O. D. von Engeln — Studies on Ice Structure. 469 



to be at all adequate. The average annual temperatures of the 

 snow fields vaiw according to latitude and elevation. As snow 

 is a poor conductor of heat it may be that quite low tempera- 

 tures are imparted to the body of the glacier from its upper 

 reservoir sources. Whether, during the course of its movement 

 from the neve source to the end of the tongue, there is enough 

 inflow of earth heat at the bottom of the ice to bring it to the 

 pressure-temperature melting point would depend on the orig- 

 inal degree of cold, the length of the glacier's course and its 

 rate of flow. In any event, earth-heat would tend to raise the 

 temperature in some measure. Since below the neve line 

 sufficient heat is supplied to melt some of the ice, it follows 

 that the surface temperatures of a glacier in summer must be 

 at the melting point. The amount of surface melting is repre- 

 sentative of the excess of heat supplied over that capable of 

 being conducted from the surface, at the melting point, to the 

 colder masses of the interior ice. Such conduction will con- 

 tinue until the interior portions have the temperature of the 

 pressure-determined melting point. But as the near-surface 

 portions will acquire this temperature earlier than those at 

 greater depths, the conduction of heat to the deeper interior 

 parts will be progressively slower from the surface downward. 

 In other words, an increasing proportion of the excess heat will 

 be utilized in near-surface melting. Thus, while in small gla- 

 ciers, like the Hintereisferner, pressure-temperature equilibrium 

 may be established for a considerable thickness of the ice 

 tongue, it does not follow that this is the case for all the mass of 

 glaciers of great thickness. Yet it may be concluded that, 

 while it is by no means certain that the bottom temperatures of 

 great ice masses are at the pressure-temperature melting point, 

 it is improbable that they exhibit any extreme degree of cold. 

 Vallot found that the process of glacier-granule growth was 

 distinctly in evidence in the neve of Mont Blanc at depths 10 

 to 13 meters below the surface (hence below the zone of sur- 

 face water infiltration) where constant annual temperatures of 

 — 16.6°C. prevailed. Emden* has clearly demonstrated that 

 ice granules as large as hazel nuts may be grown from origi- 

 nally small, snow-slush nuclei, kept in sealed vessels for several 

 weeks at a constant temperature of 0°C. This growth he 

 contends is due to the absorption of small granules by adjacent 

 larger ones with consequent molecular readjustment. Deeleyf 

 also insists that the growth of glacier grains results from 

 the transference of molecules from crystal to crystal under dif- 

 ferential pressures. 



*Emden, R. : Uber das Gletscherkorn, Denkschriften d. schweiz. naturf. 

 Ges. xxxiii, Zurich, 1892. Separates published by Ztircher and Furrer, 

 Zurich. 



fDeeley, R. M. and Fletcher, G.: Structure of Glacier Ice, Geol. Mag., 

 Decade IV, ii, p. 155, 1895. 



