Calkins. J 



Petrography of the John Day I'm sin. 



127 



refraction, the characteristic crystal forms, and by a chemical 

 test with nitric acid and ammonium molybdate. 



Pyroxene andesites of the Hald's Canon type also occur 

 at Fox, on Beach Creek, and in the Cherry Creek region. 

 The Cherry Creek rock also exhibits the mioropoikilitic ground- 

 mass in almost complete development. In the Beach Creek 

 specimen, the base is glass, which is locally altered to a semi- 

 opaque brownish white mass of uncertain composition. The 

 hypersthene in both cases is only slightly decomposed. 



A chemical analysis by the writer of the freshest material 

 from Hald's Canon is given here: 



Analysis of Hypersthene Andesite, Hold's Canon. 



Si0 2 



57.49% 



A1 2 3 



17.22 



Fe 2 3 



4.34 



FeO 



3.01 



MgO 



3.38 



CaO 



6.48 



NaoO 



3.90 



K.O 



1.30 



H-.O at 110° 



0.62 



HjO above 110° 



1.37 



Ti0 2 



1.12 



P.O., 



0.28 



MnO 



0.22 



BaO 



trace 



Total 



100.73 



The rather high percentage of combined water is probably 

 derived mainly from the iddingsite, which probably contains 

 some of the ferric iron, though a larger proportion of the latter 

 is probably in the form of limonite and hematite. The analysis 

 shows no other remarkable features, except perhaps the 

 rather large amount of titanium, which is presumably in the 

 iron ore. The potash, somewhat contrary to the writer's expec- 

 tations, is present in about the usual amount for this type of 

 rock; it even falls somewhat below the average. Yet when we 

 calculate the corresponding amount of orthoclase, we obtain a per- 

 centage of about 7.7, supposing the orthoclase to be free from the 

 albite molecules. This is, however, rarely true, and supposing the 

 alkali feldspar to have the formula AbiOri, the percentage would 



