ORDER VI. TRISMATIC FELD-SPAR. 255 



important in regard to the idea of genus in the Natural His- 

 tory of the Mineral Kingdom, to which it imparts an addi- 

 tional degree of evidence, and clearly demonstrates the ne- 

 cessity of a systematic; nomenclature : for there can be as 

 little doubt of their being particular species, as there 

 can be of their being species belonging to one and the 

 same genus, Feld-spar. Mr G. ROSE has ascertained 

 some of them with great precision, in the memoir quoted 

 above. Inquiries of this kind are among the nicest in Mi- 

 neralogy, and require great precautions to guard us from 

 new species, whose admission might depend upon an er- 

 ror of observation. These ought therefore to be often re- 

 peated before finally settling the new species of the present 

 genus. The following enumeration of some of them is 

 intended merely for engaging mineralogists to inquire far- 

 ther into their nature ; and I have therefore abstained from 

 quoting any observations from the papers published on this 

 subject. 



i. Alltte. 



The synonymes are included in those mentioned for 

 prismatic Feld-spar. 



Fundamental form. Scalene four-sided pyramid, the axis of 

 which is inclined in the planes of both diagonals. VoL L 

 Fig. 42. 



Si-aplefonn, {?},_;<!{- }, jC^f J.J, 



Pr + os (M). 

 Char, of Comb. Tetarto-prismatic. 



Combinations. 1. ?L r. *-^. r , ( Pr + -^)L Pr + os. 



24 2*2 



Fig. 84.^ 



T ~~T~ 4 " 2"* 



r (P_+ J=T p p r + ^^ FJg 85> 



