10 THE ROYAL SOCIETY OF CANADA 



but dykes of this material are not infrequent in the serpentine of the 

 district. The relation of the pegmatite veins to the granite, as just 

 cited, is, as has been pointed out by Graham in other occurrences 

 (op. cit.) analagous to that of the granite dykes of the larger peridotite- 

 serpentine bodies and may be regarded as acid residues of the original 

 magma filling contraction cracks in the cooling granite. 



To cite another occurrence, a mass of granite 17 feet in diameter 

 is exposed in one of the underground workings in such a position as 

 to assure its isolation. It is a hornblende granite of the prevailing 

 type. Hornblende is more abundant in the centre than near the sides 

 of the mass. The actual edge is composed chiefly of coarse grained 

 feldspar, and may be designated as a pegmatite. In places along the 

 contact of pegmatite and serpentine, the feldspar of the former is 

 partially replaced by diopside, which appears to indicate that some 

 measure of chemical action took place between the acid residue and 

 the basic wall rock. In general then, the contacts are sharp, and in 

 the larger bodies there is a tendency toward the development of 

 pegmatite along the margin of the granitic masses, and lime silicates 

 in contact with the wall rock. 



Lime Silicates: Certain lime-silicate bodies previously referred 

 to have been recently studied by Graham, Harvie and Poitevin.^ 

 These are often in the form of dykes. They consist in part or even 

 entirely of lime garnet, vesuvianite or diopside. Graham thus sum- 

 marizes the field evidences with regard to their relation to the granite. 

 "Aplitic dykes of normal character, containing little or no garnet are 

 frequent, but occasionally they are found to be highly garnetiferous, 

 while in other cases aplite is traversed by narrow veinlets of fine 

 granular garnet. It is difficult to avoid the impression that all grada- 

 tions exist between normal aplite and the dykes composed entirely of 

 garnet, and that an aplite might be found to gradually change into a 

 garnet (or vesuvianite or diopside) dyke in one and the same continu- 

 ous fissure, although such an ideal case has hitherto not been observed." 



It is probable that these unusual rock types are developed mainly 

 in the local dykes. Irregular, but not necessarily isolated masses 

 of them have been observed. Further study is required to definitely 

 settle this point. 



1 Economic Geology, Vol. XII, No. 2, 1917, R. P. D. Graham. 

 Summary Report, Geol. Survey Can., R. Harvie. 



Museum Bulletin, No. 27 Geol. Survey Can., 1918, E. Poitevin and R. P. D. 

 Graham. 



