304 GLACIAL GRAVELS OP MAINE. 



down to the bottom of the previous winter's snow so as to raise it all to 32°. 

 If so, this very cold snow, sinking down toward the ground beneath the 

 pressure of later snows, would cause a tenipei'ature below 32° to prevail 

 ■downward to some unknown depth, where the heat of the earth would just 

 suffice to overcome it and cause a temperature of 32°. The isogeotherm of 

 32° might here lie not far above the ground, or even beneath it. 



It has sometimes been assumed that because the surface portions of 

 the highest parts of the n^v^ were found dry and powdery there is no 

 melting in that region. I am satisfied that inferences founded on observa- 

 tion of only the surface of snow are to be received with caution. I have 

 seen several places in the Rocky Mountains where water of surface melt- 

 ing filtered down through the snow, leaving the surface dry and powdery 

 and with no sign of surface melting, or with only a thin crust which 

 the wind soon blew away. In one such case a drift about 20 feet deep 

 had formed on the frozen ground. Soon after a warm wind melted con- 

 siderable snow, and then followed two weeks of very cold weather, when the 

 mercury stood at or below zero most of the time. The temperature of the air 

 was still below the freezing point when an excavation accidentally revealed 

 the fact that the lower part of the drift to a depth of 4 feet was moist and 

 part of it was almost slush. No stream or spring was here and the earth 

 beneath was frozen. It was evident that the moisture was due to water of 

 surface melting that had seeped down through the snow, leaving ho sign of 

 its former presence to the eye ot an unguarded observer. No limit can be 

 set to the distance that water will pass into snow as into sand, provided it 

 does not reach a stratum having a temperature below the freezing point. 



Summary. — All tliose parts of glaclers where there is enough melting to 

 furnish water and store more of it in summer than freezes in winter have 

 the constant temperature of melting ice irrespective of season. Over all 

 the zone of waste the glacier has the internal temperature of 32°, while the 

 temperature of the di-y surface ice varies with the seasons, but can never 

 rise, above 32°. Under this part of an ice-sheet the bottom of the ice is 

 never frozen to the ground, biit is bathed by at least a molecular film of 

 water. The ground and the subglacial till are here unfrozen. As we go 

 above into the area of accumulation the internal and basal temperatures 

 .are variable and uncertain. 



