PT. m. OR POISONED BY VEGETABLE GROWTH ? 173 



wliicli would otherwise travel more slowly along the 

 valley, and out of the valley, by the same force which 

 brought it into the valley ^rain. 



I believe that in many cases where the country is 

 composed of soft and porous materials, as chalk, the 

 depth of the valleys and channels scooped out by rain 

 ''ays open the springs, and forms the rivers, instead of 

 'Jie rivers forming the valleys. How many dry valleys 

 are there sloping to the sea without having laid open a 

 spring, and therefore without any stream ? And what 

 formed these valleys? How many lateral or branch 

 dry valleys are there falling into the main longitudinal 

 dry valley whose lower end joins the upper end of the 

 river valley without any step or inequality ? And how 

 many lateral or branch dry valleys fall into the main 

 river valley ? And what formed these countless myriads 

 of dry valleys ? 



These valleys exist even in volcanic countries, where 

 the sea could not have formed them while the land was 

 emerging ; and the gradients of the river valleys and 

 dry valleys, and the whole form of the ridges and fur- 

 rows, of the entire surface-drainage of a volcanic region 

 (say, of Madeira), are so precisely the same as those of 

 any other mountainous district, that no eye can glance 

 over the two and doubt for an instant that the same 

 cause caused the form of the drainage of both. 



In fact, rain, which we consider only as a produc- 

 tive power, is the destroyer, the dissolver, of continents. 

 Subterranean igneous action, which we consider only 



