MUSEUM OF COMPARATIVE ZOOLOGY. 215 



the facts of the ice movement in the Swiss glaciers, that pressure melt- 

 ing plays a considerable part in determining the movement of those 

 relatively small ice streams. So far as I am aware, however, no in- 

 quirers have endeavored to ascertain the efiect of pressure melting on 

 wide-spread and deep sheets of ice. 



It is now tolerably clear that during the last glacial epoch a large 

 part of the field occupied by the continental glacier was buried to the 

 depth of about a mile beneath the accumulations of frozen water. If it 

 ■were necessary for our purpose, it could readily be shown that the thick- 

 ness of the sheet was probably much greater than six thousand feet, but 

 the pressure which a mass no more than a mile in depth would bring 

 upon the surface of the earth would be sufficient to lower the freezing 

 point to about 30° Fahrenheit. We cannot ascertain at what tempera- 

 ture the accumulations of snow were built into the mass of the glacier. 

 There is, however, reason to believe that the initial heat. was not much 

 below the freezing point of water. It would not, however, militate 

 against the hypothesis to suppose that the mean annual temperature of 

 the surface of the glacier, and consequently that of the accumulating ice 

 sheet,- was as low as 25° or even 20°. AVe have next to note, that, with 

 the progressive deposition of snow, the layers formed each year would be 

 brought nearer to the bed rock, which process would lead to a constant 

 increment in the pressure which they sustained from the superincumbent 

 material. Thus the melting point of the ice would be progressively 

 lowered. 



Not only does the progressive descent of the ice towards the bed rock 

 serve, through the influence of pressure, to bring the material ever nearer 

 the melting point, but with each stage of the down-going the particles 

 come nearer to that portion of the mass where several different causes 

 act together to produce a positive increase in temperature. There is 

 little doubt that the shearing movement of the ice due to the friction 

 of its mass upon the surface of the earth progressively, and at last very 

 rapidly, increases as we approach the base of the glacier. This inter- 

 stitial motion is necessarily attended by the conversion of a great part 

 of the energy of position of the mass into heat, which is^commuuicated 

 to the neighboring ice, and on account of the slight conductivity of the 

 material escapes towards the surface in a very slow manner. Next 

 the bed rock the actual friction of the ice upon the base over which it 

 moves, and the abrasion of the rock, convert yet more of the force 

 which leads to the motion of the glacier into heat. To these sources of 

 temperature we must add the slight but not unimportant -effect of the 



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