84 JOURNAL OF GEOLOGY— SUPPLEMENT 



border phase occurring about the nephelite syenite of Red Hill, 

 New Hampshire, was formed in this manner. Pirsson and Wash- 

 ington point out that contact chilling is shown by the finer grain 

 of this quartzose phase. "^ A more salic nature of the border phase 

 when formed by chilling (not by the squeezing out of a pegmatite- 

 like fringe) should in fact be common in rocks belonging to this 

 stage of precipitation, i.e., to associations of nordmarkite, pulaskite, 

 foyaite, and related types. It is apparently principally among 

 such types that acid border phases have been observed, an espe- 

 cially clear case being that at Almunge in Sweden where nord- 

 markite occurs as a border phase about umptekite and nephelite 

 syenite.^ 



THE r6lE op assimilation 



In the foregoing discussion of the igneous rocks assimilation has 

 not been mentioned as an essential process in the production of 

 diversity of rock types. It is believed that differentiation of 

 basaltic magma after the method proposed is the essential process 

 and that a basaltic magma may and commonly does give a salic 

 differentiate, say granitic, without having assimilated salic material. 

 If, as is believed, basaltic magma is the original material of all 

 igneous action with which we are acquainted, then, in the beginning, 

 there could, of course, have been no salic material available for 

 assimilation. Nevertheless, after the lapse of time, the formation 

 of a great diversity of rocks" by differentiation from basaltic magma 

 was accomplished and then a magma might at some stage in its 

 career make contact with, say, a salic rock. A certain amount 

 of assimilation might ensue. A magma might also assimilate 

 some material formed from igneous rocks by atmospheric agencies, 

 i.e., sedimentary rocks. It is the purpose of this part of the paper 

 to discuss the effects of such assimilation. 



As a matter of fact, plain evidence is to be found in the field 

 that magmas do assimilate, especially when they occur in the large 

 bodies commonly termed batholiths. Sometimes batholiths have, 

 indeed, a chilled contact against their country rock and in such a 

 ^ase little assimilation can be admitted. This appears to be 



' Am. Jour. Sci. (4), XXIII (1907), 276. 

 2 Quensel, op. ciL, p. 161. 



