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. [Illustration: +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. { Present. { Pleistocene. { Pliocene. +Cenozoic+ { Miocene. { Oligocene. { Eocene. : Transition (Arapahoe and Denver). { Upper Cretaceous. +Mesozoic+ { Lower Cretaceous (Comanche or Shastan). { Jurassic. { Triassic.