NATURE OF BaTHOLITHIC INTRUSIONS 



371 



one section more than half a mile in length the granite can be seen actually 

 underlying a large section of the Snowy Mountain pendant. 



In short, the fact of magmatic replacement and the related fact of 

 downward enlargement of the great magma chambers seem to be well 

 established. So fundamental are these facts that their evidence has been 

 presented somewhat at length and with considerable illustration. 





Figure 14. — Outcrop of the intrusive Contact Surface shown in Figure 13. 

 The vertical distance between the two ends of the contact line as drawn is 1,100 feet. 

 The granite is on the right ; quartzite and schist on the left. Drawn from a photo- 

 graph. Looking west. 



METHODS OF MAGMATIO REPLACEMENT ; THE* ASSIMILATION-DIFFERENTIA- 

 TION THEORY 



The chief petrologic problem consists in discovering the essential 

 processes engaged in the magmatic replacement. In several publications 

 the writer has treated of the various solutions of the problem.* On ac- 

 count of their cornplexity, the data for the best solution, attributing the 

 replacement of the invaded formation to magmatic assimilation, may not 

 here be stated in full. A very brief outline must suffice. Magmatic 

 assimilation is regarded as of a double sort, consisting, first, of the pro- 



♦American Journal of Science, vol. 15, 1903, p. 269 ; vol. 16, 1903, p. 107 ; vol. 20, 

 1905, p. 185. 



