506 GEOLOGY. 



are exposed about many of the mountains of the west, and over con- 

 siderable areas outside the mountains. The system often rests on 

 the Cambrian 1 or Ordovician, 2 as well as the Devonian, and it some- 

 times overlaps all earlier Paleozoic systems and lies upon the Pro- 

 terozoic 3 (Algonkian), and frequently attains a thickness of several 

 thousand feet. 



In Colorado 4 the Mississippian is represented chiefly by limestone 

 and dolomite (the Leadville in central Colorado, the Ouray in the 

 San Juan region, and the Millsap in the Front range), 200 to 800 feet 

 thick, and is one of the richest ore horizons of the state. The faunas 

 indicate the reference of these beds to the Early Mississippian. The 

 later beds of this system were probably removed in the post-Mississippian 

 erosion interval. 



Mississippian limestone occurs in northeastern Washington 5 and 

 in the Canadian Rockies, where the series is between 6000 and 7000 

 feet thick, and includes 5100 feet of limestone. 6 The system is very 

 generally exposed in the foot-hills of the Rocky mountains. In many 

 parts of the west it is unconformable beneath the Upper Carboniferous 

 or Pennsylvanian, and in many places there is an unconformity in the 

 undifferentiated Carboniferous which may prove to divide the two 

 systems. The system is also extensively developed in Alaska, 7 where 

 the Carboniferous proper is less wide-spread, so far as present knowl- 

 edge is concerned, and also along the Arctic Ocean farther east (Fig. 

 228). In the last-named region the beds are coal-bearing. 8 The Lower 

 Carboniferous (Mississippian) has also been recognized on Melville 

 Island. 



1 Spring Mountain range of South Nevada, Walcott, cited by Spurr, Bull. 208, 

 U. S. Geol. Surv., pp. 166-172; and also at various points in the Black Hills of South 

 Dakota, Darton, 21st Ann. Rept., U. S. Geol. Surv., Pt. IV. 



2 Snake Range, Nevada, Weeks, cited by Spurr, Bull. 208, U. S. Geol. Surv., p. 32. 



3 Smith and Darton, Hartville, Wyo. folio, U. S. Geol. Surv. 



4 Eldridge, Anthracite-Crested Butte folio, U. S. Geol. Surv. ; and Girty, Prof. 

 Paper No. 16, pp. 162, 163, and 217. 



5 Landes, unpublished evidence. 



6 Dawson, Bull. Geol. Soc. Am., Vol. XII, p. 69. Willis thinks some of the beds 

 correlated by Dawson with the Carboniferous are Algonkian; Bull. Geol. Soc. Am., 

 Vol. XIII, pp. 323-5. 



7 Dawson, op. cit. , p. 85. 



8 Dawson, Can. Geol. Surv., 1886, pp. 12R, 46-49R, and Map 



