58 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
probably hornblende, and of irregular fragments of plagioclase feld- 
spar. 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 corun- 
dum. 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, and are composed essentially of garnet and 
plagioclase feldspar or of garnet and biotite. Chalcopyrite occurs 
very sparingly 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 follow^ing para- 
graphs : 
In thin section the microscope disclosed tlie minerals hornblende, plagioclase 
feldspar, garnet, biotite, muscovite, staurolite, and rutile. Hornblende is the 
most common, forming about two-fifths of the section, while of the remainder 
plagioclase and garnet occur in about equal quantities and the others in com- 
paratively 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 
tendency 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 everywhere dotted by the small grains of garnet, which rarely show good 
crystal form. The garnet occurs associated also with the plagioclase. 
The plagioclase occurs t\yinned according to the albite law only. In sections 
perpendicular to 010 the lamellae show extinction as great as 80°, and the plagio- 
clase is therefore rich in lime and as basic as labradorite, which it probably is. 
It shows strong evidence of shearing movement in the rock ; it is often broken, 
exhibits rolling extinction, and the albite lamelhie are curved and bent. It runs 
along the planes of schistosity between the feldspars and forms a mosaic of 
angular broken grains. 
Staurolite was found in rather broad, irregular grains, and rutile in small, 
irreguiar grains and well-crystallized prisms. 
Professor Pirsson has indicated that the character and structure of 
this rock, composed chiefly of amphibole, labradorite, and garnet, 
suggest most strongly that it is a metamorphosed igneous rock of the 
gabbroid family. During metamorphism the augite of the gabbro 
w^ould be converted into the brown hornblende; any iron ore that was 
present would be taken up by the hornblende and the garnet. The 
rutile would have resulted from the titanic acid that is a regular com- 
ponent of the iron ores in these gabbro or diabase rocks. Staurolite 
IS a mineral that Avould be rather naturally expected, as it is usually 
a mineral of metamorphism, and its natural home is in the schistose 
rocks. The feldspar has suffered the least (except the corundum) 
chemically, and shows only the shearing of dynamic processes. 
The corundum does not occur in crystals, but in small fragments 
and in elongated nodules, which are cracked and seamed and appear 
