Was Mount Royal an Active Volcano ? 327 



the dykes, frequently cutting each other, show that the 

 activity was successive and long continued, although 

 proceeding far below the surface of the country. 



But while the mountain was thus being formed in the 

 depths beneath, a different process was going on at the 

 surface. The comparatively soft sedimentary rock was 

 being dissolved and eroded by the action of the elements, 

 until in course of time the limestone had been removed 

 to nearly its present level, and the great mass of lava, 

 being hard enough to resist the action which dissolved 

 the limestone, stood out as a mountain possibly of much 

 greater height than at present. 



This process of denudation must have continued for a 

 long period to have removed the great thickness of strata 

 which probably existed, and it might possibly have begun 

 about the middle of the period of 6,000,000 years already 

 referred to. 



Following this erosion came a subsidence of the land, 

 during which the conglomerate of the Lower Helderberg 

 group was laid down, of which St. Helen's Island and 

 E,ound Island, with the' exception of a few scattered out- 

 lying patches, present the only examples in the neighbor- 

 hood of Montreal. 



This formation, according to Logan, " Geology of 

 Canada," pp. 669, 358, rests unconformably on the Lower 

 Silurian strata, and is cut by dykes of dolorite, which 

 show that the volcanic activity had not ceased even at 

 that period. This formation found on St. Helen's and 

 Eound Islands presents a most diflicult question for solu- 

 tion. The conglomerate is composed of fragments, some- 

 times rounded, but for the most part angular, of Laurentian 

 gneiss, white sandstone, Trenton limestone, black and red 

 shales, and red sandstone, besides fragments of igneous 

 rocks, the whole cemented together by a paste of gray 

 dolomite, covering the island, and rising into a hill, the 

 cn-eatest heidit of which is about 125 feet above the. river. 



