CORRELATION OP THE EOCKS. 145 



dikes, and stock rocks, and stows that a great number of intermediate steps 

 can be recognized between tire most glassy andesite and the coarsest diorite. 

 It shows that the dike rocks furnish the connecting link between these two 

 extremes, and that the dike rocks of Electric Peak have the same range of 

 grain as the majority of those of Sepulchre Mountain. But many of those 

 at Sepulchre Mountain are still finer grained, and some are glassy, being 

 vesicular also. Between these rocks there is the closest possible resemblance 

 megascopically, and the two groups might have been described conjointly, 

 so far as their petrographical characters are concerned. The variation of 

 grain within each of the four principal divisions is very significant when 

 taken in connection with the geological occurrence of the different rocks. 

 The limited range of variation in the first group is in accord with the fact 

 that all of these rocks are surface ejectamenta. The range in the third 

 group from more crystalline basic rocks to less crystalline acid rocks, as 

 already pointed out, shows the greater tendency of the basic rocks to 

 crystallize. And since the dikes here represented are of nearly the same 

 size, this variation of grain corresponds to differences in the chemical com- 

 position of the rocks. On the contrary, the variations in the second group 

 indicate a slightly greater crystallization of the acid rocks. This, however, 

 is due to the fact that the basic rocks in this group, with a few exceptions, 

 occur in small dikes, while the acid rocks for the most part form broad 

 intruded bodies several hundred feet wide. In these cases the size of 

 the mass has had more influence on the degree of crystallization than has 

 the chemical composition of the magma. In the fourth group the basic 

 rocks exhibit a wider range of grain than the acid, being much coarser 

 and also considerably finer grained than the latter. This arises from the 

 fact that the basic rocks form a much larger mass and exhibit great variation 

 of grain, having fine-grained modifications that have been fully discussed 

 in an earlier part of this chapter. 



These diorites and others that cut the volcanic lavas in several locali- 

 ties in this region correspond to the andesdiorites and andesgranites of 

 Stelzner, who described stocks of granular rocks penetrating the andesitic 

 tuffs in Argentina. The study of these Tertiary granular rocks led him 

 to the conclusion that the degree of crystallization of eruptive rocks is 

 in no way dependent on their age, but depends on the physical conditions 



MON XXXII, PT II 10 



