CLASSIFICATION OF ROCKS. 9 



All the natural cavities of a fossil, even down to the minutest micro- 

 scopic pores or tubes, may in this way become filled with some such 

 infiltrated material, the two commonest agents in this process being 

 lime and flint. If the skeleton of the fossil be a calcareous one, 

 while the infiltrating material has been some less soluble substance, 

 such as silica or some silicate, then the skeleton may be artificially 

 or naturally dissolved away, leaving a cast of the internal cavities of 

 the fossil formed of the infiltrated matter. Thus the minute shells 

 of Foraminifera are often infiltrated with the silicate glauconite, and 

 exquisitely perfect casts of their interior cavities are subsequently 

 formed by dissolution of the shell itself. In this way, as we shall 

 see hereafter, deposits of greensand have been sometimes produced. 



Definition of Rock. 



The crust of the earth consists of various different materials, pro- 

 duced at different successive periods, occupying certain definite 

 spaces, and not confusedly mixed together, but, on the contrary, 

 exhibiting a definite and discoverable order of arrangement. 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 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 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 owe their struc- 

 ture and origin to the action of heat ; and 2. the Aqueous or 

 Sedimentary Rocks, which are formed at the surface of the earth, 

 and owe their structure, at any rate in part, to the mechanical 

 action of water. The Igneous Rocks 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 Aqueous and Sedimentary Rocks 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 Aqueous Rocks, as containing fossils, are the only rocks with 

 which it is essential for the palaeontologist to be acquainted, and we 

 shall very briefly consider their leading physical characters, their 



