182 THE CANADIAN NATURALIST. June 



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DiORTTES. 



Yamaska. — It now remains to describe thediorites which hav« 

 already been noticed as forming several important masses among 

 the intrusive rocks of the Montreal group. In the first place may 

 be considered that of Yamaska. The greater part of this mountain 

 consists, as already described, of a micaceous granitoid trachyte; 

 but the southeastern portion is entirely different, being a diorite 

 made up of a pearly white crystnlline translucent feldspar, with 

 black brilliant hornblende, ilmenite, and magnetic iron. This 

 rock is sometimes rather fine-grained, though the elements are 

 always very distinct to the naked eye. In other parts are seen 

 large cleavage-surfaces of feldspar half an inch in breadth, 

 which exhibit in a very beautiful manner the striae character- 

 istic of the poljsynthetic macles of the 'triclinic feldspars. The 

 associated crystals of hornblende are always much smaller and 

 less distinct, forming with grains of feldspar, a b ise, to which 

 the larger feldspar crystals give a porphyritic aspect. Finer- 

 grained bands, in which magnetite and ilmenite predominate, 

 traverse the coarser portion <, often reticulating; and the whole 

 mass is also occasionally cut by dvkis of a whitish or brown. sh- 

 gray trachytic rock, which are often porphyritic, and may 

 perhaps be branches from the trachytic part of the mountain. 



A portion of th coarse-grained diorite sdected for examination, 

 eontained, besides th- minerals air. ady enumerated, small por ions 

 of blackish mica, with grains of pyrites, and a little disseminated 

 ■carbonate of lime, which caused the mass to . ffervesce slightly with 

 nitric aci I. The macled feldspar crystals, sometimes half an inch 

 in length, were so much penetrated by hornblen e that they were 

 n t fit for analysis ; but by crushing and washing the root, a por- 

 tion of the feldspar was obtained, which did not effervesce with 

 nitric acid, and contained no visible impurity, except a few scales of 

 mica; its specific gravity was 2.756 — 2.763. It was decomposed 

 hy hydrochloric acid, with separation of pulverul nt silica; and its 

 analysis, which is given under XX and xxi, shows it to be near 

 lo anorthite, and identical in composition with the feldspar of a 

 •diorite from Bogoslowsk, in the Ural Mountains. This is absocia- 

 Aed with a greenish-black hornblende containing some titanic acid, 

 with a little mica, and some quartz. (R. H. Scott, L. E. and D, 

 Philos. Magazine [4], xv, 518.) 



MoNNOiR. — Monnoir or Mount Johnson is composed of a diorite^ 



