326 Mr. E, J. Chapman on the 'Notation of Crystals. 



face has been chosen for the basis of the notation which cuts 

 the horizontal axes, for the time being, at equal distances; so 

 that, with the exception of the hexakis-octahedrons, in which 

 a single face of this kind does not occur, these horizontal axes 

 may always be considered unity. The symbol of the leuci- 

 toids becomes thus -O. and that of the trisoctahedrons mO. 



The hexakis-octahedrons form the intermediate planes i of 

 the French crystallographers ; and as in the combinations in 

 which they occur these planes are usually subordinate to the 

 others, their necessarily small size in drawings often prevents 

 the complete symbol, which is exactly similar to that of Nau- 

 mann, from being employed in their lettering; for which 

 reason any conventional sign may be used for that purpose, 

 and the exact notation placed beneath the figure : as, " or 

 # = 204, &c. 



Fiir. 3. 



Dimetric System. 



Monaxial Planes : — 

 Basal planes, P. 

 Vertical planes, M. 



Diaxial Prismatic Planes : — 



Vertical planes alternating in position with 

 those of the ordinary square prisms, D. 

 Ditetragonal prisms, T>m. 



Diaxial Pyramidal Planes : — 



Ordinary square-based octahedrons. A, 



iA, wA. 



Triaxial Planes : — 



Octahedral planes alternating in position 



with the above, O, -O, mO. 

 Dioctahedrons, mOn. 



Zircon : Frederiksvarn. 



Observations. — In this system, and in those which follow, a 

 fundamental form must be chosen for each mineral as in the 

 usual notations, but with this difference : that whereas the 

 German crystallographers choose invariably pyramidal forms 

 for this purpose, often quite opposed to the crystallization of 



Apophyllite: Faroe- 

 Fiji. 4. 



