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174 J. F. KEMP — THE LEUCITE HILLS OF WYOMING. 



tion, because, so far as observed, float pieces are restricted to a zone, but 

 one or two hundred yards broad, around the mesa itself. The Laramie 

 sandstones on which the lava rests strike nearly east and west and dip 

 from 10 to 15 degrees north. They are cream-colored sandstone with 

 interbedded shales. 



Additional Notes. 



The leucite-rock forms several different and separated areas. The 

 southerly one is much the largest, and extends about 10 miles, with its 

 long axis running a little north of west. Its outline is irregular, so that 

 it may be two miles wide in places and less than one mile in others. Its 

 area is 10 or 12 square miles. About three miles to the north of its east- 

 erly end is Orenda butte, another flow of the lava, but of smaller area. 

 It extends two or three miles in greatest length and varies from a maxi- 

 mum of two miles in width. Five miles northeast from Orenda butte is 

 Black Rock butte, much smaller still. It is rectangular in outline, with 

 reentrant coves on the northwest and northeast, and is perhaps a quar- 

 ter of a mile on a side. The sides are precipitous and the top is almost 

 inaccessible. 



Some 10 miles west of the west end of the Leucite hills proper, mean- 

 ing by this the large south flow, are two small buttes in the valley of 

 Killpacker creek, called the Boar's tusks, from which no material was 

 obtained. Some 15 miles northwest of them and on the plateau west of 

 Killpacker creek is Pilot butte, a small area of igneous rock, of which 

 more will be said later on. It is about 1,500 feet from north to south 

 and from 800 to 900 feet from east to west. North again from the Boar's 

 tusks, a distance of about 10 miles, is North Pilot butte, presumably 

 igneous, but from which no specimens were obtained. 



Geological Characters. 



Each of these buttes or mesas consists of a more or less dissected lava 

 sheet, or aggregate of sheets, which presents an abrupt wall from 50 to 

 150 feet high, dependent on locality, and which is in the smaller buttes 

 only accessible at one or two points. From the upper surface of Orenda 

 butte and of the large south mesa arise cones of varying sizes, the apex 

 of the largest being about 300 feet above the general surface. The others 

 are smaller, some being quite low. They give the impression of craters 

 from a distance, but all are solid, and they were evidently produced 

 in the later stages of eruption by the welling up of a viscous lava, 



