesser periods or epochs, and these again are subdivided more and more closely according to the degrees of refinement to which studies are carried. The chief of these subdivisions are given in the table on page 19, and others will come under consideration in the historical chapters.

In these four great series of sedimentary rocks there are, here and there, intrusions of igneous rocks, and in some places the sedimentary beds have been metamorphosed into crystalline rocks by heat and pressure. This is particularly true in the lowest of these series, the Proterozoic, where a large part of the sediment is metamorphosed, and where there is much igneous rock, but it is still clear that the main portion of this series was originally water-laid sediment, and so it belongs to the sedimentary series rather than the Archean, in which the sediments are the minor rather than the main factor. It has, however, usually been classed with the Archean, and it is certainly not always easy to draw the dividing line. In a sense it may be regarded as a transition series.

The Archean complex.—Beneath the dominantly sedimentary but partly metamorphic and igneous series there is a very complex group of rocks largely of metamorphosed igneous origin, though containing some metamorphosed sediments. These extend downwards to unknown depths. While all the great formations are occasionally bent and broken, these lowest ones are almost everywhere warped, folded, and contorted, often in the most intricate way. They have been very generally mashed and sheared by enormous pressure, so that they have become foliated, and their original character is much masked. They therefore form a series of great obscurity and complexity. As they are at the bottom of the known series, they have been called the “Fundamental gneiss” and the “Basement complex,” but as the part which we see is not the true base nor the true foundation, it is safer to call them simply the Archean (very ancient) complex. As life appears to have been present during a part at least of the period of its formation is referred to the Archeozoic era.

Beyond and below this series, the structure of the earth is a matter of inference. Vast as are the preceding series, they together form relatively but a thin shell on the outer surface of the globe.

The foregoing series are diagrammatically expressed in Fig. 4, and systematically presented to the eye in the following table.

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.