60 VULCANISM ITS NATURE AND FUNCTION. 



and fauna of the region in which they occur. A few hun- 

 dred feet of elevation or depression in the higher latitudes 

 of the north is tantamount to a loss of several degrees of 

 annual temperature ; and we can readily conceive what 

 would be the effect of another thousand feet of uprise on 

 the existing flora and fauna of Siberia, Greenland, or the 

 Arctic islands of America. In fine, it requires no great 

 stretch of the imagination to conceive to what extent the 

 climatology of the globe may be influenced by a system of 

 extensive elevations and depressions of its surface ; how 

 the cold and warm currents of the ocean might be diverted; 

 how one region might be elevated so as to be permanently 

 enveloped in snow and ice, while another in the same lati- 

 tude might lie at so low a level as to enjoy the amenities 

 of a temperate climate. On the whole, whatever may be 

 the origin of these slow and gradual crust-motions, we be- 

 hold in them a system by which the distribution of sea and 

 land is changed, by which climate is modified, and conse- 

 quently by which the plant -life and animal -life of our 

 planet is materially affected. 



Such are the three great manifestations of vulcanism 

 the volcano, the earthquake, and the gradual crust-motion; 

 and though their origin be obscure, the human mind sel- 

 dom rests satisfied with mere description, but must attempt 

 a solution of cause and origin. There are two principal 

 hypotheses that have been advanced to account for vulcanic 

 phenomena, and which may be respectively termed the 

 mechanical and the chemical. By the former, the whole 

 is resolved into an aboriginal igneous condition of the 

 earth's mass, on which, after the lapse of ages, a cooled and 

 rocky crust has been formed over a molten interior. To 

 oscillations in this molten interior, to its reactions upon 

 the crust, to the cavernous structure of the crust, and to 



