8 ROYAL SOCIETY OF CANADA 



completely surrounded by the sedimentary series in question, while going 

 north, toward the limit of the area at present under discussion, the Fun- 

 damental Gneiss becomes relatively more al)undant and finally underlies 

 the whole region, containing, however, everywhere shreds and patches of 

 the limestone series which is frayed away into it and partly absorbed by 

 it. 



The nepheline and their associated alkali syenites are found either 

 along the actual junction of the granite and the limestone, or in the 

 limestone itself near the granite contact. There is only a single excep- 

 tion to this in the area under discussion, namely, the nepheline syenite 

 mass in the township of Methuen, which occurs between a great granite 

 intrusion and a body of amphibolite, containing a few small bands of 

 limestone. 



These nepheline rocks are of the same age as the general complex in 

 which they occur, that is, they belong to the Pre-Cambrian or Pro- 

 terozoic rocks of the Laurentian Protaxis. They are intruded into the 

 crystalline limestones and associated sedimentary rocks of the Grenville 

 Series on one hand, and at several points where they are well exposed, 

 they pass gradually over into the Fundamental Gneiss on the other hand. 

 Elsewhere, however, dykes of the nepheline syenite or associated alkali 

 syenites can be seen to cut the Fundamental Gneiss. A careful study 

 of the whole area shows that the nepheline syenite and its associated 

 alkali syenites represent a peripheral differentiation phase of the granite 

 (Fundamental Gneiss), and that in the few cases where these rocks 

 are seen to cut the Fundamental Gneiss, they are of the nature of dykes 

 of differentiated material intruded into a more acid phase of the same 

 magma which was already consolidated, very much in the same way as 

 in the occurrence of ordinary granite pegmatite dykes, are found repre- 

 senting the last product of consolidation of a common magma. 



General Petrographical Character. 



It would far exceed the scope of the present paper to describe in 

 detail the exact composition of these syenites at the many localities 

 where they are exposed, for their extreme and rapid variation in com- 

 position is one of the most noteworthy features of their development. 

 In fact, too much emphasis can hardly be given to this fact, for no other 

 rocks show an equally great diversity of types within such short dis- 

 tances. It is quite possible to obtain hand specimens from the same ex- 

 posure, and even from contiguous bands, which exhibit such a wide differ- 

 ence in their mineralogical composition as to be classed as separate and 

 distinct types of rock. All these, however, are differentiation products 

 of one highly alkaline and aluminous magma, representing one phase of 



