70 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



thick, of dense aphanitic gray and red rock (100), which might be mistaken 

 at first glance for a fine-grained sandstone. It is at about the same horizon 

 as the sheet at Bighorn Pass, and proves to be of similar rock. Its 

 resemblance to this rock was not recognized in the field, and though associ- 

 ated with dikes of dacite-porphyry its relation to them was not noted. 



Under the microscope the rock from Bighorn Pass is seen to consist of 

 a holocrystalline gronndmass of feldspar, mostly plagioelase, with quartz 

 and some orthoclase, and larger crystals of angite, biotite, and occasional 

 hornblende, with abundant magnetite, besides chlorite and calcite. It is 

 not fresh, the angite and hornblende being partly decomposed. Its chemical 

 composition is as follows: 



Analysis of Tcersantite from Bighorn Pass. 



[Analyst, J. E. Whitfield.] 



Constituent. 



1. 



2. 



3. 



SiO., 



48.73 



1.34 



11. 92 



4.79 



4.56 



.36 



5.93 



9.24 



None. 



Trace. 



Trace. 



2.62 



2.47 



.32 



.34 



.11 



5.80 



1.52 



47.73 



49.82 



TiO a 



AWOa 



10.07 

 7.39 

 4.29 

 .23 

 7.66 

 6.97 



14. 50 

 8.06 



FeO; 



FeO 



MnO 





MgO 



CaO 



5.81 

 7.69 



SrO 



BaO 







Li-0 







to 



3.78 

 1.22 



3.03 



3.50 

 Trace. 

 Trace. 



P.,0, 



S0 3 





CI 





CO 



5.88 

 4.46 



4.42 

 2.54 



HO 



Leas O for CI 



100. 05 

 .02 



99.68 



99. 37 





100. 03 







l = Kersantite, Bighorn Pass. 



2=Minette, Eichelberg, Heidelberg. 1 



3=Kersantite, between Falkenstein and Steiubach Miihle, Fichtelgebirge. 2 



1 From Roth's Tables of chemical analyses, Beitriige zur Petrographie der plutonischen Oesteine, 

 4°, Berlin, 1873, xxvi. 

 - Ibid, 1884, xxiv. 



