592 /. E. SPURR 



in condition wliicli brought about the separation of the coarse- 

 grained from the fine-grained rocks were relatively small. 

 These differences are plainly longer cooling periods (and hence 

 longer crystallization intervals) in some portions of the rock, as 

 compared with others. The coarse granular rocks, judging from 

 their almost complete and uniform crystallization, have crystal- 

 lized entirely in their present position ; the segregation from 

 them of alaskite on the one hand, and hornblendite on the other, 

 is additional evidence of this. The porphyritic forms, however, 

 show generally two distinct generations, and this, with the fact 

 that some of the phenocrysts of the first generation, especially 

 the quartz, show corrosion and resorbtion, indicates a change of 

 conditions ; also the slight fluxional arrangement, sometimes 

 observed, shows at least local movement. Yet the contempo- 

 raneity of the fine-grained porphyritic rocks with the granu- 

 lar forms, shows that the crystallization break registered by 

 the former did not arise from any important change of posi- 

 tion. 



Composition of rocks. — The general mineral composition of 

 rocks of all structures (leaving out the alaskite and hornblen- 

 dite, which are plainly segregation products from the hornblende 

 quartz-diorite), is the same, being a hornblende-quartz-diorite or 

 quartz-andesite. In the andesites the quartz, where present, was 

 plainly in process of resorbtion by the groundmass at the time 

 of solidification, so that it is probable (from the fact the horn- 

 blende andesites and the hornblende-quartz-andesites often occur 

 as variations of a single bed) that in those andesites which do 

 not show free quartz the composition is the same as where they 

 do, the quartz belonging to the first period of crystallization 

 having been entirely resorbed. 



