36 DESOKIPTIVE GEOLOGY. • 



marked horizon on Horse Creek, and interstratified betwe.en coarse sand- 

 stones, were found on examination to contain a considerable amount of 

 calcareous substance, which is probably, like the bed from the Big Thomp- 

 son, largely dolomitic. They have a fine-grained arenaceous texture, with 

 a deep reddish-yellow color. 



The limestone beds in the Triassic series along the base of the Colorado 

 Range are, in many localities, well marked, but do not appear to be per- 

 sistent over very wide areas ; at least, they were not always observed in 

 crossing the formation, but, where noticed, always preserved the same gen- 

 eral characters. Two distinct beds, but a few feet in width, are recognized. 

 They are clearly defined north of the Chugwater, and are shown in the sec- 

 tion already given, where the lower bed consists of a compact, siliceous, 

 granular rock, and the upper one a bluish-white cherty band with a brittle 

 fracture, not unlike the characteristic limestone stratum so persistent in the 

 Jurassic beds. 



The deposits of gypsum, which form so marked a feature of the Red 

 Beds, are largely developed in many localities along the base of the Colorado 

 Range. They occur in irregular beds, interstratified in the deej) red sand- 

 stones and shales, and, although they may possess a wide range throughout 

 the entire horizon, are much more prominent in the upper formations. 

 Although quite distinct, and separated from the limestone strata by interven- 

 ing sandstone layers, the heavier gypsum beds are usually at no very wide 

 interval from the limestones. The deposits vary in thickness from 2 feet 

 up to at least 25 feet; the heavier bodies frequently occurring as broad 

 beds, with a considerable thickness in the centre, but thinning out toward 

 the edges. The gypsum occurs both massive and crystallized ; is nearly 

 white in color, with a light reddish tinge, and frequently reddish-gray from 

 the admixture of slighf impurities; the lustre is usually pearly, and often 

 brilliant. The compact massive gypsum, where not discolored by exposure, 

 is generally snow-white, presenting a striking contrast with the brick-red 

 dolomitic shales. 



Gypsum from Red Valley, near the northern end of the Laramie Hills, 

 is quite pure, possessing very nearly the required theoretical composition, 

 an analysis yielding — 



