320 THE CANADIAN NATURALIST. [Vol. viii. 



the augite. It is rather dull green and can scarcely be said to 

 exhibit dichroism. In places it shows numerous fine lines run- 

 ning in several directions. 



Figures 1 and 2. — Grains of magnetite penetrated by blades of 



feldspar, (x 78.) 

 Figure 3. — Group showing a few of the varied forms which the 



magnetite assumes. (X 78.) 



IV. River Gagnon, Terrebonne County. (Plate, fig. 3.) The 

 specimen from this locality is coarser in texture than the last, 

 and of a dark grey colour. Its specific gravity is 3.013. The 

 dyke where observed by Mr. Selwyn (to whom I am indebted 

 for the specimen) cuts a band of gneiss, and is in all probability 

 of the same age as the Grenville ones, though it has not been 

 traced out. The examination of a thin section of the rock 

 shows it to be composed of plagioclase feldspar, augite, magnetite, 

 apatite and a little mica and viridite. The plagioclase shows 

 evidence of but little alteration, and much of it is striated as in 

 the case of the River St. Simon rock, and with polarised light 

 beautifully banded. The blades run in all directions, but do not 

 constitute as continuous a network as in the last specimen, since 

 the augite is much more abundant. Blades of the feldspar 

 frequently penetrate the augite, and occasionally also the magne- 

 tite. The augite is pale brown in colour, perfectly fresh, and 

 often dotted with what appear to be gas-or vapour-cavities. Its 

 cleavage is often well-marked and it occasionally shows twinning 

 (see figure). The magnetite is not very abundant and occurs in 

 irregular and often fantastic forms. The apatite and mica are 

 present in very small quantity, as is also the viridite. The 

 latter chiefly accompanies a brown somewhat decomposed mineral 

 which has not been determined. With polarised light the section 

 forms a beautiful object. 



