276 GEOLOGY. 



The summer wave. — The warm wave follows the analogy of the 

 summer wave of ice-free land surfaces much less closely. This is be- 

 cause of the low melting temperature of ice as compared with other 

 forms of sohd earth-matter. On this account the summer wave is bi- 

 fold. The one part travels downward by conduclion, the other by the 

 descent of water; the one has to do primarily with the temperature 

 before the melting-point of ice is reached; the other^ with the tempera- 

 ture after that point is reached; the first conforms measurably to the 

 warm wave affecting other sohd earth-matter, while the second is 

 governed by special laws. After the surface portion of the ice is 

 brought to the melting temperature, the additional heat which it receives 

 melts the ice and is transformed from sensible into potential heat. Ice 

 charged with water is potentially, but not sensibly, warmer than ice 

 which has just reached the melting temperature. 



The warm wave of conduction dies out below hke the cold wave. 

 The warm wave descending by the flow of water stops where the freezing 

 temperature of water is reached. In regions where the average tem- 

 perature is below freezing, the water-wave does not descend so far as 

 the wave of conduction, since the latter descends below the zone where 

 the melting temperature is found. 



The foregoing considerations warrant the generalization that glaciers 

 normally consist of two zones (1) an outer or upper zone of fluctuating 

 temperature, and (2) an under zone of nearly constant temperature. 

 The under zone obviously does not exist where the thickness of the ice 

 is less than the thickness of the zone of fluctuating temperature. This 

 may be the case in very thin glaciers in very cold regions, and in the 

 thin ends and edges of all glaciers. 



The temperature of the bottom. — The internal heat of the earth is 

 slowly conducted to the base of a glacier where it melts the ice at the 

 estimated average rate of about one-fourth of an inch per year. The 

 temperature of melting is a little below 32° Fahr. since pressure lowers 

 the mehing-point at the rate of .0133° Fahr. (.0075° C.) for one atmos- 

 phere of pressure. At the bottom of a mile of ice therefore the melt- 

 ing temperature is about 30.2° Fahr. (-1° C.) It is probable that in 

 all thick glaciers the temperature of the bottom is constantly main- 

 tained at the mehing-point. This may be indicated by the streams 

 which issue from beneath glaciers during the winter, though this cri- 

 terion is hardly decisive since the issuing waters may be derived partly 



