152 CHARACTERISTIC. 



6. When the mark of interrogation follows the numbers which 

 express the hardness, specific gravity or dimensions of particular 

 forms, it signifies that such numbers are only to be regarded as ap- 

 proximating to the truth, in the respective cases. 



7. The sign IT placed before the name of a species indicates that 

 localities of it are known in the United States. 



8. The sign || signifies that those species before which it is placed, 

 are of comparatively rare occurrence. 



9. The number of species whose individuals never occur, except 

 in the crystallized state, being extremely small, it has been deemed 

 expedient to introduce them into the uncrystallized class; since it 

 sometimes happens that it is necessary to determine a crystallized 

 mineral, whose crystalline form we may be incapable of ascertain- 

 ing from its natural imperfection, or some artificial alteration it has 

 suffered. Such minerals will be preceded by the sign . 



10. In the semi-crystallized class, those species which have already 

 occurred in the crystallized class, will be denoted simply by their 

 trivial names. In connexion with the names of such species, how- 

 ever, will be found, a reference back to the page of the crystallized 

 class where they are first mentioned. The same rule will also be 

 gbserved-in the first order of the uncrystallized class, 



