MUSEUM OF COMPARATIVE ZOOLOGY. 203 



To the above mentioned sources of heat we have clearly to add a 

 third element of action ; that which arises from the constant escape of 

 temperature from the earth's interior. Hhe amount of this is not well 

 ascertained. It is probably not sufficient to melt more than a few 

 millimeters of ice each year, but it has to be reckoned on in estimating 

 the influences which are at work to bring about melting at the base of 

 the glacier. From these three direct heat-giving actions, we are entitled 

 to assume that a certain amonnt of the ice at the base of a moving mass 

 of the material will inevitably be melted. In addition to these direct 

 influences we must also take into account the probability, indeed, we 

 may say the certainty, that the pressure of the superincumbent glacier 

 indirectly favors the liquefaction of the ice at the lower parts of the 

 section. As long ago as 1849, James Thomson theoretically estab- 

 lished the conclusion that the eff"ect of pressure was to lower the freezing 

 point of water.* Shortly after, Wm. Thomson experimentally proved 

 the verity of these theoretical considerations, and it now appears that 

 where a glacial sheet has a thickness of three thousand feet, the freezing 

 point of water is lowered to about 30°, or approximately two degrees 

 below that at which it congeals in ordinary atmospheric conditions. 



It may be regarded as fairly well established that the ice in several 

 points in Eastern New England attained a thickness of more than three 

 thousand feet ; it is possible, indeed, that when the glacial envelope was 

 most completely developed its depth much exceeded this amount. As 

 we do not yet know the temperature at which ice enters a glacial mass 

 such as recently covered the northern portion of this continent, we can- 

 not make sure that this pressure alone would be sufficient to bring 

 about melting at the base of the mass, even if it had the thickness of 

 a mile. There can be no doubt, however, that the effect arising from 

 the lowering of the melting point would be greatly to favor the liquefac- 

 tion arising from the sources of heat before mentioned. It seems to me, 

 therefore, that we are justified in assuming, at least as a working hy- 

 pothesis, the existence of large amounts of molten water next the bed 

 rock on which a deep glacier lies. This hypothesis seems to be ade- 

 quately supported by the physical conditions of our drift deposits. 

 Wherever these have been observed, we note the existence of many 

 accumulations of washed sand and gravel, commonly known as kames or 

 osars, which distinctly map out the position of subglacial streams of 

 fluid water, which often coursed for scores if not hundreds of miles 

 under the ice arches before they escaped at the margin of the glacier. 



* See Transactions of the Royal Society of Edinburgh, Vol. XVI., Part 5. 



