LOCATION AND DESCRIPTION. 173 



The wide extent of these richly alkaline magmas along the eastern 

 Rocky mountains is a most interesting phenomenon, and is rendered 

 more so by the presence of phonolites in the Black hills, at Cripple 

 Creek, Colorado, and in the Davis mountains, Texas, showing that erup- 

 tions of related rocks have occurred over a wide area. 



Additional interest has attached to leucite in the past because it was 

 so long regarded as a distinctively pre-Tertiar} 7, mineral and as fortifying 

 the now abandoned view that Tertiary and later volcanics should be 

 separated from earlier ones. Its remarkable mineralogical and crystal- 

 lographical mimetic properties have likewise attracted to it special at- 

 tention. 



Geographical Location of the Leucite Hills. 



The Leucite hills are situated in southwestern Wyoming, about 60 miles 

 north of the Colorado line. They are within the drainage basin of Green 

 river, from 10 to 15 miles north of Bitter creek, one of the eastern tribu- 

 taries of Green river and the one which is followed by the Union Pacific 

 railroad. The nearest station is Point of Rocks, from which a wagon road 

 leads north past the hills to several mining camps in the Wind River 

 mountains. The country is arid, with, however, several springs at in- 

 tervals of 10 miles or less, and is on the western rim of the so-called 

 Red desert. The accompanying sketch-map affords an idea of the loca- 

 tion. 



Previous Description of the Leucite Hills by S. F. Emmons. 



Mr S. F. Emmons describes the Leucite hills as follows : * 



"The Leucite hills consist of a number of little conical peaks protruded through 

 the beds of the Laramie Cretaceous, which form the plateau country to the north 

 of the railroad, near the Point of Rocks station. 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 de- 

 fined Tertiary beds were found in actual contact with these eruptive rocks, it is 

 evident, from their position directly over upturned Cretaceous sandstones and ad- 

 joining Green River beds, where the underlying, unconformable Vermillion 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." 



The observations of the writer corroborate this view of their late erup- 



* Vol. ii of the Fortieth Parallel Survey, p. 236. 



