XI PHENOMENA OF ORGANIC NATURE 335 



finer particles of this detritus, or mud as we call 

 it, sinks to the bottom. 



Or, again, if you take a river, rushing down 

 from its mountain sources, brawling over the 

 stones and rocks that intersect its path, loosening, 

 removing, and carrying with it in its downward 

 course the pebbles and lighter matters from its 

 banks, it crushes and pounds down the rocks and 

 earths in precisely the same way as the wearing 

 action of the sea waves. The matters forming the 

 deposit are torn from the mountain-side and 

 whirled impetuously into the valley, more slowly 

 over the plain, thence into the estuary, and from 

 the estuary they are swept into the sea. The 

 coarser and heavier fragments are obviously 

 deposited first, that is, as soon as the current 

 begins to lose its force by becoming amalgamated 

 with the stiller depths of the ocean, but the finer 

 and lighter particles are carried further on, and 

 eventually deposited in a deeper and stiller portion 

 of the ocean. 



It clearly follows from this that mud gives us a 

 chronology ; for it is evident that supposing this, 

 which I now sketch, to be the sea bottom, and 

 supposing this to be a coast-line ; from the wash- 

 ing action of the sea upon the rock, wearing and 

 grinding it down into a sediment of mud, the mud 

 will be carried down, and, at length, deposited in 

 the deeper parts of this sea bottom, where it will 

 form a layer ; and then, while that first layer is 



