398 HISTORICAL GEOLOGY 



lava flows, as well as the long periods represented by the uncon- 

 formities, it seems probable that the Proterozoic was very much 

 longer than all of Paleozoic time. In fact, if the degree 6( life 

 development is taken as a basis by which to measure time, it is 

 thought that the appearance of the Cambrian fauna, although many 

 millions of years ago, was a comparatively recent event. 



Climate. 1 — Little can be said of the climatic conditions of this 

 remote age. The presence of fossils in Montana, Arizona, and On- 

 tario indicates a climate that was certainly not frigid. The presence 

 of scratched bowlders in formations believed to be Proterozoic in 

 Norway, China, and Australia, and perhaps in southern Africa, some- 

 times resting upon a striated rock pavement possessing such char- 

 acters as to make the glacial origin of the deposits undoubted, leads to 

 the surprising conclusion that even at this time the earth was visited 

 by periods of glaciation such as that of the Great Ice Age. There is 

 some question as to the age of these glacial formations, some investi- 

 gators believing that they belong to the Lower Cambrian. According 

 to the theory of a cooling earth, with an atmosphere that was at first 

 heavy, it is difficult to explain the presence of continental ice sheets 

 in this early era. 



Life before Fossils. — The earliest rocks in which an abundance 

 of fossils of which any records have been found occur in the Cambrian. 2 

 These fossils are highly organized and are not the simple, unspecialized 

 ancestors of modern animals that the theory of evolution demands. 

 They are of a degree of specialization which indicates a long period 

 of preceding life. Can the life which antedates the first known fos- 

 sils be inferred ? 



In the seas of to-day the number and aggregate bulk of minute 

 and microscopic soft-bodied animals and plants which live near or 

 at the surface of the ocean is astonishing. Small jellyfish sometimes 

 cover the ocean for many miles, tiny crustaceans live in myriads and 

 microscropic animals in countless numbers. The reasons for the 

 abundance of microscropic life near the surface {i.e., within a few hun- 

 dred feet of the surface) are evidently to be found in the abundance 

 and uniform distribution of mineral food in solution, in the presence 

 of sunlight, and in the uniformity of temperature. Practically all 

 of the life of the ocean depends upon these simple forms, either directly 



1 Schuchert, Chas., — Climates of Geologic Time, Carnegie Institution of Washington 

 Publication 192, iqr4, pp. 263-298. 



Walcott, C. I)., — Smithsonian Misc. Coll., Vol. 64, 1914, pp. 80-84. 



2 Unless the problematical Eozoon proves to be organic. 



