CLASSIFICATION OF ROCKS. 9 



certain definite spaces, and not confusedly mixed together, 

 but, on tHe contrary, exhibiting a definite and discoverable 

 order of' arrangement. All these materials, however dif- 

 ferent in appearance, texture, or mineral composition, are 

 called "rocks" by the geologist. The term "rock." then. 

 is to be understood as applying 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 KOCKS. 



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 : 1. The Igneous 

 Eocks, 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 Eocks, which are 

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

 structure to the mechanical action of water. The Igneous 

 Eocks are principally 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 Aque- 

 ous and Sedimentary Eocks are formed at the surface by 

 the disintegration and reconstruction of previously existing 

 rocks, or by the vital chemistry of animals or plants, are 

 mostly fossiliferous, and are stratified i.e., are arranged in 

 distinct layers or " strata." The Sedimentary Eocks, as con- 

 taining fossils, are the only rocks which it is essential for the 

 palaeontologist to be acquainted with, and we shall very 

 briefly consider their leading physical characters, their chief 

 varieties, their mode of origin, and their historical succession. 



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