92 ALASKA GEOLOGY 



complex in their forms. They are combinations of the 

 cube (100), the pyritohedron (210), and the octahedron 

 (in), with the less common forms n(2ii) and 01(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(uo),T(no), z(i3o), 

 f(i3o), x(ioi), r(403), o(in), 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 



