10 



CHAPTEE II. 



THE FOSSILIFEROUS ROCKS. 



THE Sedimentary or Fossiliferous Eocks form the greater 

 portion of that part of the earth's crust which is open to 

 our examination, and are distinguished by the fact that 

 they are regularly " stratified/' or arranged in distinct and 

 definite layers or " strata." These layers may consist of a 

 single _material, as in a block of sandstone, or they may 

 consist of different materials. When examined on a large 

 scale, they are always found to consist of alternations of 

 layers of different mineral composition. We may examine 

 any given area,. and find in it nothing but one kind of rock 

 sandstone, perhaps, or limestone. In all cases, however, 

 if we extend our examination sufficiently far, we shall ulti- 

 mately 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 Eocks, they are 

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

 wear and tear of pre- existent rock. Sometimes, however, 

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

 would more properly be spoken of simply as Aqueous Eocks. 

 As to their mode of deposition, we are enabled to infer that 

 the materials 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 



