324 Mr. E. J. Chapman on the Notation of Crystals. 



although of D and B various series may occur, as D^, D|, 

 D, D2, D3, &c. To this, however, we shall revert as we 

 proceed. 



Diaxial pyramidal planes are of one kind in the monome- 

 tric, dimetric, and hexagonal systems, and are lettered A. In 

 the trimetric or rhombic system they are of two kinds, denoted 

 respectively by A and E ; the former being situated in front 

 when the crystals are in position, and the latter at the sides, 

 or upon the acute angles of the rhombic prisms. A is there- 

 fore associated in position with the prismatic planes M, and 

 E with L. In the monoclinic system the diaxial pyramidal 

 planes are of three kinds — A, E, and I. The planes E, in 

 sets of four, are situated at the sides ; A is the front plane at 

 the top of the crystal, and the back plane below ; I is opposed 

 to A. Finally, the triclinic system possesses four distinct sets 

 of diaxial pyramidal planes: — A, A A, E, EE. These are, 

 however, common to but few minerals ; and they are very 

 rarely, if ever, all present at the same time. 



Triaxial pyramidal planes are of one kind in the mono- 

 metric, dimetric, hexagonal, and trimetric systems, and are 

 designated by the symbol O. In the monoclinic system they 

 are of two kinds, O and U ; the former being situated in the 

 principal position, that is to say, in front and above. The 

 planes U are therefore the upper planes behind, and the lower 

 planes in front. Lastly, in the triclinic system, these planes 

 may be of four kinds : — O, OO, U and W — but they rarely 

 occur together in the same crystal. 



The tetraxial planes of the hexagonal system are of one 

 kind, and are lettered Y. 



Pyramidal planes are consequently denoted by 



A; E; I; A A; EE; O, U, OO, W; and Y; 

 of all of which various series may occur, but more especially 

 of A, E, I, O, U, and Y ; as, J^O, |0, ^O, lO, O, 20, 30, &c. 

 The relations and notation of these different planes are 

 exhibited more distinctly in the following table ; by which it 

 will be perceived, that by the enunciation of a single letter, 

 the position of the designated plane is immediately called up 

 before us : because, by this letter, we know at once whether 

 the plane be a prismatic or a pyramidal one ; and furthermore, 

 the number of axes which ft cuts, and consequently its po- 

 sition. 



