CH. IX.] THE WORK OF RAIN AND RIVERS. 131 



are very light may be kept suspended in the water until they 

 are carried by the river right out to sea; but a time at length 

 comes, when even these will quietly settle down upon the 

 sea-bottom. If a little of the muddy sediment, deposited by 

 the water, be dried by exposure to the air, it will be found 

 to harden into a substance not unlike day. Clay is, in 

 fact, nothing but such mud, hardened and perhaps otherwise 

 altered. 



Very little thought is necessary to convince any one that 

 the fine particles of solid matter, which form mud, are pro- 

 duced by the mechanical waste of the land. After a heavy 

 shower of rain has fallen in the street, you observe dirty 

 streams coursing along the gutters, and every one knows that 

 the muddy matter in these streams is merely the dirt washed 

 from the roofs of the houses and the stones of the street. 

 In like manner, every shower of rain that falls in the open 

 country washes something off the surface of the land. This 

 removal of matter is termed denudation^ since the rocks are 

 laid bare by having their superficial covering thus peeled off. 

 The particular kind of denudation effected by means of rain 

 is called //z^z'/fl/^ denudation. A heavy shower, falling upon 

 a field, washes away some of the soil, and carries it off in 

 muddy runnels to the nearest stream, whence it passes to 

 the river. Where the rain comes down in a deluge, as often 

 happens in the tropics, its power as a denuding agent is 

 almost incredible; and even in this country, especially 

 among the hills of Wales and Cumberland, we occasionally 

 hear of torrents of rain tearing up rocks and sweeping 

 everything before them. It has been held by Mr. A. Tylor 

 and some other geologists, that the rainfall was formerly 

 very much greater than it is at present ; and, if this be 

 admitted, it follows that the work done by rain in destroying 

 ^ Pluvial, from the Lat. pluvia, rain. 



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