DISSECTED VOLCANO OF CR AND ALL BASIN. 609 



and dust of hornblende-mica-andesite, hornblende-andesite, and 

 hornblende-pyroxene-andesite. They are partly glassy and 

 partly holocrystalline. In some places they appear to pass into 

 the overlying breccia, but in others they have been eroded and 

 weathered before the latter were thrown over them. 



The upper breccia, which constitutes the main mass of the 

 volcano, is basaltic as a whole. It consists of pyroxene-andesite 

 and basalt, the latter predominating in the upper part of the 

 accumulation. The massive flows, as far as investigated, are all 

 basalt. The composition varies constantly within narrow limits. 

 A greater part of these rocks contain glassy groundmass. 



The rocks constituting the dikes exhibit more variation than 

 the breccias, though the majority of them are like the breccias in 

 composition and habit, being basalt. They are generally more 

 crystalline. A great many dike rocks resemble the basalts in 

 outward appearance, but have little or no olivine, and are more 

 crystalline. The absence of olivine from the more crystalline 

 forms of these rocks appears to be due to the conditions which 

 influenced the crystallization of the rocks and not to their chem- 

 ical composition. For in some cases what appear in hand speci- 

 mens to be decomposed olivines are found to be paramorphs 

 after this mineral, consisting of grains of augite, magnetite, and 

 biotite. As the rocks become more crystalline biotite becomes 

 an essential constituent ; the porphyritical minerals lose their 

 sharpness of outline and assume some of the microscopical char- 

 acteristics which they possess in gabbro. 



Within the core the coarest grained forms are gabbro. The 

 composition varies in different parts of one continuous rock mass, 

 and also between different intrusions within the core. The tran- 

 sition is from gabbro to diorite with biotite and quartz ; and the 

 extreme variety is that form of granite called aplite ; the range 

 in silica being from 51.81 to 71.62 per cent. 



Fine grained, andesitic equivalents of diorite occur in dikes 

 outside of the core, but none of the most silicious varieties have 

 been found outside of it. From this it appears that toward the 

 end of volcanic activity near the core the composition of the 



