THE ROCKS OF MOUNT RAINIER 



usually twinned. Both are light colored, and the 

 pleochroism of the hypersthene is sometimes quite 

 faint. According to the relative importance of 

 these two pyroxenes, the lavas belong to different 

 types, hypersthene-andesite, pyroxene-andesite, or 

 augite-andesite. 



Olivine occurs in certain of the Rainier lavas, in 

 stout prisms somewhat rounded and often with red- 

 dened borders. The usual association with apatite 

 and magnetite crystals is noted. The olivine varies 

 much in relative abundance, so as to be considered now 

 an accessory and now an essential constituent, and in 

 the latter case the rock is a basalt. 



Hornblende is not abundant in any of the rocks 

 studied, although typical hornblende-andesite has been 

 described among the specimens collected by Professor 

 Zittel. Where it occurs it is in brown crystals, which 

 have usually suffered magmatic alteration. In one 

 case, where this alteration is less marked, the idiomor- 

 phic hornblende is found to inclose a crystal of labra- 

 dorite, and thus must have been one of the latest 

 phenocrysts to crystallize. It also surrounds olivine 

 in this same rock, 1 which is a hypersthene-andesite, 

 the hornblende and olivine being only accessory. 



The different textures of these lavas are doubtless 

 expressive primarily of diversity in the physical con- 

 ditions of consolidation, but also in part of variations 

 in chemical composition. The variations in miner- 

 alogical composition are likewise referable to these two 

 factors, but here the latter is the more important. 

 The hypersthene-augite olivine variation, already re- 

 ferred to, doubtless well expresses the chemical compo- 

 sition of the magma, and deserves to be taken as the 

 chief criterion in the classification of the lavas. As was 

 noted by Hague and Iddings, the hypersthene and 

 olivine play a like role, the former occurring when the 

 silica percentage is somewhat higher than in basalt. 



1 Observed by Iddings: Twelfth Ann. Kept. U. S. Geol. Survey, p. 612. 



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