CHEMICAL COMPOSITION OF ELECTRIC PEAK ROCKS. 1 15 



It is not possible to draw a line of demarcation anywhere in the scale 

 based on the degree of crystallization between rocks that occur in narrow 

 dikes and those that form parts of much larger bodies. A relation between 

 the degree of crystallization and the size of the rock body does not at first 

 appear when all of these occurrences are considered together. The very 

 important influence of several other factors, however, becomes apparent. 

 One is the chemical character of the magma, the more basic magmas tend- 

 ing to crystallize coarser than the more siliceous ones under similar physical 

 conditions. Another factor is the previous temperature of the rocks into 

 which the molten magmas were injected, and the consecpient differences in 

 the rate of cooling which the molten magmas experience. There may also 

 be other factors which influence the crystallization in certain cases, but 

 they are not evident in the occurrences at Electric Peak. In this locality 

 the chief factor influencing the crystallization appears to have been the 

 temperature of the inclosing rocks at the time of the different intrusions. 

 The next most influential factor appears to have been the chemical character 

 of the magma itself, and the third the size of the intruded mass. In another 

 region the relative importance of these factors may be different. 



CHEMICAL COMPOSITION. 



The chemical composition of the intrusive rocks at Electric Peak is 

 shown by the analyses in Table IX. Nos. 272 and 309 were made by Mr. 

 W. H. Melville, the remainder by Mr. J. E. Whitfield. All are from rocks 

 occurring in the stock and its immediate apophyses. They represent the 

 composition of various forms of the diorite and diorite-porphyry. The 

 first four analyses, Nos. 295, 267, 273, and 272, are from the main body of 

 the stock, and belong to Subgroup Ila. The next four analyses, Nos. 309, 

 313, 311, and 303, are from modifications of the main body of the diorite 

 and from one of the lighter-colored veins or dikes which traverse it. They 

 belong to Subgroup lift. Two more varieties of the main stock are repre- 

 sented by analyses Nos. 323 and 321. They are quite siliceous, and belong 

 to Subgroup He. Analyses Nos. 329 and 326 are from the large bodv of 

 quartz-mica-diorite-porphyry, the first being a basic variety of it, and the 

 second corresponding more nearly to the general character of the body of 

 the rock. 



