11)4 THE PAST AND FUTURE OF GEOLOGY. 



depressed below its present level to au exteut iu proportion to tbe dura- 

 tion of the glacial cold. On tbe return of the present more temperate 

 climate, that portion of the crust of the earth, measuring certainly many 

 hundreds, and possibly some thousands of feet in depth, which had 

 suffered from this abnormal loss of heat, would have to recover its 

 equilibrium with existing conditions by another change in the isother- 

 mal planes, and, until that was effected, little or no loss by radiation would 

 take place. 



Or to look at it in another way, let us suppose periods of equal tem- 

 perature before and after the glacial epoch. As the radiation of heat 

 is in proportion to the difference of temperature between the warm body 

 and the surroundiug medium, the loss of heat by the earth would, if no 

 colder period had intervened, have been nearly equal in equal times ; 

 but with the greater cold of the glacial epoch, the same result would be 

 effected in a shorter time, or, what is tantamount, the loss in the same 

 time during the glacial period would be greater than in the other two 

 periods. Thus supposing we take any given time of the glacial period 

 to be productive of a refrigeration of the crust equal to that which would 

 be effected iu a certain longer time of the pre-glacial or post-glacial 

 periods, then for a term of time — of length having a certain relation to 

 the difference between the two — succeeding the glacial epoch, the earth 

 would, with its outer crust so much below the normal, loose little or no 

 heat by radiation, so that during that subsequent period the thermo- 

 dynamical effects due to cooling would be reduced to a minimum or 

 cease altogether, and a period of nearly stable equilibrium, such as now 

 prevails, obtain. 



This last great change in the long geological record is one of so ex- 

 ceptional a nature, that, as I have formerly elsewhere observed,* it 

 deeply impresses me with the belief of great purpose and all-wise de- 

 sign, in staying that progressive refrigeration and contraction on which 

 the movements of the crust of the earth depend, and which has thus had 

 imi)arted to it that rigidity and stability which now render it so fit and 

 suitable for the habitation of civilized man ; for, without that immobil- 

 ity, the slow and constantly-recurring changes would, apart from the 

 rarer and greater catastrophes, have rendered our rivers unnavigable, 

 our harbors inaccessible, our edifices insecure, our springs ever-varying, 

 and our climates ever-changing ; and, while some districts would have 

 been gradually uplifted, other whole countries must have been gradually 

 submerged ; and against this inevitable destiny no human foresight 

 could luive prevailed. 



*rhilosophical Transactions for 18(54, p. 305. 



