41 



mitting the metallic ores to a place among rocks, 

 as they appertain to a classification of minerals ; 

 and there are still greater objections to excluding* 

 the simple rocks, on account of their perpetual 

 transition into the compound, and the incurable 

 confusion which their absence would produce in 

 the arrangement. 



The popular sense of the term rock is well 

 understood, and it is here adopted in that sense ; 

 comprising all the mineral earthy compounds, 

 whether simple or mixed, that form conspicuous 

 masses in nature, and enter into that which may 

 be considered the structure of the earth. Rock 

 salt has not been included ; but there is added an 

 appendix consisting of substances to which the 

 term rock cannot be applied, but which have ge- 

 nerally been described by writers on this subject. 

 It is convenient to notice these, because they do 

 not enter into a system of mineralogy, and are 

 not sufficiently numerous to admit of a separate 

 treatise; while they form an important part of 

 *he structure of the earth. 



