bsmich.] EAST OF WIND EIVER EANGE — JURA. 83 



In the lower, heavy hanks of dark limestone as well as in the higher 

 ones alternating with shale, we find innmnerable fossils. Though the 

 most diligent search was made at a number of localities, we could find 

 but very few species. Dr. Hayden in 1857 obtained a good series in 

 the Black Hills. During 1872, additional collections were made by the 

 members of his survey in the Northern Territories. From these it ap- 

 pears that the conditions requisite for sustaining a more varied fauna 

 were favorable in those regions as compared to the one at present under 

 consideration. We found large numbers of a Pecten ( Gamjptonectes) closely 

 resembling P. glaber of the European lower Lias. Two species of Belem- 

 nites, resembling B. brevis and B. acutus, were associated with the former, 

 and very abundant. Gryphcea, related to G. arcuata, but smaller, occurred 

 sparingly. Comstock, farther north, obtained Lingula, Rhynchonella, and 

 Modiola. Although the fossils found are not what might be expected or 

 hoped for, they are sufficiently characteristic to characterize the age of 

 the formation — if taken in conjunction with other points — beyond doubt. 

 Although we found Jurassic exposures many miles apart, the arrange- 

 ment and mode of occurrence of the fossils did not vary. 



In its surface appearances the Jurassic series closely follows the Eed 

 Beds. "Wherever the latter are exposed, there we find the former. Tak- 

 ing part in the various plications and flexures to which the older series 

 is exposed, we always find the characteristic dark limestones and light- 

 colored marls associated with it. Toward the east a slight increase of 

 thickness may be observed, but so far as the strata were exposed within 

 our district it is unimportant. 



Besides the regions which have heretofore been mentioned as showing 

 outcrops of Mesozoic formations, there is still another within the area of 

 Wind Eiver drainage. This is located along the edge of the Sweetwater 

 Plateau, about north of the western terminus of the Granite Hills. 

 There we find an anticlinal fold which has produced very marked effect. 

 The Palsezoic rocks have been raised to considerable elevation, and now 

 are seen as hills on the plateau. A little north of the edge of the latter 

 the upheaval terminates, and there we find the Mesozoic strata resting 

 on the Carboniferous, in a partiversal arrangement of dip. As this is a 

 steep one, the beds soon disappear under younger ones, and the former 

 continuity of the Cretaceous beds is reestablished. In the Sweetwater 

 Hills we find the southeastern extension of both Trias and Jura, occur- 

 ring there in the same relative positions as directly east of the Wind 

 Eiver Mountains. 



A section (Section H) extending from Sheep Mountain along Beaver 

 Creek down stream as far as our work carried us, will represent the east- 

 ern slope of the great anticlinal fold. The lettering is the same as on 

 Section I, new designations having been added for those groups not 

 above included. We find in this section a decided amplification of the 

 Triassic group (7c). Above the bright-red shales we observe an extensive 

 series of light red, and yellow, and some white sandstones. While the 

 lower mass remains tolerably constant in its vertical dimension, this addi- 

 tion increases the total thickness about 300 to 4Q0 feet. Above the varie- 

 gated sandstones and shales of this upper group we find the Jurassic 

 limestones and marls (*) directly overlying. From the section will be 

 seen the gradual decrease of dip as we proceed northeastward. This 

 line of sections carries us beyond a point where the effect of the upheaval 

 near the Sweetwater Plateau could be felt. Besting on the Jurassic beds 

 we observe the three groups of the Cretaceous formation. They are con- 

 formable throughout with older rocks, and tend to assume horizontal 

 stratification as they pass into the low country nearer the main drainage. 



