64 GEOLOGKX OV Till'. DENVEB BASIN. 



Fire-clay. Tllis OCCUTS ill loCal I 1 eVclo] HUellt ill tile shales si •] III l';l t ill"' 



the lienvN sandstone benches i>t' the Dakota. There are generally two 

 horizons of the shales, and consequently of the fire-clays, one about mid- 

 \v;i\- in the formation and another nearer the summit. The fire-clay may 

 occupy the entire space between the sandstones, usually from 2 to 8 feet. 

 or may be interrupted by intercalations of hard, white, or bluish-white, 

 quartzose shale. The typical fire-clay is blue or blue-gray, of fine, even 

 texture, hard and compact, jointed or concretionary, and very pure. It 

 becomes impure through the presence of oxide of iron, l>v a varying amount 

 nt' sand in fine grains, disseminated or in thin layers, or by carbonaceous 

 matter. The iron weathers out in the form of minute brown spots, distinc- 

 tive of the horizon. 



LIFE. 



Animal. — Xo trace of animal life has been discovered in the Dakota of 

 the I >enver Basin. 



Plant. — Plant remains, chiefly leaves of deciduous trees and enormous 

 fucoids, abound in certain localities from base to summit of the Dakota. 



Wood tissue in minute fragments, or the impressions of the same, or the 



resulting stains, are of general occurrence. 



In a comparison of the Dakota of the Denver Held, which may he 

 regarded as typical for the eastern base of the Rocky .Mountains, with that 

 in the far distant and widely separated regions of Dakota, Nebraska, ami 



Kansas, the similarity of the beds in composition, manner of occurrence, 

 and flora is remarkable, the thickness of the formation alone being the only 

 point of material difference. 



col i>i; I in) GROUP. 



The two members of this group, the Benton and Niobrara, are broadly 

 distinguishable from each other both in their sedimentation and in their 



fos>ils, hut from the existence between the two of a zone of gradual 

 transition in sediment, and from the common occurrence of many of their 

 more abundant fossil forms, especially within the transitional /one itself, a 

 definite line el' demarcation can not lie drawn. This relationship is in no 

 manner a local one, hut prevails in a greater or less degree wherever the 

 two formation- occur: it obtains even in the Montana and Dakota sections. 



