THE REGION ABOUT GOLDEN. 93 



to the north of the present position of Clear Creek. The rise of the arch, 

 as indicated by its upper beds, was apparently aboul 120 feet, but subse- 

 quent erosion must have planed it down from its original height and shape 

 to approximately the level line drawn across it in the figure as the base of 

 the Jurassic formation. 



The evidence for the occurrence of the unconformity at this horizon 

 and the fold which preceded it is found in the disappearance * > t' most of the 

 upper members of the Trias within the region of its influence, and in the 

 divergence between the present strikes <>t' the formations on either side of 

 the line of unconformity — a divergence in strikes, it being remembered, 

 corresponding to an equivalent discrepancy in the ancient dips, as shown in 

 the profiles. 



This line of unconformity is naturally somewhat wavy, and it is 

 possible, indeed, that at some points along the middle portion of the exist- 

 ing arch, through insufficient erosion, the deposition of a pan or even of 

 the whole of the Jura may not have taken place. The weight of the 

 evidence, however, is in favor of nearly complete deposition over the 

 entire section, from the fact that wherever the formation now exists it 

 displays no tendency whatever to a protracted, gradual thinning, a- is 

 the case in the disappearance of certain of the other formations, hut. on the 

 contrary, disappears by the sudden truncation of it> strata in almost their 

 full normal thickness, clearly the effect of subsequent erosion. 



The movement which brought about the elevation of the Triassic 

 strata must he regarded as synchronous with at least a portion of that more 

 prolonged or extensive movement by which the sea was sooner or later 

 shut out from certain areas in the Rocky Mountain region of Colorado, 

 causing either a partial or an entire absence of marine beds, according to 

 circumstances, with a succeeding deposition of fresh-water strata in which 

 a lacustrine life appeared. In the area under discussion the fresh-water 

 Jurassic alone was laid down. 



General subsidence of the entire region continued during tin- deposi- 

 tion of the Jura upon and against the sides of the Triassic eminence, and 

 at its close the second period in the geological development of the area 

 was completed. 



Third period. — This opened with still another uplift of all the preexisting 



