GENERAL REVIEW. 229 



over the outer depths of the ocean, while the river-sediment 

 is merely fringing the inner shores. 



Such is a brief review of the various aspects in which ice 

 occurs, and the more prominent functions it appears to per- 

 form in the economy of nature. As snow in the lower 

 grounds, it acts as a protecting blanket against the severity 

 of long-continued frosts ; as snow in the higher regions, it 

 passes into neve and glacier to grind and round the rocky 

 surface in its descent, and to smooth into gentler outlines 

 the asperities over which it passes in its slow but irresis- 

 tible progress. As the liquid stream erodes and deepens its 

 channel, so the ice-stream rasps and chisels the function 

 of both being to wear and degrade the old rocks, and to 

 transport the material for the formation of the new. As 

 ice on water, its greater bulk, as compared with that of the 

 water from which it is formed, enables it to float as a 

 protecting surface, preventing the water below from being 

 entirely frozen, and thus preserving a habitable medium, no 

 matter how intense the cold, or however long it may be 

 continued. As ice on water also (the iceberg) it becomes 

 a geological carrier, transporting to the outer depths of the 

 ocean the gravel and shingle and boulders of the rocky 

 shores, and piling them up in long submarine reaches ac- 

 cording to the set of the tides and currents by which they 

 are mainly directed. As ice in the rocks and soils, it is ever 

 splitting and disintegrating; unless within the limits of 

 perpetually frozen ground, as in the tundras of Siberia and 

 the swamps of Arctic America, and there it exercises a con- 

 servative effect* binding the softest soils as hard as rocks, 



* Even within these icy flats the power of frost is sometimes curiously 

 destructive. " The influence of the cold," says Von Wrangell, speaking 

 of the December temperature of Siberia, which was 58 below freezing, 

 " extends even to inanimate nature. The thickest trunks of trees are 

 rent asunder with a loud sound, which in those deserts falls on the ear 



