NORTH PILOT MESA 319 



with which the softer portions of the leucite rocks weather. Ahout the 

 scarp there are numerous fissures, varying from a few inches to several 

 feet in width, which have been made by the gradual giving away of the 

 soft rock on which the flow rests. One can easily trace them in all 

 stages of development from a tiny crack to an opening several feet across, 

 and finally to the stage in which the block has toppled over to join the 

 vast accumulation of large and exceedingly angular fragments below 

 (see plate 40, figure 3, and plate 41, figure 1). The talus has accumu- 

 lated in almost impassable masses about the greater portion of the scarp? 

 and extends down the slopes of the mesa for a distance of over 700 ver- 

 tical feet. The lower portion of the lava in this mesa has a reddish 

 color and is quite porous. Usually it breaks with a conchoidal frac- 

 ture. Along the western face of the scarp there are great zonal develop- 

 ments, in which there are well defined concentric rings that have a 

 diameter of 20 feet or more. The mesa rests on Tertiary rocks, but on 

 account of the obscuring talus it is impossible to say exactly on what 

 formation. From the elevation of the eruptive mass they appear to be 

 the Green River shales. At a point three or four hundred feet below the 

 crest there are typical Laramie rocks exposed along the eastern slope 

 and two workable seams of coal, which dip only a few degrees to the 

 northward. Doctor Cross seems to refer to this mesa in his paper as 

 " North Table butte,""^' while Endlich states that it is marked on some 

 maps as "Black butte."t We think that from the crevice figured on 

 plate 41, figure 1, Doctor Cross obtained his potash niter. 



In some respects the rocks of this mesa are, petrographically, among 

 the most interesting of the hills. Specimens gathered along the north 

 side and at the base of the escarpment are very near madupite — that is, 

 they are without sanidine and with moderate powers show phlogopite, 

 diopside, and apatite in a base which is so fine and dense that it is diffi- 

 cult to make it thin enough for study. With very high powers, how- 

 ever, it reveals minute diopsides and swarms of little leucites about 

 .002 millimeter in diameter. Some glass appears between them, but 

 with such small leucites it is very difficult to make a distinction as be- 

 tween them and the glass. The rock was doubtless developed by the 

 relatively quick chill at the base of the flow. At the east end on the 

 crest wyomingite was collected with some hornblende. Along the north 

 side from the talus blocks typical orendite was secured, the pink varie- 

 ties of which are very rich in the peculiar yellow hornblende. The latter 

 in one of our specimens practically replaces the diopside entirely. In 

 the sections, too, olivine will be found forming the center of phlogopite 



*Amer. Jour. Sci., Aug., 1897, pp. 116, 117. 

 fHayden Survey, 1876, p. 138. 



