Tin; i;i:<;iun about golden. 91 



STRUCTURAL DF.VF.Lnl'MF.NT "1 TUB LREA, 



Introductory. Tile ill )1 1< HM I i;i 1 ('( )ll(li tiOl IS which ll;l \ <■ 1 leell lloted ill file 



relations of tlie several formations to one another arc directly traceable to a 

 series of unconformities that exist at the particular horizons at which these 

 conditions occur. Excluding the higher ones of general occurrence along 

 the base of the mountains in this portion of ( Jolorado — that is, those between 

 the Laramie and Arapahoe, and the latter formation and the Denver beds — 

 there are still to be found four which are in some respect- peculiar to this 



locality: One between the Axchean and Trias, of special development in 



this area; a second at or near the (dose of the Trias; a third at the top of 

 the dura; and a fourth in the ( 'retaceoiis at the close of the Colorado. 



Entering most prominently into the history of these unconformities 

 are as many folds, all of which occurred prior to the general uplift of the 

 Rocky Mountains, and hence, with the erosion going on at the time, repre- 

 sented a topography for the region completely different from that of the 

 present day. When the greal uplift of the Rocky Mountains brought 

 the beds into the position they now have, all hills resulting from previous 

 folding were changed in their individual positions from one in which the 

 plane of their bases was horizontal to one in which it became vertical, or at 

 least inclined at a high angle, and parallel to the direction of the mountains. 

 In the subsequent erosion of the region, therefore, what would originally 

 have been a profile section of the strata constituting these folds now appears 

 in plan on the present surface of the ground, all originally north and smith 

 dips becoming present north-and-south strikes — in some cases slightly 

 altered in character by incidental variations in the amount of folding in the 

 general uplift of later times. 



The detailed character and the contours of these ancient elevations 

 can not he determined, the two dimensions given in the profiles being nat- 

 urally the only ones admitting of observation. The profiles, however, afford 

 data quite sufficient to furnish a clear insight into the general character of 

 the unconformities and the movements in the earth's crust which led up 

 to them. 



First period. — The several events in the ecological history of this region 



