66 GEOLOGY OF THE DENVER BASIN. 



the form of concretions, from 1 to 3 feet in diameter, are very hard, break 

 in angular fragments, and on fresh surfaces resemble in color the shales, 

 though their exterior weathers either bright-yellow or rusty-brown. They 

 are usually somewhat calcareous. A partial analysis of an ironstone 

 follows: ! 



Analysis of ironstone. 



Per cent- 

 Si lira 26.31 



Iron (metallic) 22.90 



Alumina 2.31 



Lime 3.22 



Magnesia 8.19 



Limestones. — These occur in more or less noi lcoiitinuous 1 lands, 1 to 3 

 feel thick, throughout the formation, but they are more numerous, thicker, 

 and of greater continuity in the upper third. One only is persistent over 

 the entire field, this lying about LOO feet below the summit. The lime- 

 stones are of coarse texture, dark color, and generally yield a bituminous 

 odor. The uppermost beds resemble the basal member of the Niobrara 

 and thus constitute a zone of transition between the two formations. 



LIFE. 



The life of the Benton seems to have been rather poorly represented 

 in this held as compared with other localities, for while there are evidences 

 of organisms in profusion, both of plants and animals, the only forms 

 especially abundant are those of the two genera of Mbllusca, Ostrea and 

 [noceramus. To these must be added some undetermined Selachians, 

 represented by their teeth, which occur mainly in the more strongly 

 bituminous limestones. 



NIOBKABA. 

 STRATIGRAPHY. 



The surface area occupied by the Niobrara formation has an average 

 width of about <i(>(» feet, except for a distance of about 3 miles north and 

 south of Golden, where, like the Benton, it lias entirely disappeared. 



The normal thickness of the Niobrara varies slightly on either side of 

 4(in feet. In sedimentation the formation is sufficiently differentiated to 



1.'. porl "i' the Colorado State School of Mines, Golden, Colo., p. 20, L883. 



