FORCE ENGAGED IN MOUNTAIN-MAKING. 813 



the radius is two inches, and the crust one twentieth of an inch. It is 

 well for those reading on the subject of the making of mountains to 

 put the fact in mind that an elevation on the earth of Jive miles, or 



Fig. 1159. 



26,400 feet, would correspond in this section to a four-hundredth of 

 an inch, or to but twice this, if the height is measured from the bot- 

 tom of an ocean five miles deep. 



1. Working Agencies. — Owing to the circumstances attending the 

 original solidification at surface, causing a gradual cooling down of 

 the liquid material to a considerable depth, there is probably a densely 

 viscid or pasty region, of great thickness, beneath the crust. In such 

 a globe undergoing continued refrigeration, the causes of mechanical 

 action are chiefly the following: (1) Gravity; (2) Vapors rising from 

 the viscid material ; (3) Lateral pressure or tangential thrust of the 

 crust, due to the contraction beneath and against it. 



1. Gravity. — In a globe of the size of the earth with a thin crust, 

 the weight of the crust is not sustained, in the opinion of physicists, 

 by its arched condition. It was hence suggested by Herschel that an 

 accumulation of sedimentary beds to a great thickness over large areas 

 might be accompanied by a gradual sinking of the crust ; and a sinking 

 equal in depth to the accumulation, and concurrent with it, has been 

 assumed by some writers. But this latter effect could be produced 

 only on the condition that the liquid beneath the crust is a fluid with 

 little friction between its particles ; and but slight effect would result 

 if it were densely viscid. 



With regard to the power of the crust to sustain weights laid upon 

 it, facts are afforded by the existence of lofty mountains in spite of 

 the yielding conditions underneath. By or before the close of the ear- 



