GENERAL CHARACTERISTICS 611 



tures which weather out in strong relief (plate 31). Somewhat less 

 common surface features are disk-shaped bodies of porcelain-like material, 

 embossed on a coarser ground-mass (plate 32, figure 2). In the lower 

 part of the formation, especially in the Teton and Gros Ventre ranges, 

 there are beds which show wavy horizontal lamination, more or less sug- 

 gestive of stromatoporoid growths. 



By examining the weathered surface (plate 30) as it generally appears, 

 with its sharp projecting points or raised welts one-half inch or more in 

 breadth, one may see that the projections and ridges correspond to a 

 slightly darker material within the rock itself. This dark material is dis- 

 tributed in sinuous upright or prostrate rods or corrugated sheets or in 

 thick branching stemlike structures. In many specimens these darker 

 structures are dull smoky gray, while the surrounding material is cream- 

 colored or white. It is also a noteworthy fact that the texture of the 

 darker parts is very dense, while that of the lighter parts is coarser, as 

 well as more porous. It is important to remark, however, that all of these 

 bodies have vague, indefinite outlines. The dark structures are nearly 

 uniform in texture and composition, but the matrix is a medley of par- 

 ticles of various sizes, including bits of recognizable fossils. 



The prevailing color of the Bighorn outcrops is cream-colored to gray- 

 ish white. Some beds are more or less mottled with grayish brown and 

 in places the rock is slightly stained pink by oxides of iron. Freshly 

 broken surfaces are generally more distinctly gray and are obscurely 

 mottled with darker gray, as noted above. Dark brown and blackish dolo- 

 mite, which are common in the overlying Devonian strata, are conspicu- 

 ously absent from the Bighorn. Colors also aid in distinguishing it from 

 the underlying Cambrian beds, because it lacks the red and ocherous 

 lenses and stripes which are characteristic of the older limestones in 

 Wyoming. 



When freshly broken, the Bighorn rock usually emits a distinct but not 

 strong odor of petroleum or hydrogen sulphide, presumably due to re- 

 sidual material from the decay of organic tissues. 



Unlike many dolomites, the Bighorn contains but little siliceous mat- 

 ter. In the lower layers there are generally thin, irregular lenses and 

 laminae of white or cream-colored chert, and locally a few round chert 

 nodules have been seen at higher horizons. 



Of the chemical analyses which have been made from the Bighorn for- 

 mation all show a comparatively pure dolomite. The following table 

 gives analyses made in the laboratory of the United Stales Geological 

 Survey : 



