XIV PREFACE. 



numerous considerations that will be found in 

 their proper places in the body of the work. 



There are two principal objects in view in 

 the description of rocks ; namely, that of enabling 

 a mineralogist to refer any given specimen to 

 some general or particular title, according to its 

 mineral characters, and that of assisting him in 

 determining the place which it holds geologically 

 in the order of nature. To accomplish the for- 

 mer object, it is evident that a description of all 

 the most important and decided varieties was 

 requisite. But even the latter could not have 

 been attained without such a knowledge of every 

 important variety as would enable the geologist 

 to assign the general character in every case, 

 however obscure. Although nature may be 

 reasoned on in the aggregate, it must be studied 

 in the details ; nor can any useful and satisfactory 

 knowledge of rocks, for the purposes of establish- 

 ing general conclusions, be acquired, without an 

 intimate acquaintance with all the parts which, 

 by being reduced under a common or leading 



