236 DESCRIPTIVE GEOLOGY. 



with the underlying beds, and have since been disturbed in regions of pre- 

 existing movement, it is not always possible at the present day to draw a 

 line of definite demarcation between the two formations. 



Volcanic Eocks. — The region of the Green River Basin is singularly 

 poor in volcanic rocks, which, with the exception of the basalt of Fortifica- 

 tion Peak, not designated by a special color on this map, were found in 

 only two localities, namely, the Leucite Hills and Pilot Butte. 



The Leucite Hills consist of a number of little conical peaks protruded 

 through the beds of the Laramie Cretaceous, which form the plateau comitry 

 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-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 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. 



This locality has a singular interest, not only from the fact of its being 

 so far removed from any other occurrence of volcanic rocks, but still more, 

 because it is, thus far, the only known locality of leucite-bearing rocks on 

 the American continent. The leucite rocks are of a light-gray to greenish- 

 gray color, and have a porous feldspathic groundmass, in which the only 

 crystals visible to the naked eye are plates of light-brown mica. The 

 crystals of mica are arranged in almost parallel plates, giving to the rock 

 a slight tendency to schistose structure. 



Under the microscope, the felspathic-looking groundmass is found to be 

 made up entirely of microscopic crystals of leucite, showing their characteristic 

 octagonal sections, which become quite dark between crossed nicols. Within 

 the leucite sections can be distinguished also small pale-green grains of augite 

 and particles of glass with enclosed bubbles. No feldspar at all is found to 

 exist in the rock, neither hornblende nor olivine ; the only other ingredients, 

 besides leucite and mica, being a small quantity of magnetite and a little 



