THE FOSSILIFEROUS ROCKS. 15 



rare instances where volcanic ashes have enveloped animals 

 or plants which were living in the sea or on the land in the 

 immediate vicinity of the volcanic focus. The second great 

 division of rocks is that of the Fossiliferous, Aqueous, or Sedi- 

 mentary Rocks. These are formed at the surface of the earth, 

 and, as implied by one of their names, are invariably deposited 

 in water. They are produced by vital or chemical action, or 

 are formed from the " sediment " produced by the disintegra- 

 tion and reconstruction of previously existing rocks, without 

 previous solution ; they mostly contain fossils ; and they are 

 arranged in distinct layers or "strata." The so-called "aerial" 

 rocks which, like beds of blown sand, have been formed by 

 the action of the atmosphere, may also contain fossils ; but 

 they are not of such importance as to require special notice 

 here. 



For all practical purposes, we may consider that the Aque- 

 ous Rocks are the natural cemetery of the animals and plants 

 of bygone ages ; and it is therefore essential that the palaeon- 

 tological student should be acquainted with some of the prin- 

 cipal facts as to their physical characters, their minute structure 

 and mode of origin, their chief varieties, and their historical 

 succession. 



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 lime- 

 stone. 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 

 materials which compose them have formerly been spread out 

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



