On the Geology of the Island of Montreal. 209 



front, north east. They are compact, highly ferruginous, but 

 occasionally in their route, (as is common) assume the por- 

 phyritic form described third. In one case the hornblende 

 prisms are low and oblique, four sided, and at the same time 

 few and small. The paste here is brown ; the whole mass 

 strongly resembling the trap on parts of the river Richelieu. 



These veins have parallel sides, and are from one to three 

 feet in thickness ; but near Lachine, there is one fourteen feet 

 thick ; it is visible, however, for so short a distance, that its 

 dimensions cannot be given precisely. They do not thin off, 

 at least very sensibly. In fact, we cannot at present trace the 

 same individual further than four hundred yards. Sometimes, 

 as at the south east angle of the race-course, they seem to 

 meet with an obstacle, and collect into an irregular knot, from 

 ten to thirty feet broad, which again projects a number of tor- 

 tuous ramifications : the straight course having been abandon- 

 ed. These veins frequently subdivide, and again unite, en- 

 closing lenticular masses of conchiferous limestone, and some- 

 times of large size ; in all cases adhering to the trap, with the 

 utmost firmness, and although the division is marked by a fine 

 clear line, the carbonate of lime is detected in the vein by ap- 

 plication of acids. These insulated fragments suffer no change 

 except being a little harder ; and contain terebratula, encri- 

 nites, &c. whose state of preservation does not differ from that 

 of the shells imbedded at a distance from the trap : even the 

 nacre remains in some cases. These divisions are sometimes 

 quite minute, and separate the limestone for short distances 

 into thin ribbon-like laminre, parallel to each other and per- 

 pendicular. Sometimes the trap has insinuated itself, 

 horizontally in sheets, between the layers of limestone. 

 This is beautifully exemplified in an abraded bank on 

 the south side of the race-course, at the base of the 

 hill. Here the upper stratum is horizontal trap of paral- 

 lel sides, nearly two feet thick, and covered with earth. It 

 rests on the limestone in its usual position, pushing through it 



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