MOUNT RAINIER 



It is exceptional to find the two in the same specimen, 

 the one being absent whenever the other is present. 

 The following analysis 1 of the typical hypersthene- 

 andesite from Crater Peak shows the lava to be a com- 

 paratively acid andesite : 



ANALYSIS OF HYPERSTHENE-ANDESITE FROM CRATER PEAK, 

 MOUNT RAINIER 



An analysis 2 of one of the light-gray, olivine-bearing 

 rocks on the northern slope of the mountain gives a 

 silica percentage of 54.86, and is doubtless represent- 

 ative of the more basic of the Rainier lavas. 



The sporadic occurrence of hornblende in these 

 andesites is principally the result of physical condi- 

 tions rather than of chemical composition. The mag- 

 matic alteration of the phenocrysts of hornblende 

 affords evidence of this variation in consolidation con- 

 ditions, a diminution of pressure with continuance of 

 slow cooling giving rise to the magmatic alteration of 

 the hornblende. That this change took place during 

 the later stages of consolidation is shown by the rela- 

 tive age of the hornblende, noted above, and also by the 

 fact that in one case a phenocryst of augite, where it 

 abuts against the hornblende, has protected the latter 

 from this alteration. The alteration is in part pseudo- 



1 Hague and Iddings : Twelfth Ann. Kept. U. S. Geol. Survey, p. 225. 

 * Oebbeke, op. cit., p. 226. 



248 



