,9o8.] THE PHYSICS OF THE EARTH. 201 



enable us to decide whether the substratum is plastic only, or 

 truly viscous. 



§ 23. Substratum Everyzvhere Quiescent Except when Disturbed 

 by Earthquakes. — The fact the large areas of the earth's surface in 

 such dry countries as Sahara, our Western Plateaus, and the interior 

 of Australia, are quite free from earthquake disturbances, shows 

 what would happen everywhere but for the presence of surface 

 water, and especially the leakage of the crust depending on the sea. 

 The quiescence of the substratum in interior regions remote from 

 the sea shows that under normal conditions this layer is quite inert. 

 It is only set in motion by the vapor of steam which slowly develops 

 stresses in the rocks of the crust and finally brings on earthquakes. 

 It might be plastic enough to yield slightly under sufficiently great 

 forces, but the loading and unloading due to meteorological and 

 geological causes going on in nature are not great enough to have 

 any appreciable effect, as we may infer from the universal quiescence 

 of inland areas, especially in desert countries. 



It seems to be true, however, that when the crust is broken and 

 upheaved, in the formation of mountains near the sea coast, some 

 slow yielding takes place beneath. Yet at present any changes of a 

 creeping nature can not be entirely separated from those depending 

 on the expansion and expulsion of lava from under the sea ; and 

 we can only feel sure of the inert character of the substratum, except 

 where disturbed by water vapor entering from wathout. Along the 

 sea coasts the stresses in the crust are constantly changing, and the 

 crust blocks yielding more or less to the stresses acting upon them ; 

 it is only when sudden yielding occurs that we experience a shock, 

 and the greatest earthquakes are characterized by molten rock ad- 

 justing itself beneath the crust. It is probable that much yielding 

 takes place which is exceedingly gradual and produces no disturb- 

 ances sensible to ordinary obser\'ation. In dry regions remote from 

 the sea there are no shocks, and therefore also no gradual yielding 

 of the crust ; hence the substratum is inherently and naturally qui- 

 escent except when disturbed by external forces. 



§ 24. Mountain Formation in the Sea and on the Land. — In the 

 paper on the " New' Theory of Earthquakes and Mountain Forma- 

 tion," we have cited certain cases of mountain formation now going 



