266 



GEOLOGY. 



stone. The Lower Cambrian also is thought by Comstock l to be present. It 

 is possible that the Lower Cambrian is present at some points and not at others. 

 The Cambrian is unconformable on its base, and according to Comstock * there 

 is also unconformity between the three main subdivisions of the Cambrian. The 

 thickness varies from point to point, the published sections giving 600 to 1000 

 feet or more. Strata nearly horizontal. 



In west central Colorado 2 (Tenmile region) 160 to 200 feet of white quartzite, 

 more or less argillaceous above, represents the Cambrian system. The forma- 

 tion carries Upper Cambrian fossils, and rests on the Archean unconformably. 

 Strata gently folded. 



In north-central Utah 3 (Tintic region) the Cambrian is represented by the 

 Tintic quartzite, which has a thickness of some 7000 feet. Since this quartzite 

 is not known to carry fossils, and since it is conformably overlain by Carbon- 

 iferous formations, it may not all be Cambrian. Strata much folded. 



In the Wasatch Mountains 4 the Cambrian rests conformably on the Protero- 

 zoic. The Lower Cambrian is represented by slate 75 to 600 feet thick. This 

 slate is overlain by the thick Ute limestone, 1000 to 2000 feet thick. The lower 

 shaly part only of this formation contains a Middle Cambrian fauna, and the 

 Upper Cambrian fauna is not known. The larger part of the Ute limestone is 

 Ordovician, and its top may be Silurian. Strata much folded and faulted. 



In tho Rocky mountains of Canada the following Cambrian section 5 occurs 



Names of series. 



Castle Mountain 

 series. 



Bow River series. 



Characteristics. 



Consists of limestone, dolomites, shales, slates, and schists. 

 The lower part carries Lower Cambrian fossils, and the 

 uppermost part fossils which appear to be Ordovician. 



Chiefly argillites with some sandstone, quartzite, and con- 

 glomerate. Lower Cambrian fossils occur 3000 feet 

 below the top. 



Summary of physical events and interpretations. — The accumu- 

 lation of a thick series of beds in certain regions and the spreading 

 of a relatively thin sheet over a large part of the American continent 

 are matters of positive determination. The immediate agency by 

 which this was accomplished is found in the wear of the land and the 

 deposit of the debris in the adjacent seas. The remoter agencies that fur- 



1 See foot-note, p. 221. 



2 Emmons, Ten Mile (Colo.) folio, U. S. Geol. Surv. 



3 Emmons, Tower, and Smith, Tintic (Utah) special folio, U. S. Geol. Surv. 



4 King, Expl. of the 40th Parallel, Vol. I, p. 156; Walcott, Bull. 81, U. S. Geol 

 Surv., pp. 157-60. and p. 328. 



'McConnell, Ann. Rept. Geol. Surv. of Canada, Vol. II (New Series), Pt. D 



