122 REPORT UNITED STATES GEOLOGICAL SURVEY. 



trict. It is true that their present prominent position is due, in a meas- 

 ure, to the same disturbances mentioned above, but they occupy an 

 extensive area and reach considerable vertical dimensions. Two dis- 

 tricts may be cited as exhibiting groups belonging to the formation. 

 The first is the region north and east of Rawlings Peak ; the second, 

 the vicinity of Salt Wells Station on the Union Pacific Railroad.- It 

 will be seen that the termination of an anticlinal upheaval, or an " oval 

 quaquaversal," has produced the outcrops at this latter locality. From 

 what has been observed within our district and regions adjoining, both 

 east and west, we can conclude that the entire Cretaceous formation 

 underlies all the younger sedimentary formations, which we shall have 

 occasion to discuss in subjoined pages. It is thoroughly well developed 

 throughout, and wherever it aj>pears shows a singular persistence of 

 typical features within each group. 



Dakota Group. 



Overlying the strata of Jurassic age we find the conglomeritic sand- 

 stones of the Dakota Group. Succeeding are shales, yellow, brown, and 

 gray. The lighter sandstones occurring near the top form a prominent 

 outcrop. It can be traced continuously, dipping conformably with older 

 formations, in three directions. South of Whiskey Gap, the Dakota 

 sandstones disappear, again to be exposed within the gap and along the 

 southwestern and southern slope of the Seminole Hills. No percepti- 

 ble variation from the thickness as found in the gap can be noticed. 

 Although the Cretaceous groups certainly show a decided amplification 

 toward the south, this is not sufficiently great at this locality to appear 

 prominently. The usual seams of carbonaceous material were found in 

 the lower shales of the group. No fossils were collected, as the out- 

 cropping strata were very much weathered, and but few badly preserved 

 plant-remains were seen. 



Colorado Group. 



In Section XIV the northern exposure of Colorado shales is given. It 

 is there produced, at the southwestern end of Whiskey Gap, by the effect 

 of the north to south anticlinal upheaval. Following the shales in a direc- 

 tion a little east of south, we remain upon their outcrop as produced by 

 the uplift. They show a steady diminishing dip to the eastward. To- 

 gether with the Dakota Group they form a prominent ridge, trending 

 almost parallel with the longitudinal anticlinal axis, swerving a little to 

 the eastward, however. The shales are very finely laminated, arenace- 

 ous in part, and show interstrata of hard calcareous beds or of sand- 

 stones. An admixture of ferric oxygen compounds imparts to them a 

 rusty brown color at a few places, but usually they are dark gray, nearly 

 black. Numerous joints and fissures traverse the shales, admitting 

 moisture, and thus giving rise to the only spring of the region, Brown's 

 Spring. In the upper portion of the shales several beds of sandstone 

 set in, forming a sharp cap near the summit of the ridge. It seems that 

 the Colorado shales of this locality are harder than elsewhere, and in 

 many instances resist erosion more effectually than is generally the case. 

 We observe, in connection with this series of outcrops, a feature that can 

 be regarded as characteristic of the group. This consists in the rapid 

 increase of dip upon reaching the upper members of the shales. In this 

 instance the inclination eastward amounts to about 8°, but near the end 

 of the Colorado shales it is increased to about 14°. Whether this be due 



