II 



CHAPTER II. 



THE FOSSILIFEROUS ROCKS. 



The Sedimentary or Fossiliferous Rocks 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 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," 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 Rocks. 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 mud, 

 sand, and rounded pebbles, derived from the waste of the rocks 

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

 impelled by the force of the moving water they sink to the bottom, 

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

 and sand next, and the finest mud last. Ultimately, therefore, as 

 might have been inferred upon theoretical grounds, and as is proved 

 by practical experience, every lake becomes a receptacle for a series 



