Calkins, | 



Petrography of ihe John lhnj Basin. 



167 



Near the base of the Maseall section at Belshaw's Ranch 

 occurs a somewhat coarser and fresher material. This is a gray 

 friable tuff composed essentially of angular, sometimes vesicular, 

 fragments of clear glass. The only other constituent is feldspar, 

 but the crystals are too rare to afford sufficient material for 

 determination. Following is a chemical analysis of the rock: 





I 



11 



SiO. 



68.22 



74.22 



A1203 



12.41 



13.50 



Fe 2 0, 



1 .110 



1.09 



FeO 



1.36 



1.49 



MgO 



0.18 



(1.19 



CaO 



0.95 



1.03 



Na,0 



3.38 



3.69 



K,0 



3.97 



4.33 



H 2 at 110° 



3.42 





H 2 above 1 1 11° 



4.82 





T1O2 



0.34 



0.35 



P2O, 



0.11 



0.11 



MnO 



not det. 





Total 



100.16 



100.00 



I. Rhyolitic tuft', Belshaw's Ranch, Analyst F. C. Calkins. 

 II. Analysis recalculated on a water-free basis to 100 per cent. 



The remarkably high percentage of both hygroscopical and 

 combined water somewhat masks the true chai^aeter of the rock, 

 by proportionately reducing the apparent percentages of the 

 other oxides. The recalculated analysis shows that the rock is 

 a rhyolite, with potash slightly in excess of soda. 



ROCKS OF THE RATTLESNAKE BEDS. 



Besides sandstone and conglomerate, the Rattlesnake beds 

 comprise considerable material similar to the common Maseall 

 tuff. It is perhaps in part worked over by water. This will 

 receive but this passing mention. 



The coarse tuff occurring near the middle of the section is a 

 porous, light gray rock, composed of angular lapilli with rather 

 numerous grains of feldspar, and comparatively few of quartz 

 and green augite. The feldspar is mostly of an acid triclinic 

 variety, showing fine shadowy striatums on the basal cleavage 

 flakes. A few also appear to be sanidine. 



