1875.] 127 [Shaler. 



rentian Mountains to the Ohio, is not necessary to the explanation of 

 the facts. 



Mr. James Thompson has already shown from theoretical consid- 

 erations, that the 'influence of pressure in causing water to melt at 

 lower temperatures than 32° Fahr., is considerable; his paper, too 

 elaborate to be considered in detail here, leads to the conclusion that 

 for each atmosphere of pressure the freezing point would be lowered 

 by the amount of 0.0075 of a degree Centigrade. Now if we sup- 

 pose the surface of any country to be buried beneath an ice sheet, it 

 is clear that insomuch as a glacial mass of great thickness is gener- 

 ally nearly level on its surface, however irregular the earth beneath it 

 may be, it follows that the pressure at different points on the floor 

 of the glacier must vary more or less, according to the difference in 

 depth between the highest and lowest points of the earth surface. 

 Now assuming that the glacial sheet has a uniform temperature 

 throughout its lower portion, the gradually increasing pressure as the 

 ice continues to heap up, will bring about melting from the pressure 

 alone at the base of the glacier. The amount of pressure necessary 

 to bring about this melting will depend upon the normal temperature 

 of the ice at the point of contact with the earth; if the temperature 

 be assumed as 30° Fahr., then the ice must be about two miles thick 

 in order to cause melting by the pressure alone. The probabilities 

 are, however, that the temperature is generally nearer 32° than 30° 

 Fahr., so that the mass of ice would have to be much less thick in 

 order to bring about this melting action. It is hardly worth while to 

 undertake calculations as to the precise thickness of required ice on 

 this basis of reckoning; for the data are not sufficiently clear to ad- 

 mit of certainty as to the precise amount of pressure necessary to 

 lower the melting point of ice of a given temperature. It is evident, 

 however, that a thickness of ice may be readily attained which will 

 cause ice having a normal temperature of 28° to 30° Fahr., to melt 

 by pressure. Let us now consider what would be the effect of melt- 

 ing under these conditions. It is evident that inasmuch as the fluid- 

 ity of any water melted by pressure depends upon that pressure 

 being continued, the passage of this melted water upwards through 

 the crevices of the ice would not be possible; water mounting through 

 the crevices of the ice would at once have its pressure removed, and 

 would freeze again. The movement would evidently have to be in the 

 direction of the least resistance, or towards the section where the ice 

 was thinner than at the point of melting. The actual amount of the 



