226 ICE ITS FORMS AND FUNCTIONS, 



there is occasionally witnessed the rarer phenomenon of 

 ground-ice, or that which gathers in thin sheets along the 

 pebbly beds of shallow lakes and streams. These pebbles, 

 losing their heat by radiation quicker than the water for 

 heat radiates through as it does from the surface of the 

 transparent water act as points for the formation of ice- 

 crystals, and these passing from stone to stone, shortly 

 convert the entire pebbly bed into a crust of ice. We have 

 seen in a shallow ford of a Scottish stream ground-ice fully 

 an inch in thickness when the stream itself was still flowing 

 and unfrozen. This ground-ice, when broken up by freshets 

 or other causes, floats down the stream, bearing with it its 

 burden of encrusted pebbles, and thus becomes in nature a 

 curious means of geological transport. Pebbly gravels may 

 thus be laid down in situations where no current of water 

 could carry them ; just as Deas and Simpson found the ice- 

 cake of the arctic shores driven forty or fifty feet above the 

 sea-level, and as it melted away, leaving long ridges of 

 beach -gravel at heights to which no ordinary wind -wave 

 could ever transport it. 



But important as the formation of ice on fresh-water 

 lakes and streams may be, it is as nothing compared with 

 the masses that accumulate on the surface of the arctic 

 and antarctic seas. Instead of forming at 32, ice does 

 not appear on salt water till the temperature has sunk 

 to 28J ; and then it goes on increasing, according to 

 Sir Edward Belcher,* at the rate of about half an inch 

 per day, during the long polar winters, often attaining 

 a thickness of 10, 15, and 20 feet. This enormous 

 crust, as it stretches unbroken over the ocean, is the 



* Sir Edward's observations were made in Wellington Channel (1852- 

 54) when in search of the missing Franklin Expedition. See his Nar- 

 rative for some curious and instructive facts respecting the formation, 

 character, and deportment of polar ice. 



