PLAINS 153 



may continue for an indefinite period with no material 

 change of surface. But, should they be upraised, the 

 elevation, by increasing the slope of the streams, 

 augments their erosive power, and enables them once 

 more to deepen their channels. Hence, plains like 

 that of the New Forest, which have been trenched by 

 the water-courses that traverse them, may with proba- 

 bility be assigned to a time when the land stood at 

 a lower level than it occupies at present. In this 

 connection the successive river-terraces of the country 

 deserve attention. They may be due not to the mere 

 unaided work of the rivers, but to the cooperation of 

 successive uplifts. It would be an interesting inquiry 

 to correlate the various river-terraces throughout the 

 country, for the purpose of discovering whether they 

 throw any light on the conditions under which the 

 most recent uprise of the country took place. That 

 the elevation proceeded intermittently, with long 

 pauses between the movements, is shown by the 

 succession of raised beaches. It may be possible to 

 establish a somewhat similar proof among our river- 

 terraces. 



The submarine plains are by far the most extensive 

 within the British area. The tendency of tidal scour 

 and deposit must modify the form of the bottom. 

 In the case of the North Sea, for example, this great 

 basin of water is obviously being slowly filled up 

 by the deposit of sediment over its floor. A vast 

 amount of mud and silt is borne into it by the rivers 

 of western continental Europe, and of the eastern 

 coast of Britain ; while at the same time the waves 

 are cutting away the land on both sides of this sea 



