324 KEMP AND KNIGHT — LEUCITE HILLS OF WYOMING 



Our specimens have been obtained from the southeastern portion of 

 the mesa. Under the microscope all proved to be orendite. They con- 

 tain leucite, sanidine, diopside, phlogopite, and hornblende, and are 

 normal representatives of this rock. 



Black Rock Mesa 



This is the most easterly outlier of the Leucite hills, and is a very 

 sj'mmetrical mesa, which rises 500 feet above the valle}^ at its base. 

 The slopes are talus covered, as are those of all the mesas of this type, 

 and just above them rises the vertical wall of the scarp. This exhibits 

 from a distance polygonal columns suggestive of the characteristic 

 basaltic structure; but on near examination the resemblance is less 

 close, although the columns are obviously due to parallel, vertical joints 

 (see figure 2, plate 43). The scarp is upwards of 50 feet high in places, 

 and the only possible opening through which one can reach the crest is 

 found on the northwestern slope, where by hard and difficult climbing 

 one can gain the upper surface. This mesa, like several others, appears 

 to be nearly level on top ; but in realit}^ there is a broad depression 

 through the center on an east-and-west line, which causes the north- 

 and-south sides of the scarp to rise considerably above the average level. 

 The leucite rock varies greatly in texture. The cap of the mesa is a 

 fine grained rock, shading from a gray to a reddish color, and weather- 

 ing ver}^ slowly. Near the base of the scarp on the north side the rock 

 is soft, porous, and in many places resembles a tuff, which, when break- 

 ing up, forms roundish talus blocks very different from those composed 

 of the harder rock. This mesa is about an eighth of a mile in diameter, 

 and has an approximate elevation of 7,600 feet. It rests on the Lara 

 mie Cretaceous, and has a diameter at its base of about a half mile. 

 The name Black rock has long been locall}^ used for the mesa. 



The fine grained variety of rock is chiefly wyomingite, but in one slide 

 of the cellular portion a little sanidine also appeared. Leucite is the 

 chief feldspathic mineral, and with it diopside, phlogopite, subordinate 

 hornblende, and rutile all occur. Olivine aj)pears as an inclusion in 

 the phlogopite. In one slide is a pale green crystal of high relief and 

 almost if not quite isotropic as cut. It strongly resembles a spinel, and 

 fails to give a recognizable interference figure. More thin-sections would 

 probably show orendite also in this mesa. 



Steamboat Mksa 



In many respects this is the most im})ressive of all of the mesas. As 

 early as 1875 F. M. Endlich, of the Hayden Survey, visited it and named 



