64 THE WORLD'S LUMBER ROOM. 



slabs pile themselves one on the other, until a pack many 

 feet high is formed, which not only forces along great 

 boulders and blocks of stone some tons in weight, but also 

 breaks huge fragments from thirty to fifty feet square from 

 the cliffs, wharves, and stone buildings between which it 

 passes. 



It is not often that the Thames is even nearly frozen 

 over ; but a few winters ago it was for some days full ot 

 broken ice, among which were fragments covered on the 

 under side with gravel, as if they had been formed in the 

 bed of the river, and had then floated to the surface. In 

 many of the Siberian rivers large stones are very commonly 

 found thus embedded, and are carried down by the ice in 

 considerable quantities. 



We were explaining, a chapter or two back, how it was 

 that ice was formed, not at the bottom, but on the surface 

 of water ; but ice cannot easily form on a running stream, 

 owing to the constant motion, which is greatest on the sur- 

 face and in the centre of the current. Then, too, the whole 

 body of water, being kept in constant agitation, becomes so 

 thoroughly mixed that the temperature is the same throughout, 

 and when it falls to o C. (32 Fahr.), the comparative still- 

 ness in the bed of the river, and the contact with the cold 

 surface of rocks and pebbles, enable the ice to form there, 

 and in the warm sun it floats to the surface with whatever 

 may be adhering to it. In the small tributaries of the 

 Thames, vast numbers of pebbles, and even stones a foot in 

 diameter, are carried away by ground-ice, and other rivers 

 transport still larger quantities in this way. 



If all ice formed thus at the bottom of the water, our 



