28 EEPORT UNITED STAIES GEOLOGICAL SURVEY. 



land Eidge, extending, indeed, almost continuously from its eastern to 

 its western end. 



Besides entering largely into the bulk of the great Midland Ridge, the 

 strata of this whole group form an almost continuous line of hogbacks 

 around the eastern end of both the Uinta and Plateau Uplifts, the 

 whole length of Midland Eidge, and thence continuously around the 

 western bases of both Section Eidge and Split Mountain. They also oc- 

 cupy the bottom of Eed Eock Basin ; and small isolated exposures of tb© 

 strata are to be seen near the bases of both Junction and Yampa 

 Mountains. 



Tbe fact that the middle and upper divisions of this group are so of- 

 ten exposed in the faces of hogback and other escarpments is doubtless 

 due to the softer and more yielding character of the lower division, 

 which, becoming disintegrated more rapidly than the others, allows the 

 superimposed strata to fall down, leaving precipitous escarpment faces 

 of the remaining rock. 



THE JURASSIC PERIOD. 



In this district, a group of strata of the Jurassic period is found every- 

 where bordering the exposures that are doubtfullj'^ referred to the 

 Triassic. They consist mainly of soft, greenish, grayish, reddish, and 

 purple bad-land sandstones, with a few feet of sandy calcareous Iryers. 

 near the base, which contain the fossils that are characteristic of the 

 group. Being soft, the strata of this group are often obscured by the 

 debris resulting from their own disintegration, while the associated 

 strata, both above and below them, being harder, resist disintegration, 

 and are often conspicuously exposed as hogbacks or other escarpments. 

 Sometimes, however, they ap|)ear in the lower portion of the face of a 

 hogback or escarpment, being protected there from further disintegra- 

 tion by a cap of the harder strata of the lowest group of the Cretaceous 

 period. The thickness of the group in this district being only about 

 GOO feet, and being usually exposed by flexures of the strata, it occupies 

 only a small portion of the surface. Its exposures are, indeed, mostly 

 confined to narrow bands that appear among the upturned groups of 

 strata that flank the principal uplifts in the district. 



It will thus be seen that all the groups hitherto described, occur in 

 the northwestern portion of the district alone, all having been upturned 

 along the flanks of .the Uinta and its accessory uplifts. The whole of 

 this large portion of the district is therefore occupied by the rocks of 

 older date than the Cretaceous period, largely by those of Carbonifer- 

 ous age. This space is, however, bordered by rocks of the Cretaceous 

 period, which, together with those of the Tertiary, occupy the remainder 

 of tbe district. 



THE CRETACEOUS PERIOD. 



The whole series of Cretaceous strata as they are known to geologists, 

 in the great Eocky Mountain region, is represented in this district, as 

 well as another series immediately above them, concerning the proper 

 age of which, geologists are divided in opinion, and to which I have 

 given the separate but provisional designation of Post-Cretaceous. Un- 

 der this head those strata are discussed upon subsequent pages. 



The original grouping of the Cretaceous strata, as they are found de- 

 veloped in the Upper Missouri Eiver region, by Meek and Hayden, is 

 shown in the table of correlated sections on page — . It is also there 

 shown that late authors agree in modifying that original grouping for 



