ARRAN. MINERALS. 431 



points of change. This might, a priori, be presumed ; 

 as the change of character is necessarily accompanied 

 by those alterations which prevent definite fragments from 

 being obtained, except at the perfectly characterized 

 extremes. 



But they are proved to be real variations of composition 

 by chemical examination. Corresponding changes are 

 thus detected in the proportions of the several earths 

 which enter into the composition of the several varieties. 

 It is plain that in such cases an analysis of a more accurate 

 nature would be attended with intolerable labour, while 

 it could lead to no determinate or useful results as to these 

 proportions ; and it is also evident that this view of the 

 varying character of prehnite will account for the dif- 

 ferences of the results that are obtained on analyzing it. 

 Mineralogists will immediately perceive the consequences 

 which hence follow, relating to definite proportions, or to 

 definite geometric forms, as infallible characteristics of 

 mineral species. 



