PREFACE. Xlll 



prevail among them, it will be found, on an at- 

 tentive examination of their characters, that they 

 admit of being placed in groups, distinguished 

 by certain prevailing mineral characters, that 

 these groups are not very numerous, and that 

 the real differences of texture and composition 

 are as limited as is the number of the minerals of 

 which they are composed. It will further be 

 seen that these different groups, thus united by 

 some prevailing associations of mineral charac- 

 ters, are also in a great measure distinguished in 

 nature by certain geological or general relations, 

 more or less constant and perfect. Thus two 

 natural methods of arrangement, the one founded 

 on mineral characters; and the other on geo- 

 logical relations, have fortunately been found 

 so far to coincide, that no very great or general 

 breach of the relations of the one kind has 

 been committed in adopting an order founded on 

 those of the other. Wherever this has occurred, 

 as in some remarkable instances has happened, 

 the expediency of the adopted plan will be jus- 

 tified, even where it is most defective, or where 

 it is incapable of embracing both views, by 



