Pirsson — Petrography of Tripyramid Mountain. 421 



Classification. — The greatest amount of variation in any 

 mineral is shown in the quartz ; in some instances it is fairly 

 abundant and these might be classed with granite aplites ; in 

 others it sinks to a negligible quantity and such are syenite 

 aplites; on the average the group contains much less of it than 

 normal granite aplites and is, therefore, classed as quartz- 

 syenite aplite. In the quantitative classification these rocks 

 are very clearly persalic and peralkalic, and vary between 

 liparase and nordmarkase ; the relation of soda to potash is not 

 known, but the feldspars show so little evidence of being 

 dosodic that it is probable the rocks vary in subraug from 

 liparose to phlegrose. These dikes are closely allied to the 

 syenite ; they differ from it in being, on the whole, somewhat 

 richer in quartz, and in their lack of hornblende. 



Lampkophyre Dikes. 



Owing to the thick covering of glacial debris, soil and vege- 

 tation on the lower slopes of Tripyramid and in the granite 

 surrounding it, there are few places exposed in the outer zone, 

 which would be the natural habitat of these rocks, where thdy 

 can be sought for. They may, or may not, exist in consider- 

 able numbers, but they have been found only in the granite at 

 Norway Rapids as camptonite, and in very narrow dikes cutting 

 the gabbro at the Black Cascade on Slide Brook, as previously 

 mentioned. 



Slack Cascade Dikes. 



Megascopic. — Holocrystalline ; fine-grained, average much 

 less than one millimeter, compact but perceptibly granular ; very 

 dark greenish gray ; shows a few rare occasional phenocrysts 

 (perpatic fabric) of dark, smoky feldspar phenocrysts with 

 nearly square to rectangular outlines 4 by 4 to 5 by 3 mm . 

 With the lens a fine intimate mixture of feldspar, sometimes 

 in minute laths, and ferromagnesian minerals, indeterminate 

 save for an occasional speck of biotite. Feldspar phenocrysts 

 show fine albite twinning. Very tough, rings under the 

 hammer, has a splintery fracture approaching conchoidal ; 

 weathers dark green, turning brown. 



Microscopic. — In the section the following minerals are 

 disclosed ; apatite, iron ore, biotite, olivine, augite, labradorite, 

 orthoclase and nephelite and serpentine. 



The apatite is freely distributed in minute, very long, slen- 

 der needles, which are enclosed in the other components and 

 peg them together. It is inferred from the section that the 

 amount of P 2 6 shown in the analysis is rather low and 

 probably because the complete enclosure of the very minute 

 apatite needles in other mineral grains prevented their going 



Am. Jour. Sci— Fourth Series, Vol. XXXI, No. 185.— Mat, 1911. 

 29 



