48 



may find : this forms almost an inevitable part 

 of his acquisitions in procuring the specimen 

 which he is afterwards to examine. To those 

 whose only object is to form or arrange a cabinet, 

 all varieties have the same value, if their mineral 

 characters are sufficiently remarkable. Their geo- 

 logical importance is here no object. 



The definition of eacji variety is, for the most 

 part, a description of its composition ; and, 

 where necessary, of its structure also. Other de- 

 tails are occasionally added, for the purpose of 

 preventing doubts, or facilitating the knowledge 

 of the varieties under each family ; utility and 

 certainty being the objects in view. 



The varieties which are here described, ir*- 

 elude all those which appear sufficiently constant 

 to merit a description ; and they are further no- 

 ticed in those states where, by gradually losing 

 their distinctive characters, they pass into some 

 other rock. As, in many cases, similar rocks 

 occur under different families, they are, for the 

 most part, noticed under each'; I an expedient 

 which, however irregular, from the repetition 



