26 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



Usually, the sandstone is irregularly bedded, sometimes massive, and 

 oblique stratification is quite common. Fossils are rare in this group, 

 but a few have been found in its upper strata, and those yet known are 

 not such as to distinguish it clearly from the other two Carboniferous 

 groups. It is this group that Mr. King has designated as Permo-Oar- 

 boniterous on his map. According to my observations, it is only its 

 superior position in relation to the other groups that would suggest such 

 a designation, because the Carboniferous groups below it contain fossils 

 that are as closely allied with the Permian of Europe as any that have 

 yet been discovered in the strata of this upper one. 



The strata of this group are exposed at the surface of much the 

 greater part of the area that is occupied by those of Carboniferous age 

 in this district. They cover nearly the whole surface of the western half 

 of Yampa Plateau, and nearly all the rugged surface through which 

 Yampa Caiion is cut, along the southeast flank of the Uinta Uplift, be- 

 sides a narrow area along the whole length of the Fox Creek Flexure. 

 Narrow bands of its upturned strata are also exposed around the bases 

 of Junction and Yampa Mountains. It will thus be seen that all the 

 Carboniferous strata of this district have been brought to view only by 

 the Plateau and Uinta Uplifts, and by the two sharp Upthrusts of Junc- 

 tion and Yampa Mountains. The remarkable peculiarities of these 

 uplifts and upthrusts will be explained in a subsequent chapter. 



THE MESOZOIC AGE. 



Strata of Mesozoic age cover a large part of the surface of this dis- 

 trict, and apparently form an unbroken series, from the base of those 

 strata that have usually been referred to the Triassic period, to the up- 

 permost Cretaceous strata, inclusive. The strata that I have, but with 

 some doubt, referred to the Triassic period, are everywhere in this dis- 

 trict and the surrounding region conformable upon those of undoubted 

 ■Carboniferous age. Therefore there appears to, be no important physi- 

 cal break in the series, as found in this district, from the base of the 

 Carboniferous to the top of the Cretaceous,* notwithstanding the fact 

 that the Lower Cretaceous of Europe seems not to be represented in 

 this, if in any, part of North America. Comparatively few fossils were 

 collected from any of the Mesozoic strata of this district during its 

 geological examination, but the facility for observation, as before ex- 

 plained, makes their identification unquestionable, even if no fossils at 

 all were obtained from them. 



TRIASSIC PERIOD. 



The strata, widely distributed over a large part of the western portion 

 ■of North America, that have until lately been referred with little or no 

 •doubt to the Triassic period, have generally been simply so referred with- 

 out a distinctive name as to groups or formations, but some geologists 

 have subdivided the series and given distinctive names to thedivisions 

 thus formed. They have been called by some simply the Eed-Beds ; 

 but Professor Powell recognized three separate divisions of the group 

 under the names, in the ascending order, of " Shinarump," " Yermilion 

 €liff," and " White Cliff" groups. These three divisions are recognized 

 among those strata in this district, but they are not thought to be there 

 of sufficient importance to be regarded as separate groups, coordinate 



* See also remarks concerning 'apparent unbroken continuity of deposition of Meso- 

 zoic and Cenozoic strata on a subsquent page. 



