242 LIGHT SCIENCE FOR LEISURE HOURS. 



of continents ; but as a matter of fact, there are few 

 causes to which geologists are disposed to ascribe more 

 importance. The very fact that enormous deltas have 

 been formed at the mouths of many rivers in other 

 words, the actual growth of continents through the 

 effects of rainfall is a proof how largely this cause 

 must tend to destroy and disintegrate the interiors of 

 our continents. Dwelling on this point, Sir Charles 

 Lyell presents the following remarkable illustration : 

 " During a tour in Spain," he writes, " I was surprised 

 to see a district of gently-undulating ground in Cata- 

 lonia, consisting of red and gray sandstone, and in some 

 parts of red marl, almost entirely denuded of herbage ; 

 while the roots of the pines, holm oaks, and some other 

 trees, were half exposed, as if the soil had been washed 

 away by a flood. Such is the state of the forests, for 

 example, between Oristo and Yich, and near San 

 Lorenzo. But, being overtaken by a violent thunder- 

 storm in the month of August, I saw the whole surface, 

 even the highest levels of some flat-topped hills, stream- 

 ing with mud, while on every declivity the devastation 

 of torrents was terrific. The peculiarities in the phys- 

 iognomy of the district were at one explained ; and I 

 was taught that, in speculating on the greater effects 

 which the direct action of rain may once have produced 

 on the surface of certain parts of England, we need 

 not revert to periods when the heat of the climate was 

 tropical." 



