156 W. O. CROSBY ARCHEAN-CAMBRIAN CONTACT IN COLORADO 



entire Manitou area. The actual exposures, revealing every minutest 

 detail, are absolutely continuous in some cases for hundreds and even 

 thousands of feet and aggregate several miles in length, and the preced- 

 ing notes faithfully describe all the irregularities of the contact I wag 

 able to make out in traversing these miles of exposures. The secondary 

 irregularities, due to faulting and folding, are seen to be readily distin- 

 guished from the true original erosion hummocks and hollows, and 

 eliminating the former, in imagination, we find the granitic and gneissic 

 floor of the Cambrian sea to have been almost as featureless as a bedding 

 plane of the sandstone deposited upon it. The specially startling fact 

 is, of course, not the infrequency, but rather the small scale of the ero- 

 sion inequalities. With the exception of the gentle undulations of the 

 contact noted in Williams canyon, I have discovered no elevations or 

 depressions de})arting more than 4 feet, and in only one instance (figure 

 25, page 152) more than 2 feet, from the general surface. Again, the 

 inequalities, with unimportant exceptions, are hummocks and not hol- 

 lows — erosion remnants and not channels, clearly marking the end and 

 not the beginning of the process of baseleveling ; and finally, such slight 

 inequalities as survived the wearing down of the granite may in general 

 be correlated with its coarsely concentric or spheroidal structure, show- 

 ing that at the last the erosive action was highly discriminating, the 

 spheroidal masses, like knots in a board, offering the greatest resistance 

 to the planing process. 



Comparison with other Regions 



It has not been the writer's privilege to make a detailed study of the 

 Archean-Cambrian contact elsewhere in the Rocky mountains, but gen- 

 eral observations in other parts of this great region over which the Pale- 

 ozoic strata present a surprisingly uniform section have convinced me 

 that the contact phenomena of the Manitou district are certainly not 

 unique, but probably widespread and characteristic. In the valley of 

 Eagle river (plate 17) and in the canyon of Grand river, above Glenwood, 

 Colorado, the contact between the granite and the white or gray sand- 

 stone at the base of the Cambrian is readily traceable for miles along the 

 mountain side as a strongly marked straight and often horizontal line, 

 with every indication that a detailed examination would reveal the same 

 })aucity of inequalities as in the Manitou emba3anent. M}'- recollections 

 of structural details in the Black hills of South Dakota are somewhat 

 dimmed b}'- the lapse of eleven years ; but I recall that the contact of the 

 Cambrian sandstone and the highly inclined Archean (or Algonkian ?) 

 schists is often approximately plane, and among my photographs I find 



