262 W. H. COLLINS 



the deformed strata and intrusive batholiths in mountain regions 

 and in the flat, Precambrian Shield, been overlooked; but the 

 apparent total absence of actual mountains in the Precambrian 

 Shield, as well as its enormous area, exceeding that of any known 

 mountain system, have led geologists to accept the theory of Pre- 

 cambrian mountain-building with reservations, or even to prefer 

 the explanation that the Precambrian Shield is part of the original 

 crust of a once molten world. 



Let us compare the mountains of North America in respect to 

 their ages, their topographic aspects and the abundance in them 

 of batholithic rocks. The accompanying illustration (Fig. 2), 

 compiled by J. F. Wright, of the Geological Survey, Ottawa, shows 

 a series of representative cross-sections through the Rocky Moun- 

 tains, the Coast Mountains, the Appalachians, the stumps of the 

 ancient Acadian Mountains of Nova Scotia, and through a part 

 of the peneplain of northeastern Ontario. The sections are 

 arranged in order of age from youngest to oldest, are all referred 

 to sea-level and drawn to the same horizontal and vertical scales, 

 the relief being exaggerated twenty-one times. 



The Rocky Mountains, which date from early Tertiary time, 

 reach elevations of 10,000 to 12,000 feet, have an extremely sharp 

 profile and reveal no bathohthic intrusions, although some distance 

 south of the line of section the Ice River syenite mass does reach 

 the surface. The Coast mountains, of Jurassic age, reach about 

 7,000 feet above sea, and more than one-third of the section is of 

 batholithic granitic rocks. These mountains are scarcely less sharp 

 in profile than the Rockies; but that is due to the fact that they 

 have been elevated twice and are now being carved a second time. 

 The Appalachians were formed about Permian time, and have 

 suffered erosion for a correspondingly longer time than the Coast 

 and Rocky mountains. They reach only 2,800 feet above sea, 

 have a comparatively gentle skyhne, and erosion has laid bare a 

 larger proportion of their granite core. The Acadian Mountains 

 can be called such only in a genetic sense. They possess a relief 

 of only a few hundred feet, and their granite core is freely exposed. 

 They are the barely recognizable roots of mountains, and not more 

 deserving of the designation than those shown in the next and last 

 section. 



