76 GEOLOGY OF THE DENVER BASIN. 



Laramie. The weathered concretions outwardly resemble limonite, but 

 in their normal condition they arc probably carbonate of iron with a 

 larger or smaller admixture of clay. They frequently show concentric 

 structure, the outer coats weathering off in succession, but more generally 

 they break up into angular fragments. They differ from the ferruginous 

 concretions of the Colorado and Montana groups in their superior size, 

 in the abundance of their plant remains, in the far greater amount of iron 

 contained, and in their primary composition. Somewhat similar concretions 

 occur in the overlying Arapahoe; they are, however, less abundant than 

 in the Laramie. An analysis of a purer variety from near Trinidad, Colo., 

 gav( — 



Analysis of ironstone.^ 



Per cent. 



Silica 9.19 



Protoxide of iron '45.01 



AJumina * «•- -'i> 



Lime 1.02 



Magnesia 1. 37 



Phosphorio acid 3 1.055 



Carbonic acid and organic - 33. 035 



too. ooo 



Although smelted in early times, the ironstones of the Laramie are no 

 longer of economic value. 



• l l: \ l [GR \rim o Rl l a l kins. 



'The stratigraphical relations of the Laramie formation to those of 

 younger age are varied. After the period during which the Laramie was 

 laid down, there followed a time of great oscillation and erosion, during 

 which were produced a series of unconformities that included all the post- 

 Laramie and Tertiary formations of the West. To the erosion is due the 

 impossibility of assigning to the Laramie a definite original thickness, much 

 of the deposit ha\ ing been removed and its surface rendered most undulating 

 before the deposition of the younger beds upon it. Upon this unevenfloor 



Preliminary notea on the iron resources of Colorado. Prof. Regis Chauvenet, Ann. Rept. 

 State School of Mines. Golden, Colo., 1885, p. 19. 

 •' Iron :: i 

 Phosphorus 0.46 by difference. 



