PIEEKE FORMATION. 69 



the field, the width of its outcrop is but little in excess of its thickness. 

 The latter, under more normal conditions, reaches a maximum of approxi- 

 mately 8,T00 feet, of which the Pierre constitutes the lower 7,700 to 7,900 

 feet and the Fox Hills the upper 1,000 or 800 feet 



STRATIGRAPHY. 



This formation is, in the main, a great body of plastic clays, carrying 

 small, lenticular bodies of impure limestone and, at a horizon aboul one- 

 third the distance from base to summit, a /one of sandstone from loo to 

 350 feet thick. A variable quantity of calcareous matter and alkaline salts 

 is also generally distributed throughout the formation, while iron in the 

 form of concretions or thin seams, tine carbonaceous matter, and gypsum 

 are of frequent occurrence. 



days. — The clays are usually leaden-gray, but may he blue or yellow. 

 Contraction cracks which reticulate the surface are characteristic of areas 

 underlain by the formation, particularly in regions of low dip. Some of 

 these have a linear extent of 30 t'eet, a width of (I inches, and a visible 



depth of 3 feet; usually, however, they are much smaller. The clays 

 retain their normal characteristics over the greater part of the Denver 

 field, hut in the immediate vicinity of the Ralston dike they have been 

 metamorphosed to hard, dark-blue or black shales. In the vicinity of .he 

 Valmont dike their metamorphism is less pronounced. 



Limestones. — These occur as small, lenticular bodies whose horizontal 

 axes aii' from 2 to 6 feet long, and whose vertical axes are from 6 inches to 

 ■_' t'eet. Their composition is between that of a clay, with little carbonate of 

 lime, and a very pure limestone, generally inclining to the more calcareous 

 variety. Their color is gray, their texture very fine-grained, ami both 

 color and texture are extremely even throughout the same body. < >xide of 

 iron is occasionally present in small amount. The limestones an- often 

 reticulated with calcite seams, the ramifications extending through the entire 

 mass, fender a hammer blow the mass flies into hundreds of small angular 

 fragments. This also happens with rocks showing no calcite reticulations, 

 but of homogeneous appearance, and indicates a predisposition of the rock 



