50 



It is presumed that every one who attempts to 

 become acquainted with rocks, has acquired a 

 competent knowledge of mineralogy : nor is it 

 possible to make any real progress in this latter 

 study, without learning the general characters at 

 least, of the principal rocks, as well as minerals. 

 The knowledge of rocks, however, whatever may 

 be imagined, is seldom communicated by mere 

 description, without the aid of some demonstra- 

 tion. Even mineralogists are perhaps not always 

 aware how much of their own peculiar acquire- 

 ments in mineralogy is traditional. It is probable 

 therefore that the student will unavoidably open 

 every work of this nature with a previous general 

 knowledge of the subject ; which will render less 

 necessary that accurate definition of the families 

 of rocks which the nature of the objects renders 

 it impossible to give. 



In terminating these introductory remarks on 

 the arrangement, it is necessary to add some ob- 

 servations respecting the terms by which the rocks 

 are designated. 



The existing nomenclature of rocks ia neither 



