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 (001) -(110) respec- 

 tively (110). The pyramid r(221) occurs 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 curva- 

 ture in g is not shown. This illustrates about the average pro- 

 portional 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 p 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 nar- 

 row and the "edges" gAp curved and of varying distance from 

 the oAg 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 r(221) appears, either as a 

 small triangular face between g and the prisms, or less com- 

 monly as a rhomboidal form bounded by the prisms, and by o 

 and p when g is narrow or absent. 



Only one crystal of the type shown in figure 4 was found. It 

 shows the front faces $(111) largely developed and cutting 

 down much farther on the prisms than the back faces. Both 

 ?'(112) and g are absent, and r and p occur as narrow strips bor- 

 dering o. 



Physical Characters. 



Hardness between 5 and 6. Density 3.18-3.19. Cleavage 

 perfect, prismatic parallel to m(110), with normal angle of 

 80° 18'. 



The crystals are black and lustrous ; in thin sheets or splinters, 

 deep blood red. Cleavage plates show strong pleochroism : c 

 deep ocreous yellow to brownish red ; a pale yellow to reddish 

 yellow in thicker sections; c'Ac=d4?5. 



