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 ex- 

 panding while still solid or while partially melting, while 

 others are at the same time crystallizing 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 ele- 

 vation or depression, the change to an opposite movement, 

 implying the substitution of a heating for a refrigerating 

 operation, or the revei'se, would not take place suddenly, 

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

 movement, or such a state of quiescence as prevails through- 

 out large areas of diy 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 refei;ence, indicate an}'- 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 and 



