108 REPORT UNITED STATES GEOLOGICAL SURVEY. 



changing a little in relative thicknesses, however. We first met with 

 Jurassic limestones in the western portion of the Sweetwater Hills. 

 After hnt a short line of exposure, during' which they remain conform- 

 able with the ivnderlying red beds, they are hidden by younger deposits. 

 Again they appear east of Elkkorn Gap, taking part in the folding which 

 is represented by Section XII. Innumerable fossils were found at this 

 locality, limited unfortunately to Belemnites, Gamptonectes, Pecten, and 

 Gryplma. Several specimens of the first-named genus were collected 

 which closely resemble B. brevis of the European Jura. At the east- 

 ern end of the Seminole Hills, Jurassic beds occupy a prominent posi- 

 tion. Besisting erosion more effectually than either the older Triassic 

 or younger CretaceOus rocks, they form a conspicuous line of sharp, 

 narrow ridges. In their course, these trend first toward, the east, then 

 turn south and flank the termination of the hills. Passing over to the 

 southern slope, they are soon lost under the accumulation of younger 

 formations. In the vicinity of Whiskey Gap an increase of the total 

 thickness may be observed over that seen farther northwest. The entire 

 series of beds here measures about 200 to 300 feet. 



Judging from the character of the strata composing the formation, 

 there is no doubt but that the Jurassic deposits of our district at one 

 time formed a connected sheet. Now they are broken, through the 

 agency of stratigraphical disturbances, and are exposed only at favor- 

 able localities. We may assume, however, that unless removed by early 

 erosion, the same beds extend through under the extensive series of 

 Tertiary deposits which have accumulated in the region of the Sweet- 

 water Eiver. 



CRETACEOUS. 



Dakota Group. 



Cretaceous groups are represented very sparingly in the region of the 

 Sweetwater. We find the lower sandstones of the Dakota Group pro- 

 truding from under Tertiary deposits at the northern portion of the Sweet- 

 water Hills. The characteristic yellow and white color, as well as the 

 features of weathering, permit it readily to be recognized. East of Elk- 

 horn Gap we again find it, very much folded by the plications occurring 

 there. Section XII illustrates the position of the group. At one point 

 we find it directly doubled, forming a synclinal fold, the horizontal ex- 

 tent of which is equal to zero. The two sides are closely joined in such 

 a way as to change the character of its usual appearance. 



In Whiskey Gap the members of this group occupy a very interesting 

 position. Curving in the same direction and manner as the older forma- 

 tions heretofore described, the strata curve around the western base of 

 the Seminole Hills with a partiversal dip. Erom indications elsewhere 

 obtained we find that a short distance farther west they take part in an 

 anticlinal upheaval. In order to make clear the stratigraphical condi- 

 tions within Whiskey Gap, I have prepared a section cutting across it 

 from east to west and one from north to south. 



Section XIII commences at its eastern end, near the summit of the 

 Seminole Hills, and extends across the gap to the eastern termination of 

 the Sweetwater Hills. Beginning with the Palseozoic series, we find that 

 the various formations along this hue show a westerly dip. Eesting 

 upon metamorjjhic granite (a) we have the Silurian beds (5), which, in 

 turn, are covered by Subcarboniferous (c). Above this occur the mas- 

 sive blue limestones (d), which, farther east occupy the highest portions 



