No. 2.] FIELD DAY AT MONTARVILLE. 225 



of mixtures of two or more of them was then explained, showing 

 the difference between granite, trachyte, syenite, dolerite, diorite, 

 and basalt, all of which are erupted rocks. Many of these from 

 their peculiar jointed structure, occasionally show at their out- 

 crops a step-like arrangement, which has procured for them the 

 common name of trap, from a Swedish word signifying a stair. 

 It is applied indiscriminately to almost all ancient erupted rocks, 

 and has therefore little scientific value. Such rocks are closely re- 

 lated to the lavas of modern volcanos, which when solidified under 

 pressure resemble trachyte, dolerite, basalt, etc. As the source of 

 lavas is many thousand feet beneath the surface, the lower parts of 

 the lava columns must always be thus solidified. Dr. Hunt then 

 proceeded to explain that Montarville was the site of an ancient and 

 extinct volcano belonging to palaeozoic times, and that the crystal- 

 line rocks there seen were the basal portion of the former eruption 

 of lava. The whole valley around, Ibsing the northward extension 

 of the valley of Lake Champlain to the St. Lawrence, had in 

 palaeozoic times been filled with soft stratified rocks to a hei"-ht 

 much above the present summits of 3Iount Royal and Bjloeil, and 

 presented a plateau, above which probably active volcanos marked 

 the sites of the mountain just named, and of Rougemont, Yamas- 

 ka, Monnoir, and Montarville. In some cases, however, there is 

 reason to suspect that there may be masses of erupted rock which 

 never came to the surface, and hence did not appear as active vol- 

 canos in subsequent ages. The eroding action of the elements, 

 air and water, cut away the soft sedimentary rocks, and swept 

 away the volcanic peaks, leaving little more than the hard cores of 

 crystalline rock below, which were better able to resist the eroding 

 agencies. He called attention to the fact that the stratified sedi- 

 ments near their contact with the erupted rocks had been much 

 hardened, and still remained in place, preserving, however, their 

 fine grain, and showing fossil shells within a few inches of the line 

 of contact with the crystalline dolerite, near the spot where the 

 company stood. The slow disintegrating action of the air, water 

 and frost was shown in the crumbling of this crystalline rock be- 

 neath their feet, a process to which the oxydation of iron pyrites 

 which had given rise to white crusts of soluble sulphates of iron 

 and magnesia, contributed. The supposed source of erupted rocks 

 was briefly alluded to as probably the fusion of deeply-buried stra- 

 tified rocks, and the fact was noticed that not only different volca- 

 nos in the same region, but the same volcanos at different periods, 

 Vol. VI. G No- 2. 



