THE FOSSILIFEROUS ROCKS. 17 



In all cases, however, if we extend our examination sufficiently 

 far, we shall ultimately come upon different rocks ; and, as 

 a general rule, the thickness of any particular set of beds is 

 comparatively small, so that different kinds of rock alternate 

 with one another in comparatively small spaces. 



As regards the origin of the Sedimentary Rocks, they are 

 for the most part " derivative " rocks, being derived from the 

 wear and tear of pre-existent rocks. Sometimes, however, they 

 owe their origin to chemical or vital action, when they would 

 more properly be spoken of simply as Aqueous Rocks. As to 

 their mode of deposition, we are enabled to infer that the mate- 

 rials which compose them have formerly been spread out by 

 the action of water, from what we see going on every day at 

 the mouths of our great rivers, and on a smaller scale wherever 

 there is running water. Every stream, where it runs into a 

 lake or into the sea, carries with it a burden of mud, sand, and 

 rounded pebbles, derived from the waste of the rocks which 



ft 



Fig. 5. Diagram to illustrate the formation of sedimentary deposits at the point 

 where a river debouches into the sea. 



form its bed and banks. When these materials cease to be im- 

 pelled by the force of the moving water, they sink to the bot- 

 tom, the heaviest pebbles, of course, sinking first, the smaller 



