LARAMIE FORMATION. 73 



The formation is from 600 to 1,200 feel thick and is divisible into two 

 parts, a lower of sandstones, and an upper, composed of claj s. The former 

 has a uniform thickness of about 200 feet; the latter varies. Both divisions 

 carry workable seams of coal. 



STRATIGRAPHY, LOWER DIVISION. 



The two sections given in figs. 12 and lf>, in the first part of 

 Chapter VI, show the general succession of strata in this division. Sand- 

 stones largely predominate, outcropping in successive benches separated 

 by bands of arenaceous and lignitic shale with their intercalated coal 

 seams. The two heavy beds of sandstone at the base of the division ami 

 the bed at the top are the only ones persistent over the entire field. The 

 intervening ones not only disappear, hut vary in the horizon at which they 

 recur. The coal beds also vary, one of the several seams being workable 

 in one locality and another in another. 



sandstones. — The sandstones are white and are composed almost exclu- 

 sively of quartz, clear and opaque white; minute grains of black chert and 

 rare traces of feldspar and mica are the only associates. The material is 

 somewhat loosely held together by a cement, usually white, but occasionally 

 tinged brown hv iron oxide. The chert is characteristic. 



The three important sandstones of the lower division of the Laramie 

 may, for convenience, lie designated from base upward. A, B, and ( '. A 

 and I! are each about 60 feet thick, are separated by from 2 to 4 feel of 

 shale, and with the underlying Fox Hills sandstone often form a, single 

 continuous outcrop. Although of nearly the same materials, the A and 15 

 sandstones are distinguishable from each other by their stratigrapliic.il 

 relations to the overlying and underlying beds; by the presence in the 

 lower bench of a greater amount of ferruginous matter, which imparts to it 

 a faint-yellow tinge, not nearly so deep, however, as that of the underlying 

 Fox Hills: by the occasional development in this bench of a concentric 

 structure; and by the occurrence near the summit of the upper bench of 

 enormous concretions of the same material as the surrounding sandstone, 

 but of extreme hardness. These concretions upon weathering out are of 

 various sizes and shapes, but a common form is one somewhat irregular in 



