46 CORUNDUM.. ITS OCCURRENCE AND DISTRIBUTION. 
completely destroyed by fusion or solution in the inclosing magma. 
When not completely destroyed, the inclusions also frequently con- 
tain corundum associated with numerous metamorphic minerals. 
The trachytes, andesites, and basalts of the Siebengebirge, in 
Germany, as described by Dannenberg," also contain corundum, 
spinel, magnetite, sillimanite, and numerous inclusions of sandstone, 
schist, and granulite. 
A hornblende-andesite of the Eifel, Germany, as descril^ed by Dr. 
J'. Vogelsang,^ contains numerous granular aggregates of cordierite, 
andalusite, sillimanite, feldspar, biotite, corundum, spinel, rutile, 
quartz, zircon, and magnetite, without recognizable inclusions of 
older rocks. 
CORUNDUM IN MONCHIQUITE. 
Near the entrance of Yogo Gulch, in Fergus County, Mont., two 
parallel dikes of igneous rock have been observed cutting through 
the limestones. These dikes are about 800 feet apart, and can be 
follow^ed for over a mile in a nearly east- west course. Their general 
width is from 6 to 20 feet, but they are occasionally 75 feet wide. 
They are much decomposed near the surface, but in working them 
for the sapphires which they contain the nearly unaltered rock has 
been encountered. 
The rock has a dark-gray, decidedly basic appearance, and is very 
tough, breaking with an uneven fracture. Scattered through it are 
light-green and white fragments, which are by far the most conspic- 
uous of any of the mineral components of the rock. These are a 
pyroxene that is more or less decomposed into calcite. Some of these 
white fragments are probably the barium carbonate witherite, for in 
the concentrates obtained from washing the decomposed portions of 
the dike a considerable quantity of this mineral was found. Num- 
berless crystals of pyrite, not over a millimeter in diameter and almost 
perfect trapezohedrons, were also found in these concentrates. A few 
scattered tablets of biotite, from 2 to 3 mm. in diameter, were 
observed. The sapphire variety of corundum is found rather spar- 
ingly in this rock in well-formed, flat, tabular crystals, some of which 
are half an inch in diameter. 
Professor Pirsson, of the Sheffield Scientific School, has made a 
petrographical examination of this rock, and describes it as follows : ^ 
In thin section the rock at once shows its character as a darlv, basic hunpro- 
phyre, consisting mainly of biotite and pyroxene. There is a little iron ore 
present, but its amount is small and much less than is usually seen in rocks of 
this class. The biotite is strongly pleochroic, varying between an almost color- 
" Tschermaks mineral, und petr. Mittheil., vol. 14, 1894, p. 17. 
" Zeitschr. Deutschen geol. Gesell., vol. 42, 1890, p. 1 ; Am. Naturalist, 1892, pp. 165-100 
(Abst). 
''Am. Jour. Sci., 4th ser., vol. 4, 1897, p. 421. 
