298 Turner — Rock-forming Biotites and Amphiboles, 



to be quite pure. The phosphorus pentoxide (P 2 5 ) shown 

 in the analysis must, however, be ascribed to apatite, and this 

 should be deducted from the amphibole analysis together with 

 a corresponding amount of lime. In addition minute black 

 grains may be noted in the amphibole itself. These are prob- 

 ably magnetite, but the amount thus included must be small, 

 since the powder was treated with an electro-magnet and the 

 more highly magnetic grains removed. No titanite or other 

 titaniferous mineral were found in the thin sections of gabbro 

 1970 or in the amphibole powder. The titanic oxide (TiO,) 

 shown in the analysis therefore probably belongs to the amphi- 

 bole. The pleochroism is strong in brown and greenish brown 

 colors with large extinction angle. 



Amphibole-gabbro 1970 is from Beaver Creek about 18 klm. 

 east of Big Trees P. O. in the Big Trees quadrangle. It is 

 composed chiefly of labradorite and amphibole but contains a 

 little pyrite and pyrrhotite. 



Amphibole 2652 is exceptionally pure. It is pleochroic in 

 dark green tints with large extinction angle. There are some 

 minute black grains included in the amphibole. These are 

 probably magnetite. One foreign particle only was noted in a 

 slide of the powder. As before stated, this amphibole comes 

 from the same block of quartz-monzonite as biotite 2652. 

 The specific gravity of amphibole 2652 at 21*5° C. is 3*203. 

 Analysis 2179 is that of an average quartz-monzonite of the 

 region. It is well known that the metasilicates of rocks rich 

 in alkalies are sometimes themselves rich in alkali. This relation 

 does not appear to obtain in the case of amphiboles 1970 and 

 2652, the former of which comes from a basic rock poor in 

 alkali, and the latter from an acid rock rather rich in alkali, 

 yet there is more alkali in the amphibole from the basic than 

 in that from the acid rock. 



There is also added to the table an analysis of an amphibole 

 from the Donegal granite* as well as one of the granite itself. 

 It is evident from the analysis of this granite that the rock is 

 really a quartz-diorite or granodiorite and not a true granite. 

 The biotite from the Donegal granite is remarkable for the 

 very high content of ferric iron — 19*11 per cent. 



* Haughton, Quart. Jour. Geol. Soc. Loud., vol. xviii, 1862, p. 416. 



