484 H. p. GUSHING ASYMMETRIC DIFFERENTIATION IN SYENITE 



the field relations to suggest that the Grenville here lies in a downfanlted 

 trough in the syenite, is "underlaid by syenite in depth, with the small 

 syenite knobs representing dikes running upward from this underlying 

 mass, and that the contacts with the syenite are therefore fault contacts. 

 If this be true, they have no significance bearing on the general purpose 

 of this paper. 



Summary of Field Kelations 



The Tupper syenite bathylith is in contact with a gabbro on two sides 

 and with granite-gneiss on the other two. The gabbro is a basic differ- 

 entiation border of anorthosite. Tlie gabbro is a much more basic, the 

 granite a somewhat more acid rock' than the syenite. The syenite grades 

 into a much more basic phase as the anorthosite is neared, but becomes 

 somewhat more acid near the granite. Its contacts with both rocks are 

 those of a later intrusive. 



Along its contact with the gabbro it seems locally to have cut away 

 the entire gabbro border from the anorthosite. It holds frequent large 

 inclusions of anorthosite gabbro, around which it becomes more basic. 

 Anorthosite gabbro soaked by syenite is found. Eocks are also foimd 

 which are so distinctly intermediate between the gabbro and the basic 

 syenite that they defy classification as either the one or the other. 

 Around a large inclusion of anorthosite gabbro in the granitic syenite a 

 fringe of basic syenite has developed. 



These relations are readily explained on the assumption that there 

 has been actual incorporation and digestion of material from the sur- 

 rounding rocks by the syenite, and that the digestion has mainly been a 

 border phenomenon. Nothing that was observed is antagonistic to such 

 an explanation. jSTo other solution that will at all account for the ob- 

 served relations suggests itself. It is therefore an instance which needs 

 to be taken into account when the general question of the possibility of 

 such assimilation taking place is under consideration. 



Chemical Evidence 



It has been frequently and cogently urged that, in case one rock had 

 been assimilated by 'another, the resultant rock should have a distinctly 

 intermediate composition. It is no part of the purpose of this paper 

 to consider the validity of this point. For purposes of argument it is 

 granted, and it remains to be seen whether it has any antagonistic bearing 

 in this specific instance. 



