On the Geology of the Island of Montreal. 205 



calcareous strata, effervesce slightly, though distinctly, on 

 exposure to acids. It is sometimes dotted with crystals of 

 white glassy feldspar, which, having increased to a good size, 

 in one case converted the mass into a porphyry. 



The third kind prevails as well on the hill, as in the veins 

 of the vicinity soon to be noticed. It is on some occasions 

 scarcely to be distinguished from the black trap of the Sabine 

 country, near Rome, containing solitary crystals of augite, 

 which I have seen in the collection of Mr. Greenhough of 

 London. It is the last described form crowded, (but some- 

 times only studded) like a porphyry, with crystals of greenish 

 black augite, alone, or interspersed with basaltic hornblende ; 

 both of large size, and well characterized;* and in weathered 

 masses, often showing their terminations. Among other lo- 

 calities, this form occurs on the declivity overlooking the 

 race-course, where it is chequered with spots of granular epi- 

 dote and white quartz. On La Prairie common, these augitic 

 crystals are almost acicular, in a brownish trap ; and also on 

 the Lachine Canal, where the rock assumes a peculiar, and 

 very handsome appearance. It is found in great quantity 

 near the country-seat of Edward Gregory, Esq. but cannot 

 now be examined, in situ, as it is covered by water. The 

 large fragments strewed around, however, show it to be 

 amorphous. It may be described as a confused assemblage 

 of short needle-shaped black crystals, cemented together, to 

 use a convenient phrase, by a fine mixture of pink feldspar 

 and white quartz. It is in parts, interspersed with numerous 

 round druses, averaging half an inch in diameter, lined with 

 octohedral iron pyrites, rhombic and pyramidal calcspar, and 

 green epidote in many but very small crystals. Mingling 

 with these druses, without regularity of situation, and in con- 



* The augite belonging to the A. triunitaire, of Hauy, as I learn from 

 Dr. Troost, whose skill in practical chemistry and crystallography has laid 

 me under frequent and pleasing obligation*. 



