330 R. A. DALY THE OKANAGAN COMPOSITE BATHOLITH 



Page 



Replacement theory and illustration 365 



Batholithic intrusion by magmatic replacement 370 



Methods of magmatic replacement; the assimilation-differentiation 



theory 371 



Skeleton history of a batholithic magma 374 



General summary . . . 375 



Introduction 



From end to end the Coastal division of the North American Cordillera, 

 including the Sierra Nevada, Cascade Mountain system, the British Co- 

 lumbia Coast range, the xllaska, Saint Elias, and other ranges, comprising 

 an area more than six times that of the Alps of Europe, is now proved to 

 inclose granitic masses of great size and importance. Most of them are 

 of post-Archean dates, and it is even probable that the greater number 

 belong to post-Paleozoic epochs.* Many of the Calif ornian stocks and 

 batholiths and a few batholithic masses in the state of Washington have 

 been carefully studied. Much work has been done, too, in the yet more 

 extensive granitic fields of Alaska and British Columbia; but this work 

 has generally been incidental to long reconnaissance surveys, wherein de- 

 tailed investigations could not be profitably undertaken. In the Sierra 

 Nevada the post-Paleozoic granitic rocks — granodiorites — are largely, if 

 not entirely, of Mesozoic age. In Alaska and northern British Columbia 

 the great "Coast Eange batholith" is reported to be of very similar com- 

 position, but details as to the development of this colossal body are still 

 largely lacking. Midway in this vast coastal mountain division lies the 

 International boundary, at the 49th parallel of latitude. 



On that line the writer has constructed a geological section which 

 throws light on the nature and origin of the granites and tends to bring 

 out the connection between the widely distant northern and southern 

 regions, where Cordilleran study is now being so energetically pursued in 

 government surveys. The results of the work on the 49th parallel cor- 

 roborate some of the leading conclusions of Dawson, Russell, Willis, G. 0. 

 Smith, Calkins, Mendenhall, and others who have carried on researches 

 in the adjacent parts of Washington and British Columbia. The solution 

 of the problems of these granites, as of all granites, primarily demands 

 the patient, careful accumulation of field facts. The big scale of the 

 phenomena, their unusually perfect exposure among these splendid moun- 

 tains, and the mere fact that much of the area described has never been 



*Cf. A. C. Lawson, Journal of Geology, Chicago, vol. i, 1893, p. 570. 



