32 CORUNDUM IN THE UNITED STATES. [bull. 180. 
of the shaft. The seams of decomposed feldspar observed in a become 
less and less kaolinized, until in i the seams are of pure plagioclase 
feldspar. In i there are two seams of corundum similar to b, d, and f, 
although in the fresh rock the corundum seams are not as pronounced 
as in the saprolitic rock. There is often a considerable amount of 
feldspar bordering the seams of the corundum. The general trend of 
the rock is about N. 5° to 10° E. 
From what could be seen of the solid and the saprolitic rocks, it is 
evident that the corundum occurs at intervals in the rock in seams a 
few feet in width. While the corundum may comprise 10 per cent 
or more of these veins, the amount in the rock that it would be neces- 
sary to mine would probably not be over a few per cent. The actual 
width of the dike is not known, but the saprolitic rock has been cut 
across for about 100 feet in a direction nearly at right angles to its 
strike. 
The fresh rock at the bottom of the shaft is somewhat varied in 
appearance, and while it does not show any definite gneissoid struc- 
ture, it sometimes closely resembles it. There are streaks in the rock 
a few inches thick, the more finely divided portions of which are dis- 
tinctly gneissoid. 
Some portions of the rock are decidedly porphyritic, and contain 
phenocrysts of a light-gray amphibole, a centimeter in diameter, in a 
groundmass of feldspar. A large part of the rock is made up, how- 
ever, of small, roughly outlined prismatic crystals of an amphibole, 
probably hornblende, and irregular fragments of plagioclase feldspar. 
The hornblende is almost black in color, but in thin splinters it has a 
bronze luster and a deep resinous color. Biotite of a deep-brown color 
occurs sparingly, and a pink garnet is rather abundant. This part of 
the rock has a gneissoid structure and contains the corundum. The 
corundum is of a light to a purplish-pink color and occurs in nodules 
up to 2 or 3 cm. in diameter. Some streaks in the rock are highly 
garnetiferous, composed essentially of the garnet and plagioclase 
feldspar or of garnet and biotite. Chalcopyrite occurs very spar- 
ingly in these portions of the rock. Small particles of graphite have 
been observed in the coarsely crystallized portions. 
Professor Pirsson has kindly made a microscopical examination of 
this rock, the results of which are embodied in the following para- 
graphs: 
In thin section the microscope disclosed the minerals hornblende, plagioclase 
feldspar, garnet, biotite, mnscovite, staurolite, and rutile. Hornblende is the most 
common, forming about two-fifths of the section, while of the remainder plagio- 
clase and garnet occur in about equal quantities and the others in comparatively 
insignificant amounts. 
The hornblende is formless, but tends to irregular columns, almost invariably 
extended in the plane of schistosity. It has very rarely a somewhat stringy tend- 
ency in its cleavage, but is usually homogeneous in broad plates. Its color is a 
clear olive brown, and it is somewhat pleochroic, but not strongly so. It is every- 
