NORTH TABLE AND SOUTH TABLE SZl 



tlie only otlier minerals. In many cases the phlogopite is thickly set 

 with included leucites. Figure 1 of plate 42 is a photomicrograph from 

 a specimen taken from this locality. Should any visitor to the hills 

 desire to collect wyomingite, this is the best of the mesas for the purpose. 

 The rock occurs both as a dark, gray platy variety and as a light greenish, 

 yellow one. The rock resembles the dikes much more strongly than it 

 does the surface flows. 



Remarks on North Pn.OT Mesa, North Table, and South Table 



These three mesas, with nearly flat tops, occurring so near each other 

 deserve some attention as regards their origin. North Pilot rests un- 

 questionably on Tertiary rocks, and the two associated mesas rest on 

 Laramie. At first one would be inclined to consider that they had been 

 originally one large mesa, and that faulting had separated them. On 

 careful examination, however, there was not the slightest evidence of 

 faulting found about them. The talus from the three mesas would 

 naturally meet in the gulch separating them, but there has not been time 

 enough since their elevation to produce it in sufficient amount to reach 

 the valle3^ Again, if they were due to faulting, there would naturally be 

 some difference in the inclination of the mesas, when in reality there 

 is very little. The only explanation to be offered for the origin of the 

 latter two mesas is that they are intruded sheets. Each one must then 

 have found a soft stratum which was capped by a hard one of some 

 thickness. The lava must have been forced laterally as an intruded 

 sheet, which was subsequently laid bare by erosion. That the latter 

 view is true for South table there is little question. If North table is 

 an intrusive, we must admit that the cover was slight, because of the 

 amygdaloidal character of the rock. On the other hand, if North table 

 is a surface flow, it must have been a later outbreak than North Pilot or 

 else the topography must have been, at the time of the outbreak, much 

 the same as now. 



Volcanic Necks and Dikes Southeast of North Pilot Mesa 



worth an dike 



Southeast of North Pilot a dike bears away to the southeast toward 

 the three volcanic necks to be next described. We name it after Dr J. L. 

 Wortman, now of the Peabody Museum, Yale University, in recognition 

 of his work in Wyoming paleontology. 



The rock of the dike is wyomingite. Phlogopite is the most promi- 

 nent mineral present and forms phenocrysts in a dense groundmass, 



XLVI— Bull. Geol. Soc. Am.. Vol. 14. 1902 



