474 GEOLOGY 



LIFE DURING THE PROTEROZOIC ERA 



While the Proterozoic rocks do not generally contain fossils, 

 there can be no doubt that life existed during the era. The lines 

 of evidence are as follows: (1) The carboniferous shales, slates, and 

 schists indicate the existence of life; (2) the occasional fossils 1 

 (especially in Montana and in the Grand Canyon of the Colorado) 

 demonstrate the existence of life at that time; (3) the iron ore of 

 these systems, although not originally deposited in its present form, 

 has been thought to imply the existence of life; and (4) the lime- 

 stone. It should be noted that limestone occurs near the base of 

 the Lower Huronian. This rock was formerly regarded as demon- 

 strative of the existence of life, but in recent years the belief has 

 gained ground that considerable formations of limestone may have 

 originated by precipitation from sea-water. This origin is suspected 

 for many limestone formations which are free from fossils, and if 

 the hypothesis is applicable to any extensive formation of lime- 

 stone, it may be applicable to that of the Proterozoic. But even 

 without reliance on this sort of rock, the occasional fossils leave no 

 doubt of the existence of life in this era. The best-preserved fossils 

 are those of Eurypterus-like crustaceans. There are also tracks of 

 two genera of worms, and other undetermined forms. Besides 

 these certain fossils, there are obscure forms which appear to be 

 referable to brachiopods and pteropods. It is significant that the 

 oldest definite fossils yet found are forms well up in the animal 

 kingdom and that they occur (in Montana) 9,000 feet below the 

 unconformity between the Proterozoic and the Cambrian. 



CLIMATE 



Since inferences concerning the climate of any period are drawn 

 chiefly from fossils, and since fossils are exceedingly rare in the 

 Proterozoic strata, they afford little warrant for any conclusion 

 concerning the climate of the era. It may be noted, however, 

 that conglomerate beds which have been interpreted as glacial are 



1 For summary of knowledge concerning pre-Cambrian fossils, see Walcott, 

 Bull. Geol. Soc. Am., Vol. 10, pp. 199-244. 



2 Coleman, Jour. Geol., Vol. XVI, p. 149. The evidence of the glacial 

 character of these beds is open to question. 



