374 R. A. DALY THE OKANAGAN COMPOSITE BATHOLITH 



diorites; and the Cathedral batholith, with its satellites, is a still later 

 product, assimilating all the earlier formations, including possibly Cre- 

 taceous arkoses. Special differentiation at various times produced the 

 Kruger Alkaline body and some of the dikes cutting the batholiths. In 

 each irruptive epoch the crush of mountain building may have facilitated 

 the liquefaction of the deeper lying portions of the invaded formations. 

 Whatever the source of the heat, it was each time present in sufficient 

 quantity to enable the magma of that stage to stope and dissolve its way 

 upward, well into formations that were not fused through orogenic crush- 

 ing. To that extent the magmas were superheated. 



The assimilation-differentiation theory thus explains the sequence of 

 the irruptions forming the Composite batholith. At each intrusive stage 

 the magma set free to eat its way upward was more basic than the average 

 of the rock invaded. At each stage a new magma, the one actually in 

 contact with the invaded formations, was generated through absorption 

 and gravitative differentiation, and the silica of the new magma was 

 higher than that of the preceding batholithic magma. The increasing 

 acidity is a function of the density, which decreases from below upward 

 in the subcrustal magma couche. This law of density is preserved in the 

 specific gravities of the batholithic rocks as now crystallized. 



SKELETON HISTORY OF A BATHOLITHIC MAGMA 



The development of any one of the batholiths may be summarized as 



follows : 



1. A period of high liquidity, conditioned by orogenic movement. This 

 period is characterized by — 



a. Contact fusion; 



b. S toping and abyssal assimilation of xenoliths, progressive modifica- 

 tion (here acidification) of magma; 



c. Injection of wide ranging apopiryses; 



d. Gravitative differentiation of the compound magma of assimilation; 



e. Possibly the beginning of basic segregation. 



2. A period of strong and increasing viscositv — a period characterized 

 by- 



a. Cessation of magmatic digestion; 



b. Some subsequent continuation of contact shattering; 



c. Completion of the observed segregation of basic materials in nodules 

 and contact zones ; development of maximum acidity in the main body of 

 the batholith ; 



