342 PROTEROZOIC ERA 



Proterozoic Formations in other Continents 



Proterozoic formations are believed to exist in all continents. 

 In more than one country where they have been studied, the pre- 

 Cambrian sedimentary rocks are thought to belong to at least two 

 unconformable systems. In Sweden, as about Lake Superior, iron 

 ore occurs in these formations, and the great bodies of iron ore in 

 Brazil probably are of similar age. 



LIFE DURING THE PROTEROZOIC ERA 



The presence of a few fossils in the Proterozoic rocks proves the 

 existence of life during this era. 1 The best-preserved fossils are 

 arthropods (p. 686) resembling Crustacea. There are also tracks 

 of two genera of worms. In addition, there are obscure forms which 

 appear to be referable to brachiopods and pteropods. It is signifi- 

 cant 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. 

 Other lines of evidence indicating life are: (i) Carboniferous shales, 

 slates, and schists, and (2) limestone, some of which occurs near 

 the base of the Lower Huronian. This rock was formerly regarded 

 as demonstrative 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 limestone, 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. 



CLIMATE 



Since inferences concerning the climate of any period are drawn 

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

 Proterozoic strata, they afford little warrant for conclusions con- 

 cerning the climate of the era as a whole. Conglomerate beds 

 which have been interpreted as glacial 2 are found at the base of 



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

 Bull. Geol. Soc. Am., Vol. 10, pp. 199244. 



2 Coleman, Jour. Geol., Vol. XVI, pp. 149-158, and Wilson, Ibid., Vol. XXI, 

 pp. 121-141. 



