60 THE WORLD'S LUMBER ROOM. 



transparently clear, that is in time of drought, when the 

 rivers are fed almost entirely by springs alone. 



The Thames, with its estuary, receives the drainings of 

 10,000 square miles; the Severn, of 8,580; the Colorado 

 of some 300,000 ; and the mighty Amazons, the " Mediter- 

 ranean of the West," of 2,048,000. It is because it is taken 

 from such wide areas that the large amount of rock annually 

 removed and carried off to the sea makes so little apparent 

 difference. Thus, more than 8,000,000 tons are invisibly 

 removed from England and Wales alone each year, and if 

 this were taken equally from every part of the surface, it 

 would be nearly thirteen years before a foot in depth was 

 carried away. Slowly, but surely, however, all land traversed 

 by streams, of whatever size, is being worn down and con- 

 veyed to the ocean ; and where the rocks are of chalk or 

 limestone, the work done is often perceptible enough. 



The rivers of the Teutoburger Wald and Haar, for in- 

 stance, annually take away more carbonate of lime than 

 would make a cube measuring 100 feet each way ; and in 

 sixty-seven days the Pader springs carry off enough to build 

 a cone 150 feet in diameter and twenty-four feet deep ; one 

 result of which is that landslips and subsidences are of con- 

 stant occurrence in their vicinity. 



Scattered about on the table-lands of Wiltshire and Dor- 

 set are accumulations of flints, sometimes several feet thick, 

 which once formed beds separated one from the other by 

 many feet of chalk, which has long since been dissolved 

 and carried away; and in many parts of the world vast 

 caverns have been hollowed out in the rocks by the agency 

 of water and carbonic acid alone. 



