9 2 
ALASKA GEOLOGY 
complex in their forms. They are combinations of the 
cube (ioo), the pyritohedron (210), and the octahedron 
(hi), with the less common forms n(2ii) and 111(311), 
trapezohedrons, and occasional planes of the diploids 
2(532) and Z(53i). The form n especially was well de¬ 
veloped with quite large planes. 
The occurrence of rhodonite has been noted (page 46) 
at the Chicago mine, Silver Bay, near Sitka, As this 
mineral has not before been reported from Alaska, it may 
be well to describe the occurrence in more detail. The 
rhodonite occurs in massive aggregates of short prismatic 
grains large enough to show the cleavage distinctly on the 
fractured surface. The color is pale pink and is empha¬ 
sized by the black stain of manganese oxide, as is so often 
the case with this mineral. The rhodonite forms a con¬ 
siderable portion of the vein-filling, in masses, up to a foot 
in diameter, of nearly pure material, but the color is too 
pale to give it value as a decorative stone, though its 
texture is well suited for polishing. It is often traversed 
by veins of calcite, and the microscope shows that more 
or less quartz is intermixed with it, even where it looks 
pure. 
Near the Treadwell mine, Douglas Island, several nar¬ 
row veins of albite and dolomite were found traversing 
the diorite. The albite is in small glassy crystals, some 
of which proved to be measurable on the goniometer. 
The crystals are simple twins on the albite law and show 
the following forms: M(oio), P(ooi), l(i 10), T(i 10), 2(130), 
f(i3o), x(ioi), ^403), 0(111), 8(112). These veins are of 
secondary origin, and the albite is probably simply a re¬ 
crystallization of the albite forming the predominant 
mineral in the diorite. 
Laumontite occurs abundantly in portions of the great 
trachyte body of St. Matthew Island (page 33). It is 
found with calcite and stilbite, filling or lining fissures 
