EASTERN FOOT-HILLS. 39 



The limestones of the Jurassic strata appear to be of two distinct 

 kinds; the one a hard, dense, cherty rock, carrying a considerable amount 

 of silica., and usually of a characteristic gray color ; the other less compact, 

 exhibiting greater variety in color and texture, and in most localities 

 probably dolomitic. 



A specimen from the latter limestone, collected at the base of the 

 mountains, near the old "Denver and Laramie Stage Road," shows a rather 

 more than usually compact texture, and a pinkish-gray color. Scattered 

 through the rock are numerous small crystals of calcite. Under the 

 microscope may be seen minute grains of silica and crystals of quartz. 



This rock was subjected to complete chemical analysis by Mr. B, E. 

 Brewster, with the following result: 



SiHca 2.77 2.95 



Alumina and ferric oxide 0.79 0.54 



Lime 29.90 29.89 



Magnesia 19,31 19.36 



Soda and potassa 0.38 0.28 



Carbonic acid ; 45.05 45.14 



Water '. 1.35 1.30 



99 55 99.46 



Combining the carbonic acid with the bases we have very nearly the 

 composition of true dolomite. 



Carbonate of lime .' 53.40 53.02 



Carbonate of magnesia 40.55 40.66 



93.95 93.68 



Dakota Cretaceous. — ^Although the base of the Jurassic is fre- 

 quently obscure, the upper horizon, which has generally been adopted as 

 separating the series from the Dakota sandstone of the Cretaceous group, 

 is usually well defined and easily recognized along the entire base of the 

 mountains. The Dakota beds are essentially a sandstone formation, and 

 as they are usually hard and compact, frequently almost a quartzite, they 

 form a well-defined horizon. Lying between the easily-eroded Jurassic 



