EASTERN FOOT-HILLS. 41 



1. Yellow sandstone ... 



2. Coarse sandy beds, with frequent clay-seams 



3. Yellowish-brown sandstone, almost a compact quartzite 



4. Coarse yellow sandstone, passing into fine conglomerate 



J 100 feet. 

 I 200 feet. 



300 feet. 



So far as examined, none of the sandstones would appear to carry any 

 carbonate of lime. A specimen of the Dakota sandstone, taken from the 

 prominent outlying ridge north of Wahlbach Spring, presents a fine-grained, 

 even texture, with a yellowish-brown color. Under the microscope, with a 

 high power, many of the quartz grains indicate the presence of glass cavi- 

 ties, while between the sand particles may be recognized an occasional 

 fragment of hornblende, and a green mineral, which appears to be chlorite. 



This rock was subjected to chemical analysis with the following result: 



Silica '. , 95.46 



Alumina 2.69 



Ferric oxide 0.18 



Lime >. 0.14 



Magnesia 0.06 



Soda and potassa 0.25 



Ignition 1.18 



99.96 



CoLOEADO Cebtaceous. — The Colorado group embraces the Middle 

 Cretaceous highly fossilifei^ous rocks, which overlie the Dakota sandstone, 

 and, in turn, underlie the sandstones of the Upper Cretaceous. The group 

 is essentially composed of clays, shales, marls-, limestones, and some impure 

 sandstone, which, taken together, form a well-defined series of beds, and 

 include the three divisions of the Middle Cretaceous, of the section made 

 by Meek and Hay den along the Upper Missouri River: the Fort Benton, 

 Niobrara, and Fort Pierre beds. 



Along the east base of the mountains, within the belt of our exploration, 

 but more especially in Wyoming, it would appear almost impossible to indi- 

 cate, with any degree of" accuracy, the divisions of the series, although their 



