CHAPTER XIIL 



SLOW MOVEMENTS OF THE LAND. 



Such movements of the land as those which accompanied 

 the South American earthquakes, referred to in the last 

 chapter (p. i86), must have been brought about by the com- 

 paratively sudden action of subterranean forces. But the 

 land is subject not only to a rapid rise and fall of this kind, 

 but also to local elevations and depressions so gradual as to 

 escape ordinary observation. Special means, indeed, are, in 

 most cases, needed to detect these slow changes of level, and 

 to measure their extent. Yet it is probable that such gradual 

 oscillations of the land are, in the long run, of far greater 

 importance in the economy of nature than those abrupt 

 movements which occur spasmodically. It will be shown 

 in a subsequent chapter, that every foot of solid ground 

 within the area of the Thames basin has, at some time or 

 other, been buried beneath the sea ; and it is therefore clear 

 that elevatory forces must have been at work to lift up the 

 sea-bed and expose it as dry land. Nor has such move- 

 ment been effected once only. Any one who seeks to read 

 the history of the rocks in the basin of the Thames will 

 be driven to conclude that the level of the land has changed 



