82 REPOET UNITED STATES GEOLOGICAL SURVEY. 



we find ourselves very nearly on a line wltli the anticlinal axis wMcli is 

 marked by such prominent results farther south. The Carboniferous 

 beds have simk out of sight with the exception of those belonging to the 

 Permian group. Near the junction of Cottonwood with Twin Creek they 

 too disappear. Approaching the Little Popo-Agie we find ourselves once 

 more standing upon Permian strata, while Triassic beds flank Twin Creek 

 on either side. Here, evidently, the force which produced the anticlinal 

 fold asserted itself with great violence. Although the Carboniferous lime- 

 stones do not again appear, the red beds occupy prominent positions on 

 the high bluffs they form near the junction of the two streams. At this 

 locality the amplification of Triassic strata can be well observed. jl!^ear 

 their base thick beds of gypsum set in, measuring sometimes 4 and 5 feet. 

 Gypsum, when stratified, is essentially a local deposit, and we therefore 

 find the beds thinning out rapidly in every direction. At the junction 

 of Twin Creek and the Little Popo-Agie the axis of the anticlinal fold 

 makes a turn to the northeast, and is soon lost entirely. On the Big 

 Popo-Agie it appears again, exposing Triassic beds to view. In subse- 

 quent papers this interesting stratigraphical phenomenon will be dis- 

 cussed more connectedly. 



About two miles below the entrance of Twin Creek into the Little 

 Popo-Agie we find a very fine petroleum spring. It is located on the 

 south side of the last-named stream, issuing from the base of a small 

 bluff there. During the hot season the oil appears to flow for some dis- 

 tance. It has saturated the sand and gravel in the valley, in some in- 

 -stances firmly cementing it. Evidently the spring has been active for a 

 long period of time, as we observe the coagulated oil on the northern 

 :side of the stream. This at the present time is shifting its course in that 

 direction ; consequently the oil at present found there must have been 

 deposited a very long time ago. For some distance the smell of petro- 

 leum indicates the presence of the spring. Its location is convenient 

 .and the oil of sufficiently good quality eventually to prove valuable. 

 So far as my examination extended, it appeared to me that the petro- 

 leum must collect from the various deposits of shale in that region, 

 thus rendering its point of outflow one rather determined by accident 

 than by an extraordinarily great impregnation of the rocks found in its 

 immediate vicinity. 



JURA. 



Generally the strata of variegated shales and marls, associated with 

 some limestones which are found superincumbent upon the red beds, 

 have been regarded as Jurassic. In Colorado and other localities south 

 of the forty-second parallel, fossils have been obtained which render this 

 determination justifiable. Within the ISTorthern Territories Dr. Hay- 

 den and others have found fossils which tend to show that the faunal 

 develoi)ment of the Jura there more closely coincides with that of Europe 

 than farther south. Several species have been collected which belong to 

 European genera and are scarcely specifically separable. 



East of the Wind Eiver Eange we observe a number of exposures 

 belonging to this formation. Eesting directly upon the uppermost red 

 .shales of the Triassic strata, we find a narrow band of dark, calcareous 

 shales. These are covered by several prominent banks of dark-blue and 

 gray limestone. For some distance upward follows a succession of 

 shales and limestones. Isolated interstrata of light yellow or white sand- 

 ,stones ax^pear above this in some yellow marls. Higher up, yellow, pink, 

 greenish, and white marls and shales close the formation. A thickening 

 of the strata may be observed as we approach Sheep Mountain. For 

 the entire Jurassic series, so far as it enters into consideration in this 

 chapter, we may accept a thickness of 200 to 220 feet. 



