Otf THE TABLES OF MODIFICATIONS. 97 



nected with the theory of decrements, will be ex- 

 plained in the Appendix, together with a system of 

 notation connected with the same theory, and capable 

 of expressing the figure of any secondary form by 

 means of certain symbols. 



But there are many who form collections of mi- 

 nerals as an amusement, who are not in the habit 

 of mathematical investigations, and who cannot avail 

 themselves of the theory of decrements, for the pur- 

 pose of tracing the relations between the secondary 

 and primary forms of crystals ; and I am not aware 

 of the existence of any published work unconnected 

 with that theory, which attempts to point out these 

 relations so as to enable the mineralogist to trace 

 in a secondary crystal the characters of the primary 

 to which it belongs.* 



I have attempted to supply this desideratum by the 

 following tables of modifications of the primary forms, 

 and by the explanations which follow them. And 

 although these may not furnish the young enquirer 

 with all the assistance he desires, but may leave him 

 still to encounter some difficulty in his pursuit, he 

 will certainly derive advantages from the opportunity 

 the tables will afford him, of comparing all the classes 

 of simple secondary forms, belonging to the several 

 



* A systematic method of describing crystals was taught by Werner ; 

 but that method has been found inconvenient even for the purpose of 

 dcsfription y and it supplies no rules for deducing the primary form of a 

 crystal from any of its secondary forms. The system of Mohs has not 

 yet been sufficiently developed to the English reader, to enable him to 

 judge fairly of its merits; but from what has been published here, it 

 appears that the purpose I have attempted to effect, may also be 

 effected by his system, although by a less direct course. 



The consideration of infinite lines which he has introduced into his 

 system, and his notation founded on this character, are parts of his 

 theory which will probably render its public reception less general than 

 it might have been from its merits in other respects. 



