334 KEMP AND KNIGHT — LEUCITE HILf.S OF WYOMING 



blisters or swellings of solid lava. Nearly all are of the former charac- 

 ter, but cone number 5 on Zirkel mesa and the three on Steamboat 

 mesa are of the latter type. We are inclined to believe that the latter 

 are protuberances which mark the places where the more or less viscous 

 eruptive welled out to the surface and from which it si)read in the flows. 

 There seems no other reasonable way in which to account for so great a 

 local thickening. Aside from the lava cones there appears little evidence 

 as to the channels through which the molten rock reached the surface, 

 unless it was so fluid that it spread in all directions from the sui)})ly 

 fissure without leaving an elevation to mark the location of the latter. 



The cinder cones in instances are markedl}^ semicircular, and impress 

 the observer as being the uneroded arcs of formerly complete circles or 

 rings. They rise abruptly from the fairly level mesa and were built up 

 by explosive outbreaks which yielded fragmental products. The cone 

 on Cross mesa, for instance, is nearh^ or quite a semicircle, and others 

 on Zirkel mesa show the same character. Again, we ma^^ have merely 

 a conical heap of fragmental debris. In the latter case, and according 

 to the hypothesis of the original ring like cone, we are forced to infer 

 more complete erosion than in the case of the semicircular cones, but 

 we have speculated somewhat while studj'ing over these phenomena us 

 to the possibilit}^ of the original production of a predominant deposit 

 of ejectamenta on one side of a vent either from inclined conduits or from 

 prevailing winds. If so, it would not be necessary to assume the entire 

 removal of one side of the ring. 



The interior area of the semicircles is so buried in fragmental matter 

 as to be in all cases fully concealed. We can not but believe, however, 

 'that under them are volcanic necks like the Boars tusk and the Badgers 

 teeth. The latter may once have been capped by surface flows and cones. 



Nature of the eruptive Mesas 



The eruptive rock of the mesas is in all cases, except perhaps the two 

 tables, clearly of the nature of surface flows. It is strongly amygdaloidal 

 with the cavities drawn out in the direction of the flow^ From even the 

 central portions of a flow it is almost impossible to secure solid speci- 

 mens; nevertheless it is true that cellular streaks alternate in the faces 

 of the escarpments with more compact ones. 



The tabular crystals of phlogopite which are universally present in all 

 the exposures are in the surface flows ranged in parallel and flat align- 

 ment, and when abundant give the rock the appearance of a mica schist. 

 In fact, its resemblance to a mica schist is at times extraordinarily close, 



