PREVIOUS PAPERS 307 



mens subsequently studied and described by F. Zirkel.* Emmons de- 

 scribes the Hills as 



" a number of little conical peaks protruded through the beds of the Laramie Cre- 

 taceous. . . . The form of some of these hills seems to indicate the outline of 

 a former large crater, while to the north the lavas are spread out horizontally, 

 capping the hills, and extend beyond the limits of our map, apparently forming 

 the summit of North Pilot butte. Although no well defined Tertiary beds were 

 found in actual contact with these eruptive rocks, it is evident from their position 

 directly over upturned Cretaceous sandstones and adjoining Green River beds, 

 where the underlying unconformable Vermilion Creek series is not seen, that they 

 have been poured out, not only since the deposition of the latter Tertiaries, but 

 since their partial removal by erosion." 



On page 238, speaking of Pilot butte, Emmons states : 



" It is evident that the soft Green River Tertiaries which once surrounded and 

 covered it must have been eroded away in a similar manner to those around Forti- 

 fication peak." 



F. M. Endlich,t of the Hayden Survey, visited the more northern mesas, 

 especially the one now called Steamboat, but named by him Essex moun- 

 tain. It is mapped, however, far larger than it is. Endlich also visited 

 what we call the Boars tusk, which he called Rock point, and which on the 

 map of the Hayden Survey is called " Sentinel." Endlich refers to the 

 rocks of some of the mesas as basalt and says they have olivine. Others 

 he describes as grayish brown and showing onl}' biotite. Zirkel's deter- 

 minations and descriptions of the rocks from the southern mesas were 

 available when Endlich wrote and are referred to by him. From a pass- 

 ing remark made in the paper of Doctor Cross, to be presently cited, it 

 appears that Major J. W. Powell also visited the hills, but we find no 

 published description from his pen. In December, 1896, one of us J read 

 before this Society a brief description, parti 3^ of their structural and 

 partly of their petrographic characters. The paper was based on observa- 

 tions made both by the author and two assistants, hut it left much yet 

 to be done. It contained a small map, Avhich was based on the state 

 map issued by the United States Land Office. The maj) is very sketchy 

 and defective, although somewhat less so than previous ones. Shortly 

 thereafter Dr Whitman Cross § published a very careful descri])tion of 

 the rocks, both from microscopic stud}^ and chemical analyses. Many 

 important points were established regarding their systematic classifica- 



*F. Zirkel : Geol. Survey of the Fortieth Parallel, vol. vi, p. 260. 



t Eleventh Annual Report of the Survey of the Territories for 1877, pp. 5, 9, 10, 119, and 132 ; see 

 also map 8 in the atlas of the xii annual report. 



X J. F. Kemp : The Leucite hills of Wyoming. Bull. Geol. Soc. Am., vol. 8, p. 169. 



§ Whitman Cross: Igneous rocks of the Leucite hills and Pilot butte, 'Wyoming. American 

 Journal of Science, August, 1897, p. 115, 



