On the Geology of the Island of Montreal. 203 



plentiful soil ; and is best brought into view about that town, 

 and at the rapids of the Recolet, and other portions of the 

 Riviere des Prairies. At a distance from the hill, it probably 

 rests on quartzose conglomerate, or sandstone ; as on the 

 former-, are based some of the islets in the Lake of the Two 

 Mountains, and the isle of St. Helen. Quartzy sandstone 

 discovers itself at the Cascades, St. Anne's, and in the bed of 

 the Lachine Canal. 



The trap may be said to consist chiefly of crystalline 

 hornblende, largely mixed with quartz, feldspar, mica, and 

 angite. The accidental minerals are those characterising 

 this order of rocks, as zeolite, &tc. Except where it assumes 

 the form of dykes or veins, as is common in the plain about 

 the hill, it is massive, and usually much weathered. On the 

 summit of the hill, and sometimes elsewhere, it rises above 

 the vegetation in naked flattish mounds, closely compacted, 

 and seldom exceeding five yards in diameter. In the small 

 ruinous ledges and cliffs, it is fissured perpendicularly, and 

 rent by cold, into large cuboid blocks. When quartz enters 

 into the composition of the trap in quantity, it becomes schis- 

 tose, but I have never observed real stratification. 



The varieties are very numerous, but may all be derived 

 from four principal forms, running into each other without 

 apparent order, and occupying indiscriminately all levels. 

 They do not only furnish a convenient mode of classification, 

 but also represent the kinds predominating. Of the four, the 

 two forms placed first in the following sketch are perhaps the 

 most plentiful. The magnet is not affected by any, except 

 when iron is visibly present. 



The first form consists of promiscuously aggregated, shin- 

 ing, black prisms, of hornblende, from half an inch to three 

 inches long, sometimes pure ; and so mutually compressed as 

 almost to disappear ; but more frequently white or ferruginous 

 masses of glassy feldspar and quartz, mixed in various pro- 

 portions, are interposed, so as to give the whole a strong it- 



