18 



GEOLOGY. 



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 crystalUne 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 be- 

 longs 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 transi- 

 tion 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 un- 

 known 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 '^Fun- 

 damental 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 forma- 

 tion is referred to the Archeozoic era. 



FiQ^ 4. — Diagram to illustrate the relations of the five great groups of formations. 

 .42= 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. 



