ORIGIN OF MOUNTAINS. 811 



solidity at centre. They demand that the crust, or the exterior, should 

 have freedom to change its surface-level through the action of forces 

 within or beneath it ; that the general refrigeration which has taken 

 place should have left the sphere with a sufficiently flexible crust, or 

 movable or expanding exterior, to allow of all the elevations and de- 

 pressions which geology makes known. 



If the crust is of such a nature and so thin as to admit of a slight 

 degree of warping, geology has then a basis for explaining the great 

 Pacific Coral Island subsidence of the Quaternary ; the slowly pro- 

 gressing subsidence of the Appalachian region, taking all of Paleozoic 

 time to bring it to an end and a crisis ; the high-latitude subsidence dur- 

 ing the era of melting of the great glacier, or in the Champlain period ; 

 the geanticlinal or upward movement, which raised the Rocky Mountain 

 region, or portions of it, over 10,000 feet since the Cretaceous period, 

 and made like movements over other continents ; the high-latitude ele- 

 vation which brought the continents up to their present level after the 

 Champlain period of the Quaternary. Again, the folds and upturnings 

 accompanying mountain-making, as illustrated in the Appalachians, 

 Juras, Alps, and other ranges, show that they were made by a lateral 

 shoving of the beds ; and lateral thrust is one of the certain results 

 from such conditions. Moreover, they were made simultaneously over 

 regions covering many thousands of square miles ; and action on so 

 grand a scale also would be a necessary consequence. Connected 

 with such foldings and upturnings, there have been great fractures, 

 and faults of 10,000 to 20,000 feet; and there also would be other 

 natural results. Further, the increasing temperature of the earth's 

 crust downward (p. 717), and the very wide distribution of volcanoes 

 and regions of igneous ejections over the globe all harmonize with 

 the idea of a thin crust. 



If, on the contrary, the crust is not thin enough to make the geosynclinals and 

 geanticlinals, which facts prove to have taken place, appeals have to be made to other 

 conditions ; and the more reasonable of those hitherto presented are the following : — 



1. Change in the temperature of great regions beneath the crust, producing expan^ 

 sion and rise with its increase, and contraction and subsidence with its decrease. 



But movements in heat could not take place without an adequate cause, and they are 

 not shown to be a possibility. They are far less explicable in a globe having a rigid 

 crust than in one having a thin crust over a liquid or plastic interior. 



In the case of the Appalachian geosynclinal, the trough became filled with sedimen- 

 tary beds as the subsidence went forward, and attained at last a depth of 30,000 of 

 40,000 feet; and it has been supposed that the heating up of the bottom beds (as ex- 

 plained on page 718) may have caused their expansion, and thus produced the folds 

 and flexures now observed in the rocks. But this expansion, and therefore the folding, 

 should have gone on cotemporaneously with the deposition, or with its later part at 

 least. Far from this, there is conformability from the bottom to the top. It has been 

 said that this heating from below might tend to raise the surface, and be a cause of 

 oscillations of level; but in the case of such geosynclinals, the subsidence in progress 

 overbalanced action of this kind if any took place. 



