28 



and may be described as gabbro-diorites although the 

 character of the hornblende frequently suggests its deriva- 

 tion from augite under the action of metamorphic agencies. 



V. Nepheline and Alkali Syenites. 



The nepheline and alkali syenites constitute one of 

 the most interesting and important groups of rock in the 

 whole area. One of their most notable characteristics 

 is that they usually display a well marked foliation which 

 coincides in direction with that of the gneissic granites 

 of the Laurentian with which they are associated, and with 

 which they are believed to have a close genetic connection. 

 They occur almost invariably throughout the area, as a 

 border facies of the granites where these come against the 

 limestones of the Grenville series, and are intrusive into 

 the latter rocks with which along the contact they are 

 frequently intimately interbanded. The nepheline syenite 

 magma has apparently transfused the limestones, of which 

 it holds many inclusions, so that a certain amount of 

 calcite, occurring as inclusions in some cases and in all 

 cases of undoubtedly primary origin, is found in almost 

 every occurrence of the nepheline syenite 



This group of rocks results from the crystallization of 

 magma rich in alkalies and alumina. The resultant rock 

 types present many varieties of nepheline rock (free or 

 almost free from feldspar), nepheline syenite and alkali 

 syenites, each accompanied by corresponding pegmatitic 

 developments. The magma was in all cases very rich in 

 soda as shown not only by the presence of nepheline but 

 also by the fact that the feldspar present in the rocks is 

 almost always albite. Even in the syenites free from nep- 

 heline, this soda feldspar usually predominates very largely, 

 although in some cases the corresponding potash feldspar 

 is the dominant constituent. These rocks are as a general 

 rule very poor in ferro-magnesian constituents, the latter 

 when present being represented by either biotite or horn- 

 blende, or by both. 



The occurrence of corundum in great abundance in 

 these nepheline rocks, more especialy in the eastern portion 

 of the area, gives to them a peculiar interest. 



