difficulties under which the subject labours, and 

 the objects yet to be accomplished. But the task 

 might appear invidious, and would lead to a dis- 

 sertation too extensive for an introduction of this 

 nature. It will nevertheless be of use to examine 

 the reasons which may be assigned in favour of 

 each of the leading principles of arrangement 

 which have divided geologists, and the objections 

 which, on the other hand, may be made to both. 

 Thus the reader of the following pages will obtain 

 a clearer view of difficulties which he perhaps has 

 never considered, and be enabled to appreciate 

 the motives which have led to the selection of that 

 classification which has here been adopted. 



The two principal bases on which an arrange- 

 ment of rocks may be formed, are a mineralogical 

 and a geological one. Each of these is, in a cer- 

 tain sense, natural ; the former consul ting, or pro- 

 fessing to consult, the nature alone of the sub- 

 stances to be arranged, the latter being founded 

 on the great relations which they bear to each 

 other and to the general structure of the earth. 

 Brongniart is the chief advocate in favour of the 



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