18 C. K. VAJf HISE THE PROBLEM OF THE PRE-CAMBRIAN 



are subordinate. The Algonkian is a series of rocks which is mainly 

 sedimentary. Volcanic rocks are subordinate. Of course, b}^ the latter 

 statement it is not meant to imply that vast masses of volcanic rocks were 

 not extruded in Algonkian time. In this respect this group is precisely 

 the same as any of the fossiliferous groups of rocks. Thus there was a 

 great outburst of vulcanism in Tertiary times and volcanic rocks covering 

 areas of hundreds of thousands of square miles and thousands of feet in 

 thickness- were poured out. Yet we characterize the Tertiary era by the 

 sediments and their fauna rather than by the igneous rocks. Similarly, 

 in stating that the Algonkian is a series of rocks which is mainly sedi- 

 mentar)^, it is meant to say that these are the rocks which give the keys 

 for their discrimination, and this is so notwithstanding the fact that, for 

 instance, in the Iveweenawan and the Grand Canyon groups volcanic 

 rocks are verv abundant. 



2. The Algonkian sediments, where not too greatly metamorphosed, 

 are similar in all essential respects to those which occur in the Paleozoic 

 and later periods. "\Yhen the Algonkian rocks were laid down essentially 

 the present conditions prevailed on the earth. The Archean rocks, on 

 the other hand, indicate that during this era the physical conditions had 

 not yet become such as to widely lead to the orderly succession of sedi- 

 mentary rocks like those being formed today. To this practical distinc- 

 tion, which I have repeatedly emphasized, Chamberlin and Salisbury in 

 their recent textbook on geology give a theoretical interpretation. Ac- 

 cording to their views, the Archean complex was formed before the 

 weathering processes and hydro-atmospheric work became prominent, 

 whereas the fact that the rocks of the Algonkian are in every essential 

 respect like those which are being now formed they regard as evidence 

 that the conditions had become favorable for mature weathering. The 

 atmosphere and hydrosphere were the dominant dynamic agents, and 

 they worked on land areas covered by vegetation.^ The Archean group, 

 on the other hand, appears to antedate the time in which the sedimentary 

 agencies were dominant. 



3. Tlie folding and metamorphism of the Archean are on the whole 

 very much further advanced than the Algonkian. The Archean complex 

 has an intricacy of structure which is approached only locally by the 

 Algonkian. Sehistosity, gneissosit}^, and otlier secondary structures are 

 very widespread. TMiile often the foldings and faultings of the Algon- 

 kian rocks are complex and the secondary structures widespread, in these 

 respects the group does not approach the intricacy of the Archean. 



For the Keewatin division of the Arcliean where katamorphic changes 



'Textbook of Geology, by T. C. Chamberlin and R. D. Salisbury, vol. 2, 1906, p. 139. 



