156 



GEOLOGY. 



there seems to be one unconformity which is neither slight nor local. 

 The beds above and below it have sometimes been known as the 

 Upper and Lower Laramie respectively. In Colorado the beds above 

 the great unconformity have also been called post-Laramic, 1 and have 

 sometimes been classed with the Cretaceous, and sometimes with the 

 Tertiary. They include the Arapahoe (below) and Denver formations. 



Fig. 399. — An outcropping ledge of clay, hardened by the burning of the coal-bed 

 below. Except in the immediate vicinity of the burnt-out coal-bed, the clay 

 is not indurated. Near Buffalo, Wyo. (Blackwelder.) 



The Arapahoe formation is of fresh-water (or subaerial) origin, and 

 500 or 600 feet thick. The Denver formation, also of non-marine origin, 

 has a maximum thickness of more than 1400 feet, the lower part being 

 derived chiefly from andesitic lavas. The Ohio and Ruby formations 

 in another part of Colorado 2 (2700 feet thick), and the Livingston 

 formation of Montana, 3 as well as local formations elsewhere, 4 occupy 

 the same stratigraphic position. The Livingston formation contains 

 brackish-water fossils below and fresh- water forms above. 5 



1 Geology of the Denver Basin of Colorado, Mono. XXVII, U. S. Geol. Surv. 



2 Anthracite and Crested Butte folio, U. S. Geol. Surv. 



3 Iddings and Weed, Livingston and Three Forks, Mont., folios, U. S. Geol. Surv. 



4 Cross, Am. Jour. Sci., 3d series, Vol. XLIV, 1892; pp. 19-42; also Mono. XXVII, 

 p. 213 et seq., and Hills, Proc. Colo. Sci. Soc, Vol. Ill, 1891, p. 359-458. 



5 Cross, Mono. XXVII, U. S. Geol. Surv., p. 221. 



