On the Geology of the Island of Montreal. 207 



The white cement often contains carbonate of lime. When 

 the black and white ingredients are distributed with some uni- 

 formity, and in equal quantity, their strongly contrasting 

 shades give great beauty. It often traverses the other forms 

 of trap, in broad and straight, tapering bands, which issue 

 from large irregular masses, whose precise relation to the 

 surrounding rocks happens to be hid by vegetation or debris. 



The four forms of trap now sketched, together with their 

 numerous gradations, occur in the veins or dykes which form 

 so singular a feature in the geology of the environs of Mon- 

 treal. The second, or homogeneous kind, is most abundant ; 

 but it is here protected from the elements by a ferruginous 

 crust, and breaks into thick oblong slabs, with sharp, rhom- 

 boidal edges. I do not recollect the fourth form to exist as 

 a dyke. Near the north end of a limestone quarry, a few 

 hundred yards north of the race-course, there is a dyke of a 

 composition I have not observed on the hill. It consists of a 

 soft, passing into powdery, cement, white and green, (each 

 colour predominating in patches) which contains numerous 

 finely marked crystals of basaltic hornblende, distributed con- 

 fusedly, but equally, and not often confluent. The cement is 

 principally calcareous ; the remainder being green earth, 

 which frequently coats the crystals. 



These dykes are derived from the hill. The presence of 

 vegetation and of soil, prevents our tracing them throughout 

 to their source ; but their origin is betrayed by the identity 

 of their materials with those of the rock of the hill, by the 

 course of the majority, and by their being detected in several 

 instances issuing thence, and from its immediate neighbour- 

 hood. 



Dr. Macculloch, in his account of the group of trap islands 

 off the west coast of Scotland, describes several large accu- 

 mulations, depots, as it were, of amphibolic rocks, from 

 whence veins proceed in all directions for several miles into 

 ihe adjacent strata. The form and position of the ramifica- 



