764 REPORT— 1893. 



Within the core the coarsest-grained forms are gabhro. The composition 

 Yaries in different parts of one rock mass, and also between different intrusions 

 within the core. The transition is from a gabbro to a diorite with biotite and quartz ; 

 and the extreme variety is that form of granite called haplite, the range in silica 

 being from 61"81 to 71'62 per cent. Fine-grained andesitic equivalents of diorite 

 occur in dikes outside of the core, but none of the most siliceous varieties haa 

 been found outside of it. 



From this it appears that towards the end of volcanic activity near the core the 

 composition of the magmas became more and more siliceous, and the volume of the 

 lava erupted smaller. But this change in composition was not xminterrupted, for 

 there are evidences of the alternate eruption of basic magmas as well. Dikes of 

 more siliceous rocks are traversed by later dikes of basic rocks. This has taken 

 place both inside and outside of the core. 



Some of these basic rocks are exceptionally low in silica for rocks of this region. 

 They are all found at some distance from the core, with one exception, which 

 is an inti'usion within it. They are lamprophyric, and approach more or less 

 closely camptonites, monchiquites, kersantites, and minettes. They are connected 

 with the basalts of the region by mineralogical and structural transitions. 



These exceptionally basic rocks are the chemical complements of the acid ones 

 in the core, and appear to be among the latest eruptions. "While they agree with 

 one another in having a low percentage of silica, they differ in the relative abun- 

 dance of magnesia, lime, and iron oxide on the one hand and of alumina, soda, 

 and potash on the other. 



As already pointed out by the writer in another place, the variability in 

 composition of all the igneous rocks in this volcano illustrates one mode of 

 differentiation of a magma at a particular centre of eruption. A comparison of the 

 chemical and mineral composition of the rocks of this district furnishes additional 

 evidence of the fact that magmas which are chemically similar will crystallise into 

 different groups of minerals, according to the conditions through which they pass. 

 Thus chemically similar magmas may form basalt under one set of conditions and 

 gabbro under others, the first composed of plagioclase, augite, olivine, magnetite, 

 and sometimes hypersthene, the second consisting of plagioclase, augite, hyper- 

 sthene, and biotite, besides some magnetite, orthoclase, and quartz, with or without 

 hornblende. Minerals, then, which are primarily functions of the chemical 

 composition of rocks are also functions of the physical conditions affecting 

 crystallisation. 



Some of the conditions under which the molten magmas solidiiied within the 

 dikes and core of the volcano of Crandall Basin may be inferred from a considera- 

 tion of the geological structure of this ancient volcano. The magmas which 

 solidified within that portion of the core now exposed and those in dikes within a 

 radius of two miles must have occupied positions at nearly the same distance 

 beneath the surface of the volcano — tliat is, at a depth of about 10,000 feet. The 

 latter rocks were as deep or as abysmal as the former, and yet their degrees of 

 crystallisation range from glassy to coarsely granular. The influence of pressure 

 on the crystallisation is not recognisable either in the size of grain or the phase of 

 crystallisation. 



Marked changes in the crystallisation may be traced horizontally in the imme- 

 diate vicinity of the core. They are rapid near the core, and are accompanied by 

 the induration and metamorphism of the surrounding rocks. They are in great 

 measure independent of the size of the rock-body, since narrow dikes within the 

 core are coarse-grained, while much broader ones in the surrounding breccias are 

 very fine-grained. It was unquestionably the differences in the temperature of the 

 core-rocks and of the outlying breccias which affected the degree of crystallisation. 

 The former must have been more highly heated than the surrounding rocks, and 

 the magmas solidifying within them cooled much more slowly than those injected 

 into the outlying parts of the volcano. In this case the depth at which the 

 magmas solidified appears to have been of little moment in comparison with the 

 temperature of the rocks by which they were surrounded. 



The core of gabbro and diorite, with an intricate system of veins of middle- 



