RoGErs. | Minerals of the Galena-Joplin District. 457 
may be divided into three distinct types, though other com- 
binations occur. 
TypE 1. p,,0'; ¢c,~O~; p,O. This is perhaps the com- 
monest of the types. Thecrystals are fairly large, black in 
color, and occur in cavities. They have a tetrahedrid aspect, 
due to the dominance of the negative tetrahedron p’. One 
crystal was seen in which the cube was the dominant form, 
though the tetrahedrid aspect was still noticeable. The posi- 
tive tetrahedron is always a subordinate form. Figure 9 
represents the average development of crystals of this type. 
Twinning is very common, in fact, almost universal, with this 
type of crystals. 
Several crystals composed of the correlative tetrahedra 
were observed. The two forms are almost equally developed, 
yet their appearance is quite different. An octahedral crystal 
in the collection of Mr. Will Picher, at Joplin, mentioned by 
Winslow, is evidently of this type. An octahedron is of 
course impossible in sphalerite, a mineral of the tetrahedral 
class. 
A single large crystal with the two forms, d and p, was 
observed. The surface of the positive tetrahedron is smooth 
though much pitted, while that of the dodecahedron is very 
rough with vicinal planes. 
Another crystal with the forms n, c, and p, much resembles 
crystals of this type. 
Type 2. M, —808; d, ~0; p, O. The crystals of this 
type are quite common at the Tennessee Prairie mines, five 
miles south of Galena, and at the Gracie Clark mines, north- 
west of Galena, where they occur as isolated crystals of a 
reddish-brown color in a dark clayey siliceous matrix. The 
peculiarity of these crystals is that they are abnormally de- 
veloped, being shortened in the direction of one of the octa- 
hedral iriteraxes. The dominant faces are those of the 
rhombic dodecahedron. In consequence of this abnormal de- 
velopment the crystals appear as combinations of a rhombo- 
hedron with a prism of the second order. On account of the 
presence of the M, faces the crystals have a decided hemi- 
morphic aspect in the direction of the above-mentioned axis. 
