OF ORGANIC NATURE. 33 



may be, is carried by the rush of the tides, or currents, 

 till it reaches the comparatively deeper parts of the 

 ocean, in which it can sink to the bottom, that is, to 

 parts where there is a depth of about fourteen or fifteen 

 fathoms, a depth at which the water is, usually, nearly 

 motionless, and in which, of course, the 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 w T aves. The 

 matters forming the deposit are torn from the mountain- 

 side and w T hirled 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 washing action of the sea 

 upon the rock, wearing and grinding it down into a 

 sediment of mud, the mud will be carried clown, and 

 at length, deposited in the deeper parts of this sea bot- 

 tom, where it will form a layer ; and then, while that 

 2* 



