480 H. p. GUSHING ASYMMETRIC DIFFERENTIATION IN SYENITE 



more basic. In this respect it contrasts sharply with the neighboring 

 anorthosite, this latter rock showing everywhere a broad, more basic 

 border, with a slow gradation from anorthosite into anorthosite-gabbro, 

 which in its turn passes into a heavy, basic gabbro at the extreme border, 

 apparently a normal and uniform differentiation border, largely inde- 

 pendent of the character of the adjoining .rock. 



The GRANITIC Syenite 



Since the normal syenite is a fairly acid rock, its acid phases do not 

 depart as widely from the type as do the more basic ones. It becomes 

 more acid by increase in quartz and by the plagioclase becoming more 

 albitic. At the same time hornblende tends to replace pyroxene, the 

 feldspar becomes red instead of green, and the quartz tends to take on 

 the lens or spindle form. All intermediate gradations are found between 

 the normal green syenite and the red hornblende granite, both in com- 

 position and in color. The granite is normally of coarser grain than 

 the syenite. 



This acid variety is also a border phase of the syenite and occurs at 

 the south and west sides of the bathylith, where the adjoining rocks are 

 granitic gneisses involved with amphibolites, of supposed Laurentian age. 

 The apparent great breadth of this acid border as seen on the map is 

 more apparent that real. There is a great amount of a later granite 

 which cuts the syenite here which can be separately mapped only in 

 part, and there is also a considerable amount of a red S3'enite which 

 differs somewhat from the other and may be a separate intrusion. This 

 also could not be separately mapped. Could these two be excluded, the 

 area of the certain acid syenite would be greatly diminished. 



Border Eelations between the Syenite and the Granite-gneiss 



These are by no means as clear as could be wished. Both sets of rocks 

 are cut by dikes and larger masses of a later granite which is exceedingly 

 like some phases of the granite-gneiss and also not unlike the granitic 

 syenite. The discrimination of the three rocks is difficult at best and 

 frequently is an impossible matter. No certain inchisions of the granite- 

 gneiss have been found in the syenite, and none of the granite dikes 

 found cutting the granite-gneiss have been definitely shown to be dikes 

 of the granitic syenite. They may all be dikes of the later granite. 

 There are some larger masses of the syenite which cut the granite-gneiss, 

 and they are all of rather acid type and become more acid at their 

 borders; yet they are of the green syenite type rather than the red 



