CHAPTER II. 



THE FOSSILIFEROUS ROCKS. 



Fossils are found in rocks, though not universally or pro- 

 miscuously; and it is therefore necessary that the palaeonto- 

 logist should possess some acquaintance with, at any rate, those 

 rocks which yield organic remains, and which are therefore 

 said to be " fossiliferous. " In geological language, all the ma- 

 terials which enter into the composition of the solid crust of 

 the earth, be their texture what it may from the most impal- 

 pable mud to the hardest granite are termed " rocks ;" and for 

 our present purpose we may divide these into two great groups. 

 In the first division are the Igneous Rocks such as the lavas and 

 ashes of volcanoes which are formed within the body of the 

 earth itself, and which owe their structure and origin to the 

 action of heat. The Igneous Rocks are formed primarily below 

 the surface of the earth, which they only reach as the result of 

 volcanic action; they are generally destitute of distinct "strati- 

 fication," or arrangement in successive layers; and they do 

 not contain fossils, except in the comparatively 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 Sedimentary 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 disintegration 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 the beds 

 of blown sand, have been formed by the action of the atmos- 

 phere, may also contain fossils; but they are not of such im- 

 portance as to require special notice here. 



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