CORRELATION OF THE ROOKS. 147 



In this region of Electric Peak and Sepulchre Mountain the greatest 

 mineralogical differences accompany the greatest differences in structure or 

 degree of crystallization; hence we may assume that the causes leading to 

 each are coexistent. The source of these causes must be sought in the 

 differences of geological environment, and these affect the rate at which 

 the heat escapes from the magmas and the pressure the) 7 experience during 

 crystallization. 



It is to be remarked that the most essential mineralogical difference 

 between the intruded rocks and their chemically equivalent extrusive forms 

 is the much greater development of biotite and quartz in the intruded rocks, 

 these minerals bein^ abundant even in the basic intrusions and absent from 

 their basic volcanic equivalents. That their simultaneous development is 

 naturally to be expected in many cases is evident from a consideration of 

 the character of their chemical molecules and that of other minerals com- 

 mon to these rocks. For if we assume that biotite is made up of two 

 molecules, corresponding respectively to K 6 Al 6 Si 6 2 i and R 13 Si 6 24 , and 

 compare these with the molecules of orthoclase, K 3 ALSi 6 16 , of olivine, 

 R 2 Si0 4 , and of hypersthene, RSi0 3 , we see that molecules which under 

 some conditions might have taken the form of olivine or hypersthene and 

 potash feldspar, which latter may have entered into combination with lime- 

 soda-feldspar molecules to form somewhat alkaline feldspars, may under 

 other conditions combine as biotite with the separation of free silica or 

 quartz ; in which case also the feldspars of the rock would be less alkaline. 



Another mineralogical difference between the two groups of rocks 

 just mentioned is the greater development of hornblende in the intruded 

 rocks in place of augite, which is chemically similar, though it has not 

 been determined whether in this case the hornblende of the diorite has 

 nearly the same composition as the augite of the andesite. The proba- 

 bility is that there are considerable differences between them. 



The crystallization of quartz, biotite, and hornblende in fused magmas, 

 according to our present knowledge, requires the assistance of a mineraliz- 

 ing agent; for it has been demonstrated by synthetical research that these 

 minerals will not crystallize in the forms they assume in igneous rocks when 

 their chemical constituents are fused and simply allowed to cool under 

 ordinary atmospheric conditions. But they have been produced artificially 

 with the aid of the mineralizing action of water vapor and of other gases. 



