THE ROCKS OF MOUNT RAINIER 



The blocks resemble pigs of iron in size and shape, 

 and where exposed in a vertical cliff these seem to be 

 piled in various positions. 



The rocks on the higher slopes of Mount Rainier are 

 in general very fresh in appearance. An exception 

 may be noted in the case of the rocks at the base of 

 Little Tahoma, where some alteration is evident. The 

 bright coloring of the surfaces of the lava blocks and 

 the general appearance of the face of the cliff may in- 

 dicate fumarole action at this point. There is also 

 some decomposition along the inner edge of the crater 

 rim, near the steam vents. On the lower slopes, some 

 distance below the snow line, the freshness of the rock 

 is not a noticeable feature, and it is seen that here 

 weathering is of the nature of chemical decomposition 

 as well as of mechanical disintegration. 



MICROSCOPIC CHARACTERS 



Microscopically these lavas show more uniformity 

 than is apparent megascopically. Rocks which in 

 color and texture appear quite diverse are found to be 

 mineralogical equivalents. The majority of these rocks 

 are andesites, the hypersthene-andesites predominat- 

 ing, as was shown by Hague and Iddings ; but over 

 large areas the andesites are decidedly basaltic, and, 

 indeed, many of the lavas are basalts. The mega- 

 scopic differences are mostly referable to groundmass 

 characters, the color of the rock being dependent upon 

 the color and proportion of glassy base present. There- 

 fore the degree of crystallization of groundmass con- 

 stituents is of more importance in determining the 

 megascopic appearance than is the mineralogical com- 

 position, and the basaltic lavas are for the most part 

 light gray in color, while the more acid hypersthene- 

 andesites are often black or red. 



In petrographic character the lavas range from hyper- 

 sthene-andesite to basalt. This variation is dependent 



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