CROSS MESA 317 



Osborn, of the American Museum of Natural Histor}^, in recognition of 

 his researches among the fossil beds of W3'oming. 



Cross Mesa 



This is the most western one of the principal flows, but the Badgers 

 teeth, the Boars tusk, and Pilot butte lie still farther in this direction. 

 Cross mesa is a lofty and commanding fragment in the shape of a dum- 

 bell about 2 miles from northwest to southeast and a half mile wide at 

 either end and less in the middle. From the valle}^ to the northwest it 

 rises 400 feet over a rather steep slope of Laramie, which is covered with 

 a scattered talus of lava. The precipitous escarpment about 40 feet in 

 height then ascends abruptly, but is broken by re-entrant gulches up 

 which one may climb. Great joints have separated blocks from the 

 main mass, and these, after splitting awa}^ 10 feet or so, remain standing 

 so that clefts and dens are afforded behind them, which are utilized by 

 various rodents. 



From its extreme western point the mesa runs eastward for about a 

 mile and a half to a point where an apparent cone some 200 feet high 

 arises. When observed closely it is found not to be a true cone, but the 

 semicircular half of an old ring, with its concavity to the south. There 

 is reason to think that the ring was once complete, but that the southern 

 half has been entirely removed by erosion. The convexity of the curve 

 is its high point; from this it slopes down to the horns of the lunette, 

 which are 200 yards apart. It is built up of fragments of pumice. The 

 angle of the outside slope was 23 degrees and of the inside 11. The 

 rock of the flow is a cellular lava, but it has granitoid inclusions of 

 coarse feldspathic rock. There was evidence of only one flow, so far as 

 noted by us, the top being very flat and regular. Only one flow ap- 

 peared in the escarpment. 



The chief rock of this mesa is orendite, Avith leucite, sanidine, diopside, 

 hornblende, phlogopite, and rutile. Near the cone and at its west foot 

 we also gathered wyomingite, or at least orendite very poor in sanidine. 

 Along the southern escarpment we found an included boulder of some 

 coarsely crystalline feldspathic rock, now" greatly altered. Under the 

 microscope it exhibits untwinned feldspar, greatl}^ strained and filled 

 with small inclusions apparently produced by the influence of the 

 lava, as in contact metamorphism. The boulder probabh^ came from 

 the ancient crj^stallines, on which the sedimentar}" rocks of this region 

 rest. 



We name this, which is one of the most striking of the group, after 

 Dr Whitman Cross, of the United States Geological Survey, to whose 



