O. D. von JEngeln — Studies on Ice Structure. 461 



surface to the center of the cake. Emden * has clearly shown 

 that ice grains as large as hazel nuts may be grown from snow 

 slush with originally small ice nuclei, kept in sealed vessels for 

 several weeks at a constant temperature of 0°C. In this case 

 growth proceeds from the center outward, the accretion and 

 orientation of each crystal being determined by the orientation 

 of the original nucleus. Accordingly the orientation of the 

 grains in the mass is irregular like that of glacier grains. 

 From these observations it seems safe to conclude that under 

 normal atmospheric pressure the orientation of ice crystals is 

 determined by the position of the original surface or center of 

 refrigeration, consequently the orientation of glacier grains is 

 determined by the position of the crystal axes in the original 

 snow and neve grains. 



As originally shown by McConnel and later more fully 

 formulated by Miigge and confirmed by the experiments of 

 Tarr and Rich, an ice crystal is made up of moleeularly thin, 

 flexible laminae lying in a plane normal to the principal axes. 

 Under pressure, with the ends of the principal axes supported, 

 these laminae will glide over one another and this gliding prop- 

 erty may account for such uniform orientation of ice crystals 

 in the basal layers of glaciers as may exist. It can not be 

 assumed that such uniformly oriented crystals develop and 

 grow in the direction of least resistance, in ice masses under 

 pressure, for similarly orientated mica flakes in slates and 

 schists which are presumed to have grown under like conditions 

 are distinctly tabular, whereas the uniformly oriented glacier 

 grains are not described as having any greater basal diameter 

 than adjacent ones that are irregularly oriented. On the other 

 hand, it is quite possible that the presence of a number of 

 similarly oriented crystals may be the result of shear along 

 the gliding planes brought about by differential pressure in 

 the general direction of the movement of the glacier. Re- 

 growth of the two parts of an individual crystal completely 

 separated by such shear would result in adjacent crystals with 

 similar orientation. This is in accord with Emden's contention 

 that the essentially progressive growth of glacier grains from 

 the neve to the ends of glacier tongues results from the 

 absorption of small granules by adjacent larger ones, and with 

 Deeley's assertion that the growth of glacier grains results 

 from the transference of molecules from crystal to crystal 

 under differential pressures in the ice. 



Water is at its maximum density at a temperature of 4 C. 

 and between that temperature and the freezing point, 0°C, it 



* Emden, Kobt. : Uber das Gletscherkorn, Denkschriften d. schweiz. 

 naturf. Ges., xxxiii, Zurich, 1892. Separates published by Ziircher and 

 Furrer, Zurich. 



