92 GEOLOGY OF THE DENVEE BASIN. 



by which it lias reached its present state of evolution were as follows: 

 First, that which brought about the unconformity between the Archean 

 and Trias. That there every where exists a general unconformity between 

 the rocks of these two ages is well known, but within the region in ques- 

 tion there is direct evidence of a special development of the unconformity, 

 which is, furthermore, borne out by the subsequent events which form 

 the successive steps in the geological history of the area. This evidence 

 consists in the observed termination of certain of the lower beds of the 

 Trias against a slightly projecting portion of the Archean; in the impos- 

 sibility on structural grounds of the whole amount of thinning which the 

 Triassic beds have undergone being attributable to disappearance from the 

 top and the consequent necessity for its having taken place from below; 

 and in the graphical development of an Archean eminence, as represented 

 in Profile 1, PI. X, by tracing backward from their present positions through 

 the series of figures given the relative movements of the rocks of the 

 several ages bv which they have been brought into these positions. The 

 evidence is found to lead directly to the following conclusions regarding 

 the first of the periods in the special history of this region. 



Prior to the deposition of the Trias there had been developed in the 

 Archean, partly by erosion and partly, perhaps, by compression, the eleva- 

 tion shown in section in Profile I, PI. X. Its height was probably 800 feet, 

 and it had a linear extent in a north-and-south direction of nearly 4 miles. 

 Against the sides of this Archean elevation were laid down the coarse 

 sediments of the lower division of the Trias — the Red Beds — which in time 

 completely capped the hill along the line of profile given, and finally buried 

 its summit deep beneath the accumulated material. General subsidence 

 and sedimentation continued uninterruptedly to, or nearly to, the close of 

 Triassic times, completing the first stage in the history of events here 

 considered. 



second period. — At the close of the Trias the region which embraced 

 the above events yielded a second time in a marked degree to the forces of 

 elevation and developed the gentle arch of Triassic and Archean strata 

 shown in Profile II, PI. X. The north-and-south extent of this arch was but 

 slightly greater than that of the one already described in Archean times, 

 its crown — coincident with that of the earlier one — lying about a half mile 



