CHAP. xiv. AND PROBABLE CAUSES OF MOVEMENTS. 287 



rocks have originated in the interior of the earth's crust. 

 All mineralogists are agreed that the passage of voluminous 

 masses, from a liquid or pasty to a solid and crystalline 

 state, must be an extremely slow process. It may often 

 happen that, in the same series of superimposed rocks, some 

 are expanding while still solid or while partially melting, while 

 others are at the same time crystallising and contracting ; so 

 that the alterations of level at the surface may be the result 

 of complicated and often of conflicting agencies. The more 

 gradually we conceive such changes to take place, the more 

 comprehensible they become in the eyes of the chemist 

 and natural philosopher who speculates on the changes of 

 the earth's interior ; and the more fertile are they in the 

 hands of the geologist in accounting for revolutions on the 

 habitable surface. 



We may presume, that after the movement has gone on for 

 a long time in one determinate direction, whether of eleva 

 tion or depression, the change to an opposite movement, 

 implying the substitution of a heating for a refrigerating 

 operation, or the reverse, would not take place suddenly ; but 

 would be marked by a period of inaction, or of slight move 

 ment, or such a state of quiescence, as prevails throughout 

 large areas of dry land in the normal condition of the 

 globe. 



I see no reason for supposing that any part of the revo 

 lutions in physical geography, to which the maps above 

 described have reference, indicate any catastrophes greater 

 than those which the present generation has witnessed. If 

 man was in existence when the Cromer forest was becoming 

 submerged, he would have felt no more alarm than the 

 Danish settlers on the east coast of Baffin's Bay, when they 

 found the poles, which they had driven into the beach to 

 secure their boats, had subsided below their original level. 



Already, perhaps, the melting ice has thrown down till 



