618 HISTORY OF GEOLOGY. 



former periods of the globe, can be judged to have 

 differed from the present state of things. 



The ingenuity and plausibility of this theory 

 cannot be doubted : and perhaps its results may 

 hereafter be found ndl quite out of the reach of 

 calculation. Some progress has already been made 

 in calculating the movement of heat into, through, 

 and out of the earth ; but when we add to this the 

 effects of the currents of the ocean and the atmo- 

 sphere, the problem, thus involving so many ther- 

 motical and atmological laws, operating under com- 

 plex conditions, is undoubtedly one of extreme 

 difficulty. Still, it is something, in this as in all 

 cases, to have the problem even stated; and none 

 of the elements of the solution appears to be of 

 such a nature, that we need allow ourselves to yield 

 to despair, respecting the possibility of dealing with 

 it in a useful manner, as our knowledge becomes 

 more complete and definite. 



