TRICLINIC SYSTEM. 43 



2 Positions of Planes. Lettering of Crystals — On account oi 



the obliquity of the crystals, the planes above and below the basal sec- 

 tion require a distinguishing mark in their lettering-, as well as in tha 

 mathematical expressions for them. One set is made minus and the 

 other plus. The plus sign is omitted in the lettering. In fig. 7 there 

 are above the basal section (or above i-i) the planes 1-i, $-i, 1, -£, but be- 

 low it, — i-i, — 1. The plus planes are those opposite the acute inter- 

 section of the basal and orthodiagonal sections, and the minus those 

 opposite the obtuse. No signs are needed for planes of the clinocliago- 

 nal section, since they are alike both above and below the basal sec- 

 tion. 



The distinction of longer and shorter lateral axis is not available in 

 this system, since either may be the clinodiagonal. The distinction of 

 clinodiagonal and orthodiagonal planes is indicated by a grave accent 

 over fche number or letters referring to the clinodiagonal. The lettering 

 for the clinodomes on fig. 6 is 1-1, 2-1 — the i (initial of infinite, with 

 the accent) signifying parallelism to the clijiodi&gonal. The hemiocta- 

 hedrons, 1, 2, etc., need no such mark, as the expression for them 

 is 1c : lb : Id, 2c : lb : Id, the planes having a unit ratio for d and b. 

 But the plane 2-2, in fig. 5, requires it, its expression being 2o : lb : 2d ; 

 the fact that the last 2 refers to the clinodiagonal is indicated by 

 the accent. If it referred to the orthodiagonal, that is, if the expres- 

 sion for the plane were 2c : 2b : la, it would be written 2-2 without the 

 accent. 



3. Cleavage. — Cleavage may be basal, or parallel to either of 

 the other diametral sections, or parallel to a vertical rhombic 

 prism, or to the planes of a hemioctahedron ; or to the planes of 

 a clinodome, or to that of a hemiorthodome. If occurring in two 

 or more directions in any species it is always different in degree 

 in each different direction, as in all the other systems. 



4. Irregularities. — Crystals of this system may be elongated 

 abnormally in the direction of either axis, and any diagonal. 

 The hemiorthodomes may be in aspect the bases of prisms, and 

 the hemioctahedrons the sides of prisms. Which ]jlane in the 

 zone of hemiorthodomes should be made the base, and which in 

 the series of hemioctahedrons should be assumed as the funda- 

 mental prism determining the direction of the vertical axis, ia 

 often decided differently by different crystallographers. Con- 

 venience of mathematical calculation is often the principal point 

 referred to in order to reach a conclusion. 



V. TRICLINIC SYSTEM. 



1. Descriptions of Forms. — In the triclinic system the thre« 

 axes are unequal and their three intersections are oblique, and 

 consequently there are never more than two planes of a kind \ 



