bxduch.] SOUTHERN AREA EEUPTIVES. 133 



occur near the base. Stratification is not very apparent in it. At some 

 points, where the granites of the lower range form gaps, the white, chalk- 

 like bluffs extend southward for a short distance. So far as could be 

 determined, they are unconformable with the Green River series, whicli 

 they join along the Little Sandy Creek. At some points the imperfectly 

 defined strata of this Pliocene series show a dip of 2° and 3° to the 

 northeast. About 5 miles east of the Little Sandy it ends, very near 

 South Pass. It seems as if portions of it had formerly extended farther 

 east and south, but had been removed by erosion. No fossils were found, 

 but some specimens of a small Pupa. At no place do we find a favora- 

 ble section of the group, and an estimate of its thickness is therefore 

 necessarily not very reliable. I regard it as about 300 feet. 



Wyoming conglom crate. 



Wherever we find Tertiary strata which have been but little disturbed 

 by subsequent erosion, we observe extensive deposits of this loose, irreg- 

 ular conglomerate. Along the southern slope of the Sweetwater Mount- 

 ains it is very abundant, totally obscuring the junction lines of the 

 different groups. On the summit of Station 21 it is found as a small 

 remnant, about 12 feet in thickness, forming the very highest point. 

 Indians had here built up a small "lookout" with the bowlders. ISTow 

 this is disturbed, as our monument occupies its place. Upon the sum- 

 mits of some of the higher Tertiary hdls this conglomerate affords the 

 only material for such purposes, as the strata weather into small frag- 

 ments, leaving no bowlders of sufficient size. Table Hills and Steam- 

 boat Buttes, a little farther west, are covered with the same material. 

 At these places some of the metamorphic bowlders are so large that 

 they may have been and probably were transported by ice. Within the 

 Pliocene valley west of South Pass the same conglomerate occurs in 

 abundance. It is scattered to a greater or less extent all over the coun- 

 try which has been subjected to extensive erosion. Having been removed 

 from the regions where it was first deposited, it was dropped in the di- 

 rection taken by the waters which so greatly modified the surface of the 

 country. Although our observations are somewhat imperfect on account 

 of the erosion following the regular deposition of Tertiary groups, I am 

 inclined to regard the maximum accumulation of this conglomerate as 

 occurring along the shores of the former Tertiary lakes. 



ERUPTIVES. 



Mention has been made of the eruptives which form a series of hills 

 and bluffs north and northeast of Salt Wells. Mount Essex is the prin- 

 cipal one of these. A high bluff is formed by volcanic material which 

 has been ejected through the strata of the Wasatch Group. Sloping 

 gently to the northward, we find that the summit is essentially a small 

 plateau, containing one prominent, conical point. To the south and east 

 the mountain presents vertical, rocky faces, which surmount a steep, fur- 

 row ed slope. The rock is dark gray to black, more or less vesicular, and 

 contains small crystals of augite and biotite. Immediately south of 

 Essex Mountain is a table-topped hill, which on some maps is marked 

 as " Black Butte.'' Here the lava has escaped through a fissure trend- 

 ing about north 45° west, and has covered the Wasatch beds with about 

 400 feet of black rock. It is somewhat columnar, and presents a nearly 

 vertical face on all sides. West of Essex Mountain, on the drainage of 

 Packer's Creek, is an isolated needle of basalt, known as Pock Point. 



