210 



GEOLOGY. 



many of them there are thick series of sedimentary or meta-sedimentary 

 rocks overlying the Archean, and overlain by Cambrian or younger 

 strata. Where the post-Archean formations are not overlain by Lower 

 Cambrian strata, their Proterozoic age is sometimes open to ques- 

 tion. In some places they are highly metamorphic, and in others but 

 slightly so. 



Rocks referable to the Proterozoic systems are found in the Medicine Bow 

 range and in some of other mountains of Wyoming; in the Bridger and Little 



Fig. 86. — Section showing the relations of the Proterozoic formations in the Telluride 

 region of Colorado. Aq, Proterozoic quartzite; J, Triassic and Jurassic; K, 

 Cretaceous; E, Eocene; sj, San Juan series (tuff, breccia, etc.); prh, Potosi 

 rhyolite; gd, gabbro diorite. Length of section, 5 miles. (U. S. Geol. Surv.) 



Belt mountains 1 (Belt series, fossiliferous 2 ) in Montana, in the Lewiston and 

 Livingston ranges, near the National Boundary, where they are fossiliferous, 

 and some 10,000 feet thick 3 (Fig. 46), and at a few other points in the same State; K 

 in British Columbia; in the Wasatch and certain lesser mountain ranges of Utah; 

 in several of the ranges of Nevada 5 (Shell Creek, Egan, Pogonip, and Pinyon) ; 

 in the Front, Sawatch, Uncompahgre, and Quartzite ranges of Colorado, 6 and. in 

 the Grand Canyon of the Colorado in Arizona. 7 In most if not all of these locali- 

 ties the sedimentary beds predominate, but are accompanied by igneous rocks 

 which are in part contemporaneous in origin. The thickness of the Protero- 

 zoic rocks in these various localities is often great, but generally unmeasured. 

 In the Canyon of the Colorado (Arizona) the succession of pre-Cambrian for- 

 mations is easily studied, because of the extensive exposures. The Grand 

 Canyon group 8 (Proterozoic), more than 10,000 feet in thickness, rests uncon- 

 formably on the Archean, and is in turn covered unconformably by the Cam- 

 brian. This great group is itself divisible into two systems by a slight uncon.- 



1 See Little Belt, Livingston, and Three Forks folios, U. S. Geol. Surv. 



2 Walcott, Bull. Geol. Soc. Am., Vol. 10, pp. 227-44. 



3 Willis, Bull. Geol. Soc. Am., Vol. 13, pp. 316-24. 



4 Weed, 22d Ann. Rept. U. S. Geol. Surv., Pt. II, p. 434. See also the Little 

 Belt, Livingston, and Three Forks (Mont.) folios, U. S. Geol. Surv. 



5 King, Geol. Expl. of the 40th Parallel, Vol. I. 



8 Proterozoic (Algonkian) formations are shown on the Pike's Peak and Telluride 

 folios (U. S. Geol. Surv.) of this State. 



7 See references in Bull. 86, U. S. Geol. Surv. 



8 Powell, Geology of the Uinta Mountains. 



