THE WORK OF SNOW AND ICE. 247 



compact and finally coherent, and grades into porous ice. This grada- 

 tion is accomplished at no great depth, though the thicknesses of snow 

 and neve are by no means constant. 



Structure of the ice. — Ice formed beneath a snow-field is in some 

 sense stratified. It is made up of successive falls of snow which tend 

 to retain the form of layers. This follows from two or three conditions. 

 The snow of one season, or of one period of precipitation, may have 

 been considerably changed before the succeeding fall of snow. So also 

 the surface of the snow-field at the end of the melting season is often 

 covered with a visible amount of earthy matter, some of which was 

 blown up and dropped on the surface during the melting season, and 

 some of which was concentrated in that position by the melting of the 

 snow in which it was originally imbedded. The amount of earthy 

 matter is often sufficient to define snows of successive years, or per- 

 haps of minor periods of precipitation, and makes distinct the stratifica- 

 tion which would otherwise remain obscure. The snowfall of successive 

 years has been estimated by this means^ where the snow is exposed in 

 crevices in the snow-field. 



In addition to its rude stratification, the ice of the deeper portions 

 often acquires a stratiform structure which may perhaps best be called 

 foliation to distinguish it from the stratification which arises from depo- 

 sition. The foHation appears to result mainly from the shearing of 

 one part over another in the course of the movements to which the 

 ice is subjected, as will be illustrated presently. 



Texture. — The ice derived from the snow is formed of interlocking 

 crystalline grains. The crystaUine character is present from the begin- 

 ning, for it is assumed by the snowflakes when they form, and the sub- 

 sequent changes seem only to modify the original crystals by building 

 up some and destroying others. By the time the snow is converted 

 into neve, the granules have become coarse, and wherever the ice derived 

 from the neve has been examined, the granular crystalline texture is 

 present. The individual crystals in the ice are usually larger than 

 those of the neve, and more closely grown together. In the fresh unex- 

 posed ice the crystals are so intimately interlocked that they are not 

 readily seen except under a polarizing microscope, but when the ice 

 has been honeycombed by partial melting, the granules become par- 



^ Russell. Nat'l Cxeog. Mag., Vol. Ill, pp. 127 and 181. 



