THE WORK OF SNOW AND ICE 227 



at a temperature of 26 to 28 Fahr., the variation being due to 

 the degree of salinity of the water. 



In polar regions the sea ice attains a depth of several feet, at 

 least as much as eight or ten. Floating ice of much greater thick- 

 ness is sometimes seen, but it is doubtful if these great thicknesses 

 represent the ice formed by the freezing of undisturbed sea-water. 

 At any rate, the ice formed in winter is often broken up in summer 

 into floating pieces called floe-ice (Fig. 185). Floe-ice is sometimes 



Fig. 185. Floe-ice on the shore of Greenland. 



crowded together in ice-packs, the separate pieces being so jammed 

 together that some of them are ended up and stand high above 

 the water. If the ice-pack of one summer is still far enough north 

 at the end of the warm season, it is frozen together, and its aggregate 

 thickness, made up as it is of blocks of ice some of which are on 

 edge, is far beyond that of normal sea ice. 



Snow-fields. Over the larger part of the land, the snow of winter 

 does not endure through the summer, and when it melts it follows 

 the same course as rain; but in cold regions where the fall of snow 

 is heavy, some of it remains unmelted from year to year, and con- 

 stitutes perennial snow-fields. 



High mountains and the lands of high latitudes are the common 

 habitats of snow-fields. In North America there are numerous 



