16 GEOLOGY OF THE DEjSTVEK BASIN. 



of the successive beds, but, as more detailed studies of the respective 

 exposures have been made, it has been found that certain members of the 

 series thin out locally or are entirely wanting'. Thus, in the Wasatch 

 Mountains, where the ocean waters were apparently deeper than in the 

 more eastern legions and the series is most complete, the Upper Cambrian 

 deposits are apparently wanting. In the Grand Canyon region, on the 

 other hand, the Devonian is represented by less that 100 feet of beds, while 

 the Silurian is wanting. Evidences of erosion are there very distinct, but 

 it is still uncertain whether the Silurian was never deposited or whether it 

 has been entirely removed by erosion. Finally, in the Rock}- Mountain 

 region, no Devonian has yet been discovered, but wherever the Lower 

 Paleozoic section has been studied in detail the Cambrian, Silurian, 

 and Lower Carboniferous beds have been found closely associated and 

 quite conformable. Evidences of erosion are most frequent, however, 

 be'tween the Silurian and Lower Carboniferous, which would favor the 

 hypothesis that during Devonian time ocean waters had retreated from this 

 region and portions of it at least had been exposed to erosion. The studies 

 that have been made of this series of rocks on the eastern flanks of the 

 Colorado Island, in the Canyon City and Manitou bays, respectively, show 

 besides a notable decrease in thickness a decided change in the character 

 of the sediments from those observed on the west side of the island on the 

 flanks of the Mosquito Range, as well as a considerable variation in either 

 respect between those in the respective bays. The Cambrian is here very 

 thin, and at some points does not appear, but the close association of its 

 sediments and fauna with those of the succeeding Silurian renders it prob- 

 able that there was no actual discontinuity in the deposition of the two 

 series, the local absence being due to overlapping by the Silurian. The 

 Silurian beds are relatively well developed and contain an abundant and 

 characteristic fauna, which has rendered it possible to distinguish three 

 distinct subdivisions or formations in this group. The local thinning out 

 or absence of the uppermost of these formations proves that a considerable 

 period of erosion must have intervened before the deposition of the suc- 

 ceeding Lower Carboniferous beds, and in so far confirms the opinion, 

 hitherto only tentatively put forth, that the absence of Devonian beds in 



