THE USEFULNESS OF EARTHQUAKES. 22 r 



But we have yet to consider the second principal 

 cause of the wasting away of the land. The cause we 

 have just been dealing with acts upon the shores or 

 outlines of islands and continents ; the one we have now 

 to consider acts upon their interior. It will, perhaps, 

 hardly be supposed that the fall of rain upon the land 

 could have any appreciable influence in the demolition 

 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 Catalonia, consisting of 

 red and grey 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 Vich, and near San Lorenzo. But, 

 being overtaken by a violent thunderstorm in the month 

 of August, I saw the whole surface, even the highest 

 levels of some flat-topped hills, streaming with mud, 

 while on every declivity the devastation of torrents 

 was terrific. The peculiarities in the physiognomy of 

 the district were at once explained ; and I was taught 



