PILOT MESA 



331 



occurs. As soon as one looks at the slides one is impressed with the 

 basaltic character of the rock, even though this impression would never 

 be given by the hand specimens. The latter are light cream to drab 

 color and of pronounced platy structure. They are usually streaked 

 with alteration products. Their specific gravity, greater by .15 to .18, as 

 compared with that of the leucite rocks, is marked when one lifts them. 

 The slides show abundant dark grains of perofskite, besides which diop- 

 side is the principal mineral. Phlogopite is, however, also present as is 

 interstitial glass. Leucites are doubtful, but we believe that small, im- 

 perfect ones occur. The rock is closely related to the ordinary augitites. 

 It is illustrated in plate 46, figures 1 and 2. 



Tabulation and Summary of Exposures 



Name. 



Character. 



Underlying 

 beds. 



Altitude. 



Number 

 of cones. 



1. Zirkel mesa 



2. Emmons mesa 



3. Osborn mesa 



4. Cross mesa 



5. Hatcher mesa 



6. North Pilot mesa . . 



7. North Table 



8. South Table 



9. Wortman dike 



Surface flows 



Surface flow 



Surface flow 



Surface flow 



Surface flow 



Surface flow 



Surface flow? 



Intruded sheet 



Dike. 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



G reen River ? . . 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie 



Laramie... . 



Laramie 



Bridger 



Laramie 



Laramie 



Laramie 



Bridger 



7,825 



7,700? 



8,200? 



7,850 



7,640 



7,650 



7,515 



7,350 



7,350 



7,000? 



7,000? 



7,000? 



7,000? 



7,250 



7,500? 



7,700 



7,500 



7,750 



7,200 



6,950 



6,700 



7,650 



5 

 1 

 

 1 

 

 

 

 

 



10. Iddings butte 



11. Iddings dike 



12. Weed butte 



13. Hallock butte 



14. EndlichhiU 



15. Hague hill 



Volcanic neck 



Dike.. 



Volcanic neck . ... 



Volcanic neck 



Residual sheet 



Residual sheet 



Surface flows 



Surface flow 



Surface flows 



Volcanic neck .... 



Volcanic neck 



Volcanic neck 



Surface flow 





 

 

 

 

 



16. Orenda mesa 



17. Black Rock mesa. . . 



18. Steamboat mesa. . . 



19. Badgers teeth 



20. Boars tusk 



21. Matthew hill..... 



22. Pilot mesa 



3 

 

 3 

 

 

 

 







From this tabulation it appears that the Steamboat and Pilot lavas 

 rest on the Bridger beds, and, as the eruptives are surface flows, they 

 must have been poured out during or after the Bridger epoch of the 

 Eocene. Now eight others of the exposures are cellular surface flows. 

 Their very open, amygdaloidal texture makes it impossible to consider 

 them anything else. 



They have, moreover, cinder cones of volcanic tuff and breccia, and, 

 although resting on Laramie strata in many instances, tliey never can 



