MAJOR DIVISIONS OF THE PRE-CAMBRIAN 17 



tion of the rocks of the pre-Cambrian into Proterozoic and Archeozoic 

 premises a fundamental difference in the fauna of the two eras for which 

 there is no evidence. In short, the zoic division is an unverified assump- 

 tion which the future may or may not justify. 



On the other hand, this objection can not be raised against tlie terms 

 Algonkian and Archean. Their use is justified by the facts in the field, 

 which inevitably lead to the dual classification of the pre-Cambrian for- 

 mations on a physical basis. The above gives sufficient reason for con- 

 tinuing to adhere to them. 



However, the great physical contrasts of the Archean and Algonkian 

 and the profound unconformity separating the two, marking one of the 

 greatest physical revolutions of the world, doubtless does correspond to 

 great life changes. There has been a tendency to place more and more 

 emphasis on the physical breaks between the Paleozoic and the Mesozoic, 

 and the Mesozoic and the Cenozoic, and to regard the differences of life of 

 these eras as largely due to physical revolutions; consequently we should 

 expect the great physical revolution separating the Archean and Algonkian 

 to have resulted in important life changes; and while I shall use the 

 terms Algonliian and Archean because they correspond by definition to 

 the physical facts on which the dual division of the pre-Cambrian is made, 

 if others prefer the terms Proterozoic and Archeozoic, I shall not quarrel 

 with them, after having pointed out that fossil evidence can not be given 

 to justify this life classification, and that the use of these terms is based 

 on faith in future progress rather than on achieve.d results. 



It is evident from the above outline of the history of the pre-Cambrian 

 stratigraphy and nomenclature that the geologists working on these for- 

 mations are much nearer to agreement than they were a score of years 

 since. The point on which there is perhaps most difference among 

 geologists, and especially between the United States geologists and some 

 Canadian geologists, is the emphasis by the former on the distinction 

 between, the Algonkian and the Arcliean. Since there has been some dis- 

 sent as to the fundamental importance of this discrimination, it may be 

 well to mention briefl}' the principles on which it is based. 



In the first place, as already indicated, between the two groups an un- 

 conformity is general. Moreover, this unconformity is believed to be the 

 most distinctive and widespread exhibited by the pre-Cambrian forma- 

 tions. However, to justify fully the emphasis placed on the di;al division 

 of the pre-Cambrian, the separating unconformity needs support by other 

 contrasts. Some of the more important of these are as follows : 



1. The Archean is a series dominantly composed of igneous rocks, 

 largely volcanic and for extensive areas probably submarine. Sediments 



II — Bull. Geol. Soc. Am., Vol. 10, 190T 



