THE GREAT ICE AGE 377 



ice acted together, and along with aqueous agencies, in produc- 

 ing the complicated formations of this remarkable age. They 

 have, however, objected strenuously to the sole employment of 

 one agent to the exclusion of others, and to attributing to that 

 agent powers and extension which obviously could not belong 

 to it, under the known laws which regulate the movement of 

 glaciers by the force of gravity, and the precipitation of 

 moisture in the form of snow on mountains and plateaus. 

 These laws show that the movement of glaciers over level 

 surfaces, or against the slope of the ground, and their moving 

 stones otherwise than down slopes, are physical impossibilities, 

 and that the accumulation of snow to form glaciers can take 

 place only on elevated and cold land, supplied with large 

 quantities of vapour from neighbouring water. Such accumu- 

 lation can under no imaginable conditions take place in the 

 interior plains and table lands of great continents. 



Applying these laws and conclusions to the whole northern 

 hemisphere, we learn that the conditions to produce a glacial 

 period are the diversion of the warm currents from the northern 

 seas, the submergence of land in the temperate regions, and 

 its invasion by cold Arctic water, and great condensation of 

 snow on the higher lands. Whether this condensation has a 

 tendency finally to rectify the state of affairs, by pressing down 

 the mountains and elevating the plains, we do not know, but I 

 should imagine that it has not ; for the high lands will, in the 

 case supposed, be lightened by denudation, while the plains 

 will be burdened with a great weight of deposit. Perhaps we 

 should rather look to this as the agency for depressing and sub- 

 merging the plains and elevating the hills, and suppose some 

 other and more general pressure proceeding from the great sea 

 basins, to effect the re-elevation of the plains. 



These questions suggest that of the date of the Glacial period. 

 This subject has recently been discussed by Prestwich and 

 others, with the result that there is no purely geological ground 



