356 University of California Publications. [Geology
of their surface is very generally impaired, a showing vertical striations and sometimes slight curvature horizontally, and m is striated both vertically and in the zone
respec-
tively
. The pyramidoccurs
only on a minority
of crystals and is generally small and usually bright.
A
rather common type is illustrated in figure 1, plate 39, which was
drawn from a doubly terminated crystal free from the matrix and 42 mm.
long by 8 mm. wide. The slight curvature in g is not shown.
This illustrates about the average proportional length of the prisms.
It is very common for the front planes to be rather small and the back
planes to cut much farther down on the prism. The width of g is very variable and the planes ρ are frequently the largest of the terminal planes.
Figure 2 is from a stout prism some 12 mm. wide and shows a high development of the unit pyramids. The g faces are narrow and the "edges"
curved and of varying distance from the
edges.
Figure
3 shows a peculiar asymmetric development and was drawn from a doubly
terminated prism 8 mm. long by 3 mm. wide, though only the planes at
one termination are shown. It illustrates the usual ways in which
appears, either as a
small triangular face between g and the prisms, or less commonly as a rhomboidal form bounded by the prisms, and by ο and ρ when g is narrow or absent.
Only one crystal of the type shown in figure 4 was found. It shows the front faces
largely developed and cutting
down much farther on the prisms than the back faces. Both 
and g are absent, and r and ρ occur as narrow strips bordering o.
Physical Characters.
Hardness between 5 and 6. Density 3.18-3.19. Cleavage perfect, prismatic parallel to
, with normal angle of
80° 18'.
The
crystals are black and lustrous; in thin sheets or splinters, deep
blood red. Cleavage plates show strong pleoehroism: c deep ocreous
yellow to brownish red; a pale yellow to reddish yellow in thicker sections;
