358 MAN AND THE GLACIAL PERIOD. 



the formation of ice takes place in two different manners ; 

 a thin crust spreads itself along the banks and over the 

 smaller bays where the current is least rapid ; but the 

 greater part is formed in the bed of the river, in the hol- 

 lows among the stones, where the weeds give it the appear- 

 ance of a greenish mud. As soon as a piece of ice of this 

 kind attains a certain size, it is detached from the ground 

 and raised to the surface by the greater specific gravity of 

 the water ; these masses, containing a quantity of gravel 

 and weeds, unite and consolidate, and in a few hours the 

 river becomes passable in sledges instead of in boats.' 

 Similar observations have been made in America ; but 

 instances need not be multiplied, as it is a common phe- 

 nomenon in all arctic countries, and is not uncommon on 

 a small scale even in our latitudes. 



" The two causes combined — torrential river-floods and 

 rafts of ground-ice, together with the rapid wear of the 

 river cliifs by frost — constituted elements of destruction 

 and erosion of which our present rivers can give a very 

 inadequate conception ; and the excavations of the valleys 

 must have proceeded with a rapidity with which the pres- 

 ent rate of erosion cannot be compared; and estimates of 

 time founded on this, like those before mentioned on sur- 

 face denudation, are therefore not to be relied upon."* 



Speaking a little later of taking the present rates of 

 river erosion as a standard to estimate the chronology of 

 the Glacial period, the same high authority remarks : " It 

 no more affords a true and sufficient guide than it Avould 

 be to take the tottering paces and weakened force of an 

 old man as the measure of what that individual was, and 

 what he could do, in his robust and active youth. It may 

 be right to take the effects at present produced by a given 

 power as the known quantity, a, but it is equally indis- 

 pensable, in all calculations relative to the degree of those 



* Prestwich's Geology, vol. ii. pp. 471, 472. 



