PREFACE. XV 



general character, are to form the basis of correct 

 geological description and reasoning. 



If even this general principle were not uni- 

 versally true, the experience of the student in 

 this department would soon convince him of that 

 which the author has deduced from his own ; 

 namely, that without a very accurate knowledge 

 of all the variations to which rocks are subject, 

 he can never feel confidence in determining the 

 character and name of any. Hence arose those 

 detailed enumerations of varieties which form 

 the bulk of the catalogues in the following work ; 

 and the apology for them will be found in these 

 considerations. 



The number of the minerals essential to the 

 composition of rocks is exceedingly limited. But 

 they are also the repositories of nearly all the 

 minerals in nature ; which, in a certain sense, 

 may therefore be considered as constituent parts 

 of the masses which are treated of in this work. 

 These unessential minerals are, however, the 

 objects of systems of mineralogy; in which they 



