SUBAERIAL VERSUS MARINE EROSION 161 



a surface entirely devoid of relief. This condition, so far as the unusually 

 satisfactory'' evidence permits us to judge, is almost realized in the Manitou 

 district, where the insignificant and almost inappreciable relief features 

 occupy but a minute fraction of a surface which is otherwise hardly 

 distinguishable from a perfect plane. 



In reaching the conclusion that the sea has at least put the finishing 

 touches upon and given character to a plane rock surface which may 

 very well have been reduced to a peneplain by prior subaerial erosion, 

 I am not unmindful of the view advanced by Hayes and Campbell* that 

 this supplementary erosion may be due to chemical agents acting in the 

 subaerial zone, or more specifically to the solution of silica by the azo- 

 humic acids of marshy tracts. But this argument appears to me inad- 

 equate and inconclusive, at least as an explanation of the continent-wide 

 phenomenon under consideration, and the same may be said of the argu- 

 ment suggested by McGee's account f of sheetflood erosion in the arid 

 districts of the west. 



No one questions the invasion of the land by the sea in Cambrian 

 times, or that, as Walcott and others have insisted, given time enough, 

 the mill of the ocean beach was competent to grind the face of the conti- 

 nent to the nearly perfect plane now preserved in the Archean-Cambrian 

 contact. Therefore it would seem illogical to endeavor to press into this 

 service localized agencies whose very existence in Cambrian times is a 

 matter of conjecture. 



Relation of^the Form op unconformable Contacts to the Character 



OF THE immediately OVERLYING SeDIMENTS 



Not only were the Archean granites of the Manitou area— alike the 

 coarse and the fine grained, the massive and the gneissoid varieties — 

 worn down by the surf of the Cambrian sea to a surface as smooth and 

 flat, in the main, as a floor, but on this floor was left hardly a recognizable 

 trace of the granite. In other words, the base, at least, of the Cambrian 

 is very seldom in any appreciable degree of arkose character, and then, 

 so far as I have observed, only in close proximity to some residual hum- 

 mock where, we may suppose, the lateral gnawing of the waves went on 

 to the last ; and it may be noted in passing that the comparative abrupt- 

 ness of these slight residuary reliefs is far more consistent with marine 

 than with subaerial erosion. With the exception of an occasional well 

 rounded pebble of vein quartz, the basal member of the Cambrian is a 

 nearly pure white quartz sandstone, usually of rather friable texture, but 



* Bull. Geol. Soc. Am., vol. 8, pp. 213-226. 

 fBull. Geol. Soc. Am., vol. 8, pp. 87-112. 



