18 GEOLOGY OF THE DENTEK BASIN. 



To what extent tins elevation and erosion affected the eastern shore-line of 

 the Colorado Island, immediately facing the mediterranean ocean, it is 

 difficult to determine, on account of the uncertainty that exists with regard 

 to the next succeeding deposits along that line. 



Following the Carboniferous elevation and erosion, there was a general 

 subsidence throughout the Rocky Mountain region, which continued in 

 some parts through Upper Carboniferous into Triassic time. On the west- 

 ern slopes immense thicknesses of conglomerates accumulated during the 

 Upper Carboniferous period, which passed upward into red sandstones, 

 gradually growing lighter in color and finer in grain, the final development 

 being the Red Beds, which have been generally considered Triassic, mainly 

 on stratigraphical grounds. 



Along the eastern front no great conglomerate series comparable to the 

 Upper Carboniferous of the western slope lias yet been observed, and the 

 beds occurring below the Jurassic have, on account of their red color, 

 hitherto been assigned to the Trias, but the red color is not an infallible 

 criterion, for red sandstones are known to occur in beds carrying Upper 

 Carboniferous fossils. 



WYOMING FORMATION. 



In the case of isolated and incomplete exposures of a series of beds 

 deposited between two nonconformities, the lithological constitution of the 

 beds is, in the absence of all paleontological evidence, the only, though 

 necessarily very uncertain, means of determining their geological horizon, 

 for the earlier formation of the series may be concealed by overlap if there 

 was a continuous subsidence during the period, and the later formations 

 may have been removed by erosion during a later elevation. 



The assignment of the lowest series of beds in the present field to the 

 Trias is based mainly on lithological correspondence with the upper part 

 of the lied Bed series as developed on the west flanks of the Rocky Moun- 

 tain uplift. There is, however, a further reason for this assignment in the 

 fact that the variation in thickness of the formation, which is very consider- 

 able, is at the bottom rather than at the top of the series, and hence is 

 mainly due to overlap rather than to erosion. Whether any considerable 

 amount of beds not represented in the actual outcrops exists under the 



