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 effect 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°. We 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 communicated 
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 
VOL. xvI.—no. 11. 3 
