260 W. J. MILLER — MAOMATlC DlPFERENTlATIOlsr AND ASSIMILATION 



tain basic border phases of the Essex County anorthosite. Much of the 

 rock, however, looks much like the normal syenite, though it is always 

 without quartz. The degree of foliation varies considerably, though it is 

 generally pronounced. The clear red garnets are not thought to be of 

 isecondary origin. Along the borders, especially to the east and south- 

 east, the rocks of this area grade into normal quartz-syenite. This area 

 of so-called basic syenite appears to have been produced by magmatic 

 assimilation where dark, basic Grenville gneisses were incorporated into 

 the normal syenite magma. A considerable inclusion, one-half mile 

 southeast of the southern end of Lake Pleasant, has an important bear- 

 ing in this connection. The inclusion contains 40 per cent hornblende; 

 30 per cent oligoclase to labradorite; 18 per cent hypersthene; 1 per cent 

 garnet, and 2 per cent magnetite and pyrite. Its borders have been well 

 fused, and the assimilation product greatly resembles the most typical 

 phase of the syenite just to the west. A complete assimilation of many 

 such basic Grenville fragments by the normal syenite magma would cer- 

 tainly account for the so-called basic syenite. The view that the syenite 

 magma did break through and actually assimilate considerable quantities 

 of such Grenville rocks is strongly supported by the following facts : The 

 very presence of the above-mentioned inclusion, which is taken to repre- 

 sent a late stage in the stoping and assimilation process; the fact of the 

 former existence of much basic Grenville gneiss over the site of the basic 

 syenite area as proved by numerous inclusions and the present occur- 

 rence of large masses just northward; the variable composition and ap- 

 pearance of the rock which would be expected because of differences in 

 amount and character of the rocks assimilated ; the gradation of the basic 

 syenite into normal syenite; and the fact that the rocks so very closely 

 resemble rocks definitely known to have been produced by the action of 

 a similar syenite magma upon similar' Grenville rocks in the Thirteenth 

 Lake quadrangle (see description above). 



GENERAL CONSIDERATIONS 



The widely different views held by petrologists regarding magmatic 

 assimilation are suggested by the following brief references to several 

 recent papers. 



Loewinson-Lessing believes that assimilation of invaded rock has a 

 great influence upon differentiation, and says: 



"Is not assimilation a phenomenon that must be expected a priori in intru- 

 sive bodies, for it is difficult to imagine a magmatic basin heating the rocky- 

 masses in contact with it for a long period without partly dissolving them." " 



^ F. Loewinson-Lessing : Geol, Mag., n. s.. Dec. V, VIII, 1911, pp. 248-257, 289-297. 



