8o THE WORLDS LUMBER ROOM. 



argument may be reasonable as far as it goes (though even 

 then there are other matters to be considered), and we 

 may be further justified in adding that it is the changes 

 taking place in this molten mass which cause the earth's 

 crust to rise and sink. 



But we cannot prove anything of the kind, and our 

 knowledge of the two thousand feet or so which we have 

 partly explored at a few scattered spots does not give 

 us any information as to what goes on, or as to the tem- 

 perature which may prevail at the depth of even a hundred 

 miles. 



All that we can say is that, whatever may be the state 

 of the interior of the globe, the heat at some spots, and 

 these comparatively near the surface, is great enough to 

 melt rock, and to keep water not only hot, but in many 

 cases boiling. 



It is possible, therefore, that the slow movements in 

 the earth's crust may be due to the cooling and re-melting 

 and consequent contraction and expansion of miles of rock ; 

 but whether this is really so we are quite unable to say. 



Neither are we much better off when we try to find 

 out the causes of those sudden and violent movements 

 to which we give the name of earthquake. These are 

 so explosive in their character that one might at times 

 imagine them to be occasioned by the sudden conversion 

 into steam of water which has filtered downwards until 

 it has come in contact with heated rock ; but whatever 

 the cause "an earthquake," says Prof. Huxley, "is just 

 such a disturbance of the ground as would result from 

 a sudden shock or blow given upwards in the interior of 



