OF ORGANIC NATURE. 33 



integrating process. And tlius, slowly but surely^ the 

 hardest rocks are gradually ground down to a powdery 

 substance ; and the mud thus formed, coarser or finer, 

 as the case 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 

 abovit fourteen or fifteen fathoms, a depth at which the 

 water is, usually, nearly motionless, and in which, of 

 coui-se, 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 waves. 

 The matters forming the deposit are torn from the 

 mountain-side and whirled impetuously into the val- 

 ley, 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 ob- 

 viously 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 



