CLASSIFICATION OF ROCKS. 5 



ment. All these materials, however different in appearance, 

 texture, or mineral composition, are called " rocks " by the 

 geologist. The term " rock," then, is to be understood as ap- 

 plying to all the materials which compose the crust of the 

 earth. In the language of geology, the finest mud, the loosest 

 sand, and the most incoherent gravel, are just as much rocks as 

 are the hardest and most compact granites or limestones. 



CLASSIFICATION OF ROCKS. 



For the purposes of the palaeontologist all the rocks which 

 enter into the composition of the solid exterior of the earth 

 may be divided into two great classes : i. The Igneous 

 Rocks, which are formed within the body of the earth itself, 

 and which owe their structure and origin to the action of heat; 

 and 2, the Aqueous or Sedimentary Rocks, which are formed 

 at the surface of the earth, and which owe their structure and 

 origin to the mechanical action of water. The Igneous Rocks 

 are formed below the surface of the earth, are as a general rule 

 destitute of organic remains or fossils, and are mostly in the 

 form of unstratified masses. The Aqueous and Sedimentary 

 Rocks are formed at the surface by the disintegration and re- 

 construction of previously existing rocks, are mostly fossilifer- 

 ous, and are stratified i.e., are arranged in distinct layers or 

 " strata." The Sedimentary Rocks, as containing fossils, are the 

 only rocks which it is essential for the palaeontologist to be 

 acquainted with, and we shall very briefly consider their lead- 

 ing physical characters, their chief varieties, their mode of ori- 

 gin, and their historical succession. 



CHAPTER II. 



SEDIMENTARY ROCKS. 



THE Sedimentary or Fossiliferous Rocks form the greater por- 

 tion 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 



