328 Mr. E. J, Chapman on the Notation of Crystals. 



than Naumann's symbol of R'. Scalenohedrons, however, 

 are not possessed by more than five or six minerals, and are 

 only abundant in one, — calcareous spar. 



Observations. — Rhombohedrons being the more common 

 fundamental forms of this group, Naumann adopts two kindred 

 systems for the notation of the derived forms ; one starting 

 with the hexagonal pyramid .rP, and the other with the rhom- 

 bohedron xK^ each producing a similar series : — OP, c» P, 

 GO Poo, — OR, 00 R, ooRoo, &c. It is however evident, that 

 in the proposed notation only one series can y\q. q, 



result, whatever be the fundamental form ; 

 for R and S are the only arbitrary symbols ; 

 and their employment, in rendering the no- 

 tation more simple, does not in any way 

 affect the other signs. The triaxial pris- 

 matic planes T should be strictly T2, 2 ; but 

 the figures are quite unnecessary, for these 

 planes being tangents to one of the horizon- 

 tal axes, it is obvious — the axes crossing 

 each other at 60° — that the other two must 

 be cut at twice the distance. The values also 

 of m and n in the dihexagonal pyramids can 

 only equal -|-2, — 2, as otherwise the form Y 

 would result. 



C ale-spar : 

 Cumberland. 



Fig. 7. 



Trimetric System. 



Monaxial Planes: — 

 Basal planes, P. 



Back and front vertical planes, M. 

 Side vertical planes, L. 



Diaxial Prismatic Planes : — 



Vertical planes of ordinary rhombic 

 prisms, D, D^, Tim. 



Diaxial Pyramidal Planes : — 



Inclined planes cutting the shorter 



horizontal, or frontal axis, A, ^ A, 



TwA. 

 Inclined planes cutting the longer 



horizontal, or right and left axis, 



E, -^E, wE. 



'to ' 



Triaxial Planes : — 

 Ordinary rhombic octahedrons, O, 



iO, mO. 

 Intermediate planes, xOx. Scorodite : Brazil. 



Epistilbite: Faroe. 

 Fiff. 8. 



