426 Pirsson — Petrography of Tripyramid Mountain. 



and the small amount of nephelite it contains would then be 

 a natural feature. Compare analysis No. V. 



If we consider the gabbro as genuine gabbro and this as an 

 attendant dike and, therefore, alkalicalcic, comparison witli 

 the analysis of the gabbro shows at once that this is aplitic (salic) 

 rather than lamprophyric in character. One must then com- 

 pare it with luciite and beerbachite; it differs from these 

 considerably in chemical composition, especially in potash. 

 See analyses Nos. VI and VII. Thus it differs both in min- 

 erals and in lacking the aplitic texture. 



On the whole, it is, perhaps, best considered as a very fine- 

 grained essexite and it will be further treated in the discussion 

 of the mutual relations of the rocks of the complex. 



Dike at Norway Rapids. 



Megascopic characters. — Holocrystalline ; dark, greenish 

 gray, dotted with dull whitish spots about 2 mm in diameter ; 

 megagranular, but fine; has a somewhat silky shimmer due to 

 abundant minute, slender feldspar laths, 2-3 mm long, diversely 

 scattered among the dark greenish granules of ferromagnesian 

 minerals, suggesting a minute ophitic texture. The whitish 

 spots are uniformly scattered, but have no definite boundaries, 

 and are neither phenocrysts nor amygdules; they have a 

 fibrous structure and are the scapolite mentioned later. Frac- 

 ture rough and hackly ; weathers brown. 



Microscopic. — The minerals observed in the section are : 

 apatite, iron ore, hornblende, and plagioclase ; these are 

 original, while epidote, scapolite, garnet, chlorite, and titanite 

 are present as alteration products. 



The hornblende was originally a rich brown, pleochroic 

 variety similar to that found in normal camptonites and 

 generally ascribed to barkevikite. It occurs in stoutly shaped 

 prismoids several millimeters long. It was originally interwoven 

 with compact laths of a plagioclase, apparently a labradorite, 

 but now too much altered for definite determination. Between 

 these, and also enclosed by them, were the grains of iron ore, 

 rather small and sparse, and prisms and grains of apatite. In 

 these respects the rock appears like a normal camptonite. 



From this condition the rock has been considerably altered, 

 the brown hornblende, save for occasional remnants, has been 

 bleached to a light green variety, and this has largely under- 

 gone a still further alteration into masses of green chlorite, a 

 pale epidote, and considerable titanite. " The occurrence of the 

 titanite is similar to that observed by the writer in the altern- 

 ation of a barkevikite-like hornblende in a dike rock from the 

 Belknap Mts.* and points to a considerable amount of titanic 

 * This Journal, vol. xxii, p. 503, 1906. 



