216 GEOLOGY. 



systems, separated by unconformities. It is worthy of note that 

 in Sweden * iron ore occurs, as in the Huronian of Lake Superior. 

 Little has been done toward the correlation of the pre-Cambrian rocks 

 of Europe and America. It is significant, however, that the terms 

 Algonkian and Proterozoic are being adopted 2 in the delineation of 

 the pre-Cambrian formations on geological maps. As in America, 

 the Proterozoic rocks of Europe contain occasional fossils, though 

 their number is small. 



In the province of Shan-si, China, there are at least two great Pro- 

 terozoic systems, separated by an unconformity. The lower system is 

 composed of meta-sedimentary rocks, closely folded; the upper is some- 

 what folded and slightly metamorphosed. The lower system rests on 

 the Archean, and the upper underlies the Middle Cambrian uncon- 

 formably. 3 



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 direct evidences 

 are as follows: (1) The carboniferous shales, slates, and schists indi- 

 cate the existence of life; (2) the occasional fossils 4 (especially in 

 Montana and in the Grand Canyon of the Colorado in Arizona) demon- 

 strate the existence of life at that time; (3) the iron ore of these sys- 

 tems, although not originally deposited in its present form, seems to 

 imply the existence of life, both when it was accumulated, and when 

 it was transformed into its present condition; and (4), the limestone. 

 It should be noted that limestone occurs in the Huronian system, 

 and that it is, in some places, near its base. The limestone was formerly 

 regarded as demonstrative of the existence of life, but in recent years 

 the belief has gained ground that considerable formations of lime- 

 stone may have originated by precipitation from sea-water. This 

 origin is suspected for many limestone formations which are free from 



1 Tornebohm, Grunddragen af det Centrale Skandinaviens Bergbyggnad, 1896. 

 a Backstrom, Vestanafaltet (The Vestana region), 1867. 



3 Willis and Blackwelder, Carnegie Institution. Unpublished data. 



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

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



