
TRICLINIC SYSTEM. 43 
2. Po itions of Planes. Lettering of Crystals.—Oon account ot 
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 the 
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 7-7) the planes 1-2, 3-2, 1, 4, but be- 
low it, —4-2, —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 clinodiago- 
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 the number or letters referring to the clinodiagonal. The lettering 
for the clinodomes on fig. 6 is 1-2, 2-t—the 2? (initial of infinite, with 
the accent) signifying parallelism to the clinodiagonal. The hemiocta- 
hedrons, 1, 2, etc., need no such mark, as the expression for them 
is le:16:1d, 2c: 160: 1d, the planes having a unit ratio for @ and b. 
But the plane 2-2, in fig. 5, requires it, its expression being 2c : 10 : 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 : 20 : 1d, 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 plane 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, is 
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 three 
axes are unequal and their three intersections are oblique, and 
consequently there are never more than two planes of a kind; 
