CHAPTER IV. 



GEOLOGY OF THE DEPRESSED SOUTHEEF ABEA. 



GENERAL STRUCTURE. 



This entire area comprises more than one-third of our district, ex- 

 tending southward from the Wind Eiver Mountains, the Sweetwater 

 and Seminole Hills, to the southern limits of our work. It is essentially 

 a low bluff country, formed by gentle undulations of the groups compos- 

 ing it. Beginning in the western end, we find bluff succeeding bluff 

 until, in the vicinity of Mount Essex, the general character is slightly 

 changed by the appearance of some eruptive rocks. From there east- 

 ward we pass through the low depression which we have called the 

 Shoshone Basin. Farther on we cross the anticlinal fold running from 

 Whiskey Gap to Eawlings Springs. Beyond that, to the eastern, limits 

 of our district, the strata dip eastward, forming bluffs with steep western 

 slopes and gentle inclines in the opposite direction. Combined with the 

 east and west flexure of the strata, we observe a steady though slight 

 dip to the southward, indicating a continuation of that observed farther 

 north in the beds of the Sweetwater Group. - 



Within this entire area we meet with mainly the younger sedimentary 

 formations. Along the anticlinal upheaval above alluded to we find 

 exposures of older groups, but they are confined to a restricted area. 

 The Tertiary formation is well represented, and to its consideration will 

 be devoted the larger portion of this chapter. 



METAMORPHICS. 



But one outcrop of metamorphic rocks in situ was observed in this 

 area. It occurs on and near Eawlings Peak, along the line of the anti- 

 clinal uplift. Forming the central mass of this hill, it extends for a 

 short distance to the southward, appearing exposed in rugged masses. 

 On all sides it is soon covered by unchanged sedimentary beds, thus 

 forming the'nucleus of a quaquaversal arrangement of strata. The rock 

 is essentially a granitic one, consisting of oligoclase, orthoclase, gray 

 and white quartz, light-green, partly fibrous hornblende, and a small 

 quantity of mica. It might be termed a stratified or stratoid granitic 

 syenite at some points. Its general color is a muddy green, red in some 

 places, is o doubt this granite extends northward for a considerable dis- 

 tance, but it does not appear on the surface again. So far as can be 

 seen, the structure of this rock conforms to the anticlinical upheaval by 

 which it has been placed in such a position as to appear upon the surface. 

 Overlying the syenite we find a series of sedimentary formations, which . 

 extend for some distance beyond it on three sides. 



SILURIAN. 



Potsdam Group. 



Directly above the metamorphic rocks we observe a series of quartzites i 

 and sandstones, dipping off to the west, north, and east. Towards the s 

 west, the dip is steep at first, amounting to about 45°, but is somewhat i 



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