SUBAERIAL VERSUS MARINE EROSION 



159 



often somewhat abrupt irregularities. The more 

 abrupt of these have an evident genetic relation 

 to the durability and general resisting power of 

 the rocks which compose them." 



He further states that these prominences, 

 in exceptional cases, rise from beneath the 

 Potsdam (Cambrian) sandstone to heights 

 of from 100 to 600 and even, in the Bara- 

 boo ranges, of 1,200 feet. Irving's more 

 general sections are reproduced in figures 

 36 and 37. Explicit descriptions are, of 

 course, far more satisfactory than the 

 highly generalized figures usually accom- 

 panying them, and the latter must be 

 quoted and used cautiously. 



Dr A. C. Lawson, on the other hand, 

 concludes that the early Paleozoic rocks 

 were laid dovs^n on a surface which did not 

 diff'er essentially from that presented by 

 the exposed Archean surface of the present 

 day; but, notwithstanding this and sim- 

 ilar statements from other good observers 

 concerning the conditions in Canada, the 

 Adirondacks, and other regions, the fact 

 remains that tlie general consensus of 

 opinion is emphatically in favor of the 

 view that the Cambrian, and especially 

 the Potsdam sediments of North America, 

 were in general deposited upon a surface 

 of pre-Cambrian rocks far more perfectly 

 baseleveled than that presented by these 

 older formations in any part of the conti- 

 nent today. All that can be claimed for 

 the Manitou area, therefore, is that this 

 almost universally flat contact has here 

 one of its most perfect developments. 



SUBAERIAL VERSUS MARINE ErOSION 



Although the competency of subaerial 

 erosion to develop a typical peneplain, as 

 defined by Davis, is still questioned by 



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