Fig. 3. The figure represents a section of the earth about 1000 miles Long. The unequally thick black line at the top represents on something like its proper scale the depth of the stratified rock. The area below represents crystalline rock, largely metamorphic.

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.

Fig. 4.—Diagram to illustrate the relations of the five great groups of formations.
AR = Archean, Pr = Proterozoic, P = Paleozoic, M = Mesozoic, C = Cenozoic.

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.

GENERAL TABLE OF GEOLOGIC DIVISIONS.
Cenozoic Present.
Pleistocene.
Pliocene.
Miocene.
Oligocene.
Eocene.
Transition (Arapahoe and Denver).
Mesozoic Upper Cretaceous.
Lower Cretaceous (Comanche or Shastan).
Jurassic.
Triassic.
Paleozoic Permian.
Coal Measures or Pennsylvanian.
Subcarboniferous, or Mississippian.
Silurian.
Devonian.
Ordovician.
Cambrian.
Great interval.
Proterozoic Keweenawan.
Interval.
Animikean or Penokean.
(Upper Huronian of some authors).
Interval.
Huronian.
Great interval.
Archeozoic Archean Complex. Great Granitoid Series.
(Intrusive in the main, Laurentian.)
Great Schist Series.
(Mona, Kitchi, Lower Keewatin, Coutchiching, Lower Huronian of some authors.)