Studies in the Geology of Montreal and Vicinity. 67 



agents of decay. The piles of volcanic ashes, the crater, 

 the lava streams and other superficial features have long- 

 since disappearad ; only the hard basal portion of the 

 mountain has survived ; even now it is year by year fall- 

 ing to pieces, as can be seen at the foot of all the steep 

 slopes on the mountain side, notably that opposite the 

 head of McTavish Street. 



There were, speaking generally, three stages in the 

 history of the activity of the mountain, marked by the 

 outpouring of three different classes of rock. Each of 

 these can still be recognized and studied, and much 

 remains to be learned concerning them. At the first 

 eruption a dark-coloured basic rock was poured out, repre- 

 sented by the main body of Mount Eoyal, as seen in all 

 the cuttings on the upper part of the mountain and in the 

 cemeteries. This is a rock found in but few other places 

 in the world, and known as Theralite or Essexite. On 

 looking at a fresh fracture it is seen to be composed of 

 constituents, some of which are colourless and some of 

 which are black. The colourless ones are feldspar and 

 nepheline, the black ones pyroxene and hornblende. The 

 relative proportion of these minerals varies from place 

 to place, in that part of the mountain occupied by this 

 rock, the rock being in some places dull gray in colour, but 

 in others black. It often shows a distinct flow structure 

 which it acquired when moving up through the throat of 

 the volcano while still in a semi-fluid condition. 



After the rock of this first eruption had cooled, becom- 

 ing hard and solid, it was rent asunder and shattered, 

 undoubtedly with the accompaniment of violent earth- 

 quakes. On a line which runs along the back of the 

 mountain and up through this shattered zone there came 

 a second eruption, of rock of a different character. This 

 is much lighter in colour, a pale gray, and is seen excel- 

 lently exposed in the great quarry worked by the Corpo- 

 ration for road material at the back of the mountain at 



