EXDLicn.l SWEETWATER DRAIXAGE COLORADO GROUP. 109 



of tlic hills ; they are succeeded by Permian (e) and Triassic strata (/). 

 Jurassic beds (//) form a prominent, sharp ridge directly underlying the 

 Dakota Group (/<). This latter dips at an angle of very nearly 45°. 

 Yellow and white sandstones, gray and brown shales, and beds of argil- 

 laceous sandstone compose the series at this locality. Its thickness is 

 about 700 feet. Overlying the Dakota we find Colorado shales (i), form- 

 ing low blufts which trend parallel to the others. Fox Hills {Jc) is the 

 youngest group cut by this section. We observe that a short synclinal 

 iV)ld is very api)arent here, followed immediately by an anticlinal (c). 

 From subsequent examinations we found that at this point we have the 

 northern termination of a great anticlinal fold running north and south 

 and ending near llawlings Springs. In Chapter IV this will be more 

 fully discussed. On the western side of the anticlinal the Fox Hills beds 

 agahi dip eastward and disappear under a large mass of drift (m), which 

 covers the end of the Sweetwater Hills. Exactly in what relation the 

 sedimentary beds stand to the granite (/) o^ these hills could not be de- 

 termined, as none of the exposures aflbrded any evidence upon this point. 

 In the section I have indicated the various groups as foUowiug in a di- 

 minishing ratio the flexures observed on the surface. Farther south, 

 we have evidence which justifies the representation of such an arrange- 

 ment. Their terminations to the westward are necessarily left doubtful. 



Section XIV runs at approximately right angles to the preceding one 

 within the gap. At its southern end we see the last sloi)o of the anti- 

 clinal {€), after which the strata resume a. southerly dip. Going north- 

 ward we cross one formation after the other in regular succession. The 

 first prominent ridge is formed by Jurassic beds (ry), and the second by 

 Carboniferous limestone {d). North of the latter the Silurian quartzites 

 appear in a naiTow band. Ko granite reaches the surface along the line 

 of this section. As it appears, however, a few miles farther east, it is 

 introduced in the section (a) as covered partly by the quartzites (b) and 

 partly by the Tertiary Sweetwater beds (S). 



On first sight the arrangement of strata in the immediate vicinity of 

 Whiskey Gap is very puzzling, but as soon as the existence of the anti- 

 clinal fold {€) is recognized the entire system becomes very clear. 



Colorado Group. 



But two localities are to be recorded where shales of the Colorado 

 Group appear within the region. The first is near Elkhorn Gap, the 

 second in Whiskey Gap. At the latter place the shales are dark gray, 

 finely laminated, and contain numerous fragments of Inoceramus and 

 O^trea. Their tliickness is about G50 to 700 feet. Withhi the gap they 

 mostly occujjy depressions, running parallel to the strike of the strata. 

 From Sections XIII and XIV, their position, as compared to that of 

 other groups, may be seen. In the structiu'e of the anticlinal (0) they 

 are of considerable importance, and wiU be treated of at greater length 

 in the succeeding chapter. 



Fox Hills Group. 



The only point where these were observed is in Whiskey Gap. Form- 

 uig sharp, low ridges, they participate in the stratigraphical disturbances 

 there occiuring. Owing to the numerous changes of position to which 

 they were subjected, it is rather difficult to estunate their total thickness. 

 It is near 1,000 feet, and seems to increase toward the south. 



