MOUNT RAINIER 



RELATION TO THE VOLCANIC ROCKS 



Along the side of the knob overlooking Carbon Gla- 

 cier the granite as seen from a distance appears to be 

 intrusive. Blocks of andesite cover the slope, de- 

 posited there by the glacier at a time when it possessed 

 greater lateral extent, and the granite talus from above 

 crosses this same slope in a narrow band. The rela- 

 tions prove less deceptive on close examination, and 

 the granite is seen to constitute an older ridge. Far- 

 ther along this ridge, at the cliffs on the north-eastern 

 edge of Moraine Park, the granitic rock is found over- 

 lain by the lava. The actual contact of the two rocks 

 is concealed by soil filling the crevice left by disinte- 

 gration along the contact plane. The granite, however, 

 exhibits no intrusive characters, while the overlying 

 andesite becomes scoriaceous in its lower portion, 

 although compact immediately above. This contact 

 is on the southern side of the granite ridge, the crest of 

 which is approximately east-west. This position of 

 the lava contact considerably below the highest occur- 

 rence of the granite indicates that the topographic fea- 

 tures of this old granite ridge were even more marked 

 at the time of the eruption of the lavas and the 

 building of the volcanic cone. Above this ridge of 

 granite on the one side tower the cliffs of bedded 

 volcanics which compose the Sluiskin Mountains, and 

 on the other is the andesite ridge bounding the can- 

 yon of Winthrop Glacier. Thus Mount Rainier, al- 

 though a volcanic peak, rests upon an elevated plat- 

 form of granite which is exposed by erosion at a few 

 points on the slopes of the mountain. 



SUMMARY 



The volcanic rocks of Mount Rainier include both 

 lavas and pyroclastics. The breccias, agglomerates, 

 and tuffs, although of striking appearance, are, per- 



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