330 KEMP AND KNIGHT — LEUCITE HILLS OF WYOMING 



fracting mineral, probably a zeolite. Leucite, diopside, and plilogopite 

 make up the fresher portions. 



Pilot Mesa 



This is located on the large plateau of Green River shales, about 12 

 miles west and north of Rock Springs. The plateau has an elevation of 

 about 1,000 feet above the town of Rock Springs, and is bounded on the 

 east and south by very precipitous bluffs. The mesa is situated about 

 3 or 4 miles back from the eastern edge of the plateau and rises to a 

 height of 350 feet above the comparatively level surface. The talus 

 slopes are quite smooth and are in great contrast with the slopes of the 

 other mesas, where they are in part impassable. They are also more 

 gentle, and it is seldom that one finds a fragment of the eruptive cap of 

 any size until the vertical escarpment is reached. The escarpment varies 

 from 25 to nearly 75 feet in height, being lowest on the eastern and 

 highest on the western side. The only place noted where one could 

 reach the top was through a narrow crevice on the eastern side, which 

 leads to a broad gulch, which in turn conducts one westward nearly to 

 the center of the mass. In shape this mesa resembles a gourd with the 

 neck extending to the westward. It is less than a quarter of a mile in 

 length. The greatest diameter is east of the center, and is less than an 

 eighth of a mile. The surface is gently undulating, but gradually rises 

 from east to west. The highest point is very near the edge of the scarj) 

 on the west, and is 7,650 feet above sea-level. From the lowest exposed 

 rock to the top on the eastern face nothing but a yellowish schistose 

 variety of eruptive was noted. This is fine grained, somewhat vesic- 

 ular, and heavily charged with small angular fragments of Green River 

 shale. The mass of the mesa is schistose, and the schistosity is par- 

 allel to the bedding planes of the underlying sediments. On the highest 

 point there is apparently a second flow that is less schistose, but 

 more porous and of a grayish green color. Judging from the slight 

 development of large talus blocks and the absence of fissures about the 

 escarpment, this mesa must rest on a very firm base, which is probably 

 a hard band in the Bridger. The fine talus has so obscured everything 

 that there are no exposures of sedimentary rocks about the mesa, so far 

 as could be discovered. In fact, the talus had spread out to a consider- 

 able distance from the base of the mesa. It is quite probable that Pilot 

 and Steamboat rest on the same formation, if not on the same stratum. 



Pilot mesa presents a different rock from any of the other exposures. 

 It is the type locality for the madupite of Cross, and, so far as rocks 

 have yet been studied, is the only place where this interesting rock 



