12 GEOLOGY OP THE DENVER BASIN. 



quartzite thai are not only older than Cambrian, but are almost entirely 

 surrounded b) granite, which also sends apophyses into them. These he 

 regards as undoubted Algonkian, while much of the granite must be of 

 e\ en later age. 



Along the h»w er canyons of South Boulder and ( !oal creeks, within the 



area of the present map, and also near Big and Little Thompson creeks 



to the north of it, there are certain beds of highly altered quartzite and 

 conglomerate, associated with schists, which occupy a position between the 

 Triassic sandstones and the more massive gneisses of the interior of the 

 range. These were first noted by Marvine, of the Harden Survey, in 

 1 s 7 ."> . who regarded the quartzites as the firsl phase in the successive 

 nietainorphism of greal series of sedimentary rocks, of which the final 

 expression, in his idea, is a structureless granite. 



The Coal ('reek occurrences have since been more particularly exam- 

 ined by A. Lakes for C. R. Van llise, in L890. In analyzing the various 

 observations Van llise- finds evidence of the existence of a general funda- 

 mental crystalline complex of pre-Cambrian rocks in the Colorado Range, 

 and regards these quartzites and conglomerates as undoubted pre-Cambrian 

 elastics, but is unable from the evidence at hand to say definitely whether 

 there is a sharply defined line between the two or only an insensible 

 gradation. 



In the present work, inasmuch as but a very limited extent of pre- 

 Cambrian rocks comes within the limits of the map and the internal 

 Structure and mutual relations of these rocks have no bearing upon the 

 subject-matter of this investigation, it was not considered advisable to 

 enter upon the necessarily lengthy and complicated study that would he 

 required to determine these relations. Hence, beyond noting the occur- 

 rence of tlie quartzites and conglomerates in the area round Coal and 

 South Boulder creek- and recognizing their distinctly sedimentary character 

 as contrasted with gneisses, granite-gneisses, and massive granite, which 

 appear successively as one approaches the center of the range, nothing 

 has been done toward definitely delimiting the areas occupied by either 



Si ■ mil Ann. Rept. 1'. S. G. and G. Survey, Washington, 1874, \k 137. 



Bull. r. s. Geol. Survej X<>. 86. Correlation Paper, Axchean and Algonkian, Washington, 

 L892, I.,.. 312, 325. 



