THE ROCKS OF MOUNT RAINIER 



haps, less important elements in the construction of 

 the composite cone. 



The lavas vary much in color and texture, but these 

 megascopic differences are referable rather to the 

 degree of crystallization of the magma than to its 

 chemical character. The variation in the chemical 

 composition of the lavas expresses itself in minera- 

 logical differences, and thus four rock types are dis- 

 tinguished hypersthene-andesite, pyroxene-andesite, 

 augite-andesite, and basalt. The distribution of these 

 types indicates a radial arrangement of lava streams, 

 and hypersthene-andesite is the more abundant variety 

 of lava. 



Granite is exposed on the slopes of Rainier where 

 erosion has cut away the overlying lava, and it is plain 

 that the volcanic cone rests upon an elevated platform 

 of older rock, approximately 8,000 feet above sea level. 



253 



