78 ROCK-METAMORPHISM. 



Mont St. Philippe and the Glassillaun dyke at Cleggan ; but 

 we have others at hand. 



It has already been stated that S terry Hunt has shown that 

 the Laurentian rocks of Canada are intersected by two classes 

 of veins, distinguished as " granitic " and " calcareous/'' " Most 

 of their characteristic minerals are common to the two classes ; 

 and it is easy to trace a gradual change from the typical granitic 

 veins to those in which carbonate of lime is the predominant 

 mineral " * ; hence " it is impossible to draw any definite line 

 between " them. The fact is, the one passes by " insensible 

 degrees" into the other through the gradual decrease of mineral 

 silicates and the increase of carbonate of lime. 



The same authority has also shown that there are certain Lau- 

 rentian beds, "generally granitoid or gneissoid in structure, which 

 may be described as pyroxenites, from the prevailing mineral/ 1 

 pyroxene (augite) : this mineral is te sometimes nearly pure, and 

 at other times mingled with mica, or with quartz, and ortho- 

 clase, and often associated with hornblende, epidote, &c." The 

 beds are occasionally of great thickness, and are " then often 

 interstratified with beds of granitoid orthoclase gneiss, into 

 which the quartzo-feldspathic pyroxenites pass, by a gradual 

 disappearance of the pyroxene." " These peculiar strata, which 

 contain at the same time the minerals of the associated gneiss 

 and of the limestones/'' also " gradually pass, by an admixture 

 of carbonate of lime, into the adjacent crystalline limestones" t 

 those whose origin is under consideration : it is noteworthy 

 that, besides serpentine, they, too, contain the mineral species 

 of the granitoid gneiss and the pyroxenites. Thus the opposite 

 extremes of the Lauren tian series of rocks, from the essen- 

 tially silacid to the highly calcareous members, are linked 

 together by intermediate gradations. 



Another remarkable point remains to be noticed. It is stated 

 by Sterry Hunt that ' ' in their mineral character the calcareous 

 veins so closely resemble the stratified limestone that the dif* 

 ferent geographical relations of the two alone enable us in some 

 examples to distinguish between them," and that " the observer 

 will often find it difficult to determine whether a detached mass; 

 or an imperfectly displayed outcrop of crystalline limestone^ 

 belongs to a bed or a vein" J. 



* Geology of Canada, 18G6 (Dr. Sterry Hunt's Report, p. 191> 

 t Ibid. p. 185. t Ibid. p. 188. 



