194 THE WORLD OF LIFE 



Thickness of the Earth's Crust 



We now have to consider a quite different set of phenomena 

 which have a very important bearing on the causes which have 

 produced the elevations and depressions which have occurred 

 over much of the land surface of the globe. It is a universal 

 fact that as we descend into the crust of the earth (in deep 

 wells or mines) the temperature rises at a tolerably uniform 

 rate, which is found to be on the average one degree Fahr. for 

 eveiy 47% feet. This rate, if continued downwards, would 

 reach the temperature of melted rock at a depth of about 20 

 miles. Hot springs in non-volcanic countries furnish an ad- 

 ditional proof of the high temperature of the interior. Below 

 the depth above indicated there would probably be some miles 

 of rock in a plastic state, while irregularities w^ould result 

 from the nature of the rock, some being more easily melted than 

 others. 



^ow, it has been ascertained that the various rocks of the 

 crust are of less specific gravity in the solid state than when 

 they are liquefied, so that the crust may be looked upon as 

 actually floating upon the liquid interior, very much as the 

 polar ice-sheets float upon the ocean. A curious confirmation 

 of this has been given by measurements of the force of gravity, 

 which show that near all great mountain masses gravity is di- 

 minished, not only by the amount due to the mass of the moun- 

 tain itself, but to about double that amount. This is so uni- 

 versally the case that it has been concluded that the weight of 

 the mountain mass is supported by a corresponding mass 

 forced down into the fluid magma, and hence termed the 

 ^^ roots of the mountains " ; just as every lofty iceberg must 

 have a mass of submerged ice about nine times as great to sup- 

 port it in the water. This, of course, proves that the crust is 

 flexible, and that just as any portion of it is upheaved or made 

 thicker by additions above, a corresponding increase in thick- 

 ness must occur below to keep it in equilibrimn. 



Thus are explained the ver^^ frequent phenomena of hori- 



