326 Canadian Record of Science. 



an extent of country estimated at 200,000 square miles is 

 covered with nearly horizontal sheets of lava to a depth 

 of 4,000 or 5,000 feet. If accompanied by explosions or 

 great force, the molten matter, with ashes and stones, 

 would be thrown out, and a cone built up out of the 

 ejected materials. 



If, on the contrary, the fracture did not extend to the 

 surface, the molten matter would be forced upwards as 

 into a mould, where it would be cooled and consolidated 

 into a mass, the shape of the cavity which contained it. 

 If, then, in course of time, the overlying crust under which 

 it cooled should be removed, the intrusive mass, being 

 harder than the surrounding rock, would remain and 

 stand out as a hill or " Boss " of trap. 



There is reason for the opinion that Mount Royal 

 belongs to the latter class. Had the ordinary volcanic 

 phenomena been present, the resulting cone or sheets of 

 lava might have protected the softer limestone strata from 

 erosion, as is the case at various points on the mountain 

 where the limestone was overlaid by the trap, and thus 

 preserved the mountain from it, to some extent at least ; 

 but the strongest reason for this opinion is found in the 

 fact that the trap of which the mountain is composed 

 presents, as already noticed, the compact and highly crys- 

 talline structure, which it could scarcely have shown had 

 it not been consolidated under great pressure. 



Assuming this view to be correct, the eruption or ejec- 

 tion of the trap which now" constitutes the mountain 

 might have been accompanied by no external phenomena, 

 unless it were by earthquakes of a more or less violent 

 character, according to the nature of the disturbances, 

 which were probably of frequent occurrence, since the 

 principal mass of the mountain is composed of two dis- 

 tinct and different materials ejected at different periods, 

 while a succession of trap dykes and floors extending 

 for great distances from the mountain, and, in the case of 



