104 J. W. DAWSON — SOME RECENT DISCUSSIONS IN GEOLOGY. 



Vesuvius, and the cone of Cotopaxi, are built up of materi3.1s ejected 

 from below in the manner of mole-hills or the dump of a mine. As I 

 do not like the modern method of inventing grandiloquent names for 

 structural features, I shall call this class " dump mountains." The most 

 curious thing about them from our present point of view is the fact that 

 they do not crush dt)wn the crust under them as sedimentary deposits 

 would, and this, as any one can easily understand, depends on the cir- 

 cumstance that the very existence of such mountains is an effect of the 

 upward pressure of matter beneath them. It may be said that such 

 mountains are modern ; but it is true that some very old elevations are 

 remnants of the denudation of ancient piled-up cones. 



Another class of mountains, which may be named "blister mountains," 

 is produced by the gentle swelling up of the crust without any folding. 

 Such mountains are the Catskills, the western Sierra, some mountains of 

 old red sandstone in Scotland, and the high chain of Lebanon, which at 

 its summit, 10,000 feet above the sea, presents horizontal beds of lime- 

 stone falling away in mural precipices. Such mountains, unless sup- 

 ported merely by the heating and expansion of matter below, must be 

 sustained by the horizontal injection of mobile matter beneath them. 

 Hence the elevation of these mountains may imply much movement of 

 softened rock beneath the crust, of a kind altogether distinct from lateral 

 pressure at the surface. 



The greater and more typical ranges of mountains, however, like the 

 Alps and the Appalachians, are mountains of crumpling, showing evi- 

 dence of enormous lateral pressure proceeding from the adjoining sea 

 basins, and to this, it is now almost universally admitted, their elevation 

 must be in great part due. We must note here, however, that in all 

 great mountain ranges all these kinds of elevation are observed, for 

 mountain-making on the great scale has implied not only plication but 

 the elevation of plateaus and tablelands and volcanic ejections as well. 



Two momentous questions arise here : Whence the pressure ; and 

 why has it acted along certain determinate lines ? 



The last of these questions comes first in order of time, for it seems . 

 established, and in this country has been well illustrated by Hall, Dana 

 and Rogers, that the main lines of folding occur where the thickest sedi- 

 ments have been deposited along the borders of the oceans, and where, 

 consequently, the lower parts of such sediments have been pushed by 

 subsidence far down toward the heated interior of the earth. Again, 

 whatever reasons may be urged against such a conclusion, it is evident 

 that the crust underlying the oceans is the strongest of all, and that it 

 must have been the pushing or resisting agent. The mountain regions 

 of western America have, according to the Geological Survey of Canada, 



