60 DESCRIPTIVE GEOLOGY. 



what difficult to estimate, owing partly to the impossibility of. determining 

 its upper limit, and in part to the uncertainty of dip. 



The Fox Hill strata pass by imperceptible gradations into the Lar- 

 amie series, offering no well-defined line of separation, both formations 

 from top to bottom consisting of coarse sandstone. It is difficult to 

 give a detailed description of either division which is not in some po- 

 sition in the series applicable to the other. In general, however, the two 

 formations present some distinctions which are recognizable in the field 

 over large areas. In the Fox Hill bed?, the sandstones are usually more 

 massive, with great uniformity of texture through a wide vertical range. 

 The Laramie beds indicate more variety, with changes, within certain 

 limits, from coarse to fine layers, frequently interstratified Avith seams of 

 argillaceous sandstone and of pure clay. In color also, the beds shoAv 

 more changes from rusty -yellow to deep red, with well-marked layers of 

 whitish-gray sandstone. Another characteristic is the occurrence of beds 

 of loose quartz-grains, like beach-sand, lightly held together by some 

 ferruginous cementing material. The Laramie formation possesses even a 

 less dip than the Fox Hill, and over wide areas dips only from 1^° to 2°;' 

 in many places it appears perfectly horizontal. In one or two localities, 

 far to the eastward, the beds indicate a slight inclination to the westward 

 toward the mountains, as if there had been a gentle oscillation in the move- 

 ments of the beds producing a wave-like structure. Such a structure, 

 however, was not clearly shown. The Fox Hill and Laramie formations 

 taken together have been roughly estimated, in Northern Colorado, as 

 measuring 3,000 feet in thickness, allowing some 1,500 feet to each. In 

 the Laramie formation occur the great deposits of coal, which have proved 

 of such great economic value to Colorado. They form one of its most 

 distinctive features, as in Northern Colorado, at least, no beds of coal have 

 as yet been found in the Fox Hill beds. Through a wide vertical horizon, 

 thin seams of coal and carbonaceous clays appear to crop out in the more 

 elevated banks and ridges, but no attempt to give the numbers of such 

 seams or their true positions in a section has as yet been made with any 

 degree of accuracy. Within the limit of our survey, no coal deposits of 

 any great value have as yet been opened, although a number of tunnels 



