74 GEOLOGY OF THE DENYEE BASIN. 



outline, of circular or elliptical cross-section, from 2 to 4 feet in diameter 

 and 30 to 40 feet long. A further feature of the upper bench of sandstone 

 (15) is the polygonal structure . developed upon its lamination planes in 

 weathering. The shale separating the A and B sandstones is frequently 

 lignitic and in a few localities, as in the bluffs north of North Boulder 

 ('reek and in the Erie coal held, is partially replaced by coal. 



Sandstone C occurs aboul 60 feet above the basal sandstones A and B. 

 Its thickness averages 8 or in feet. 'The upper half is usually the whiter 

 and more solid: the lower is somewhat ferruginous and also strongly ripple- 

 marked. As there are several hands at about this horizon, it is not always 

 possible to recognize this particular bed, but the presence of the one or the 



other is sufficient for the determination of the general horizon where a g 1 



section is exposed. 'Phis sandstone is of importance in coal exploration, all 

 the workable beds in the lower Laramie of the Denver field occurring 

 below it. 



ostrea bed. — This is a sandstone occurring from L2 to 1 ."> I'cet above 

 sandstone B. It is but slightly developed in many parts of the held, but 

 wherever observed consists of the same material as the other sandstones, 

 with the addition of a considerable amount of lime. It often contains in 

 abundance one of the characteristic Laramie species, Ostrea glabra, besides 

 a lew indeterminable fragments of shells of other types. Its position is of 

 special importance in working out the coal horizons. 



shales ana coai beds. — The remaining portion of the lower Laramie con- 

 sists, besides the coal beds, of shales which are almost always more or 



less lignitic and generally somewhal arenaceous. They contain abundant 



partially carbonized plant remains — Lark, wood, leaves, and their impres- 

 sions. From the presence of the carbonaceous material the shales are 

 frequently dark-gray or brown, and upon its prevalence to the exclusion of 

 clay and sand a bed may become true coal of economic importance. 



Ironstones are sometimes present, but they are not so abundant as in 

 the upper division of the Laramie. 



Local alteration of Laramie beds. Tile salldstolles alld shales of tile lower Lai'a- 



mie have locally undergone considerable alteration from the burning out of 

 associated coal beds, the result of spontaneous combustion or prairie fires. 



