REVIEWS 455 



Cross' maps and describes the geology of the Telluride quadrangle, 

 Colorado, and briefly slcetches the geology of the San Juan region, of 

 which the Telluride quadrangle is a part. 



In the Telluride quadrangle, along Canyon Creek north of Stony 

 Mountain, is a small body of upturned quartzites, with an intercalated 

 rhyolite sheet, which have been referred to the Algonkian. The 

 quartzites are coarse and grade into fine conglomerate. 



Ancient granites, gneisses, and schists are known in the Animas 

 Valley and in the Uncompahgre plateau. These rocks have usually 

 been considered as belonging to the Archean, but some of them are 

 probably younger than the great series of quartzites exhibited in the 

 Needle Mountains to the south, and younger than the quartzites 

 beneath the volcanics in the canyons of the Uncompahgre, above 

 Ouray, which have been referred to the Algonkian. These quartzites 

 stand on edge or have been greatly disturbed. The relations of these 

 isolated exposures to contemporaneous formations elsewhere are 

 unknown. 



Spurr^ maps and briefly describes the Archean ^ granite of the 

 Aspen district of Colorado. This is unconformably below and in 

 direct contact with sediments of upper Cambrian age. 



Davis'* in a general account of a trip through the Colorado Canyon 

 district briefly describes certain features of the pre-Cambrian geology. 

 Recalls attention to the extraordinary evenness of the floor of schists 

 with granite dikes (Archean) upon which the Chuar and Unkar ter- 

 ranes (Algonkian) rest. The floor for the Paleozoic strata is somewhat 

 less regular than the floor for the Unkar. In two places the pre-Cam- 

 brian rocks rise higher than the basal Tonto (Cambrian) sandstone. 

 The Archean schists beneath the Unkar have a steep and regular slope, 

 indicating uniform resistance to erosion. Where, beneath the Tonto, 

 they show a bench, it is taken to indicate a softer character at 

 this point, probably due to a longer period of pre-Tonto weather- 

 ing. 



' Telluride Folio, Colorado, by Whitman Cross : Geol. Atlas of the U. S., No. 

 57, 1899- 



^Geology of the Aspen Mining District, Colorado, with Atlas, by J. E. Spurr : 

 Men. U. S. Geol. Surv., No. 31, 1898, pp. 1-4, 



3 The term Archean is evidently used in the sense of pre-Cambrian. 



■t Notes on the Colorado Canyon District, by W. M. Davis : Am. Jour. Sci., 4th 

 ser., Vol. X, 1900, pp. 251-259. 



