72 THE WORLD'S LUMBER ROOM. 



rocks over and between which it passed, dropped them in a 

 heap at Muswell Hill and Finchley. 



Glacial drift of this kind has been found all over the 

 northern part of Europe and America, and one of the 

 huge old glaciers which then descended from Mont 

 Blanc, filling the whole valley of Aosta, a hundred miles 

 in length, has left behind it a " dust-heap," or moraine, 

 which is 1,600 feet high, and measures sixty miles in cir- 

 cumference. 



What was the effect upon the ocean of the withdrawal 

 and locking up of this vast quantity of water can only be 

 guessed ; but its depth must certainly have been reduced 

 considerably, some say by 600 feet, and some by as many 

 as 1,000. 



And when the great thaw came, and the water was set 

 free, there is good reason to believe that at least one continent 

 was not merely inundated, but altogether swallowed up, 

 and that what we call the West Indian Islands are really 

 just the highlands and mountain tops of an unknown 

 region which then disappeared beneath the waves, and may 

 have given rise to the various fables about the beautiful 

 enchanted land beneath the sea, which is variously known 

 as Atlantis, Tir-na-n-oge, &c. 



We have still to say something about icebergs as dust, 

 carriers. 



True icebergs, according to Professor Nordenskjold, 

 are those which rise more than 300 feet out of the water, 

 and are found only where the bed of the glacier is so steep 

 and uneven, and its motion so rapid, that it is really split up 

 into bergs long before it reaches the sea (Fig. 17). Even the 



