MINERAL COMPOSITION OF DIKE EOCKS. 105 



is a gradual transition from the sharply idiomorphic quartz phenocrysts of 

 the fine-grained porphyries to allotriomorphic crystals, such as occur in 

 granites and in the ( coarser-grained porphyries. This shows that the por- 

 phyritical quartzes were the last phenocrysts to crystallize, and that their 

 crystallization in the coarser-grained varieties continued into the period 

 of crystallization of the groundmass without evidence of interruption. 



GENERAL CONSIDERATION OF THE MINERAL AND CHEMICAL COMPOSITION 

 OF THE INTRUSIVE ROCKS IN ELECTRIC PEAK. 



MINERAL COMPOSITION. 



In order to convey an idea of the variations in mineral composition of 

 the rocks just described, recourse has been had to tabular statements of the 

 relative abundance of the constituent minerals in the different groups of 

 rocks. The tables serve not only to condense into very compact space 

 many data, but place them in such form that they may be comprehended at 

 a glance, while at the same time the formal arrangement of the data often 

 conveys ideas of transitional relationships not easily expressed otherwise. 

 The same is true of data relative to degree of crystallization, which will be 

 explained later on. 



In the dike rocks constituting Group I, and in the dikes of Group III, 

 the mineral variations most readily noted affect the phenocrysts, and more 

 particularly those of the ferromagnesian minerals and quartz. Variations 

 in the feldspars may be recognized, but with more difficulty, and may be 

 expressed in general terms by saying that those in the more basic rocks are 

 richer in calcium, while those in the more siliceous rocks are richer in 

 sodium, and that the transition between the extremes appears to be gradual. 

 The variations in the phenocrysts other than the feldspars are indicated in 

 Table III, in which the first column contains symbols of mineralogical 

 subdivisions to be used for correlation in subsequent tables. In this table 

 no .account is taken of the degree of crystallization. 



