116 W. Cross — Igneous Hocks in Wyoming. 



of his geological explorations, and collected specimens identi- 

 cal in character with those described by Kemp and in the 

 course of the present paper. For the opportunity of examin- 

 ing Major Powell's specimens I am indebted to Mr. J. S. Dil- 

 ler, in whose hands they had been placed for study. 



The observations communicated in the following pages are 

 based upon a trip to the Leucite Hills made a number of years 

 ago. That their publication has been so long deferred is partly 

 due to the pressure of other work, and largely to a desire — 

 thus far not realized — to revisit the region before publication 

 in order to greatly extend my observations. In view of the 

 unusual interest attaching to the rock types to be described, I 

 feel that an explanation of the inadequate character of my 

 field observations in the Leucite Hills is not out of place. The 

 visit to this locality was made late in the fall of 1884, with an 

 assistant, W. B. Smith, for the express purpose of collecting a 

 large number of specimens of the leucite rock for the ''Educa- 

 tional Series" then being assembled by the Geological Survey. 

 No mineralogical variation in the rocks of the region was 

 indicated by the descriptions of Emmons and Zirkel, and, as 

 none could be detected by the naked eye, such variation was 

 not suspected at the time of my visit. The weather became 

 very stormy soon after our arrival, and snow covered the hills 

 a part of the time. For these reasons my observations do not 

 make plain the field relations of the types collected, whose 

 differences were chiefly evident only after microscopical study. 

 Pilot Butte was visited during one of the terrific sand storms 

 for which this part of Wyoming is noted at certain times of 

 the year, and I was only able to gather specimens of the rock 

 without determining definitely the manner of its occurrence. 



Occurrence of the Rocks of the Leucite Hills. 



Inasmuch as Professor Kemp has so recently presented a 

 general sketch of the Leucite Hills, together with illustrations 

 from photographs and an outline map of the region, I shall in 

 the main content myself in the following pages with some 

 details in regard to the portions of the area visited in which 

 the rocks to be described were found. 



The mesa. — The principal area of leucite-bearing rocks is in 

 a low, irregular mesa of perhaps 15 square miles in extent, 

 bounded by a scarp usually not more than 50 feet in height, 

 representing the surface flow of the leucite rock resting, for 

 the most part, on upper Cretaceous strata. On the north and 

 northeast of this mesa are several isolated hills, flat- topped, 

 with scarps, which were once connected with the principal 

 mesa. Of these outlying hills only two were visited, namely, 

 Orenda Butte and North Table Butte. 



