28 CORUNDUM IN THE UNITED STATES. [bull. 180. 
This gradual transition of the pleonaste and iron ore into t lie nor- 
mal norite and the occurrence of the norite minerals in the contact 
ore are strong evidence that these ore bodies were formed by the dif- 
ferentiation of the molten norite magma. 
In the vicinity of these norites there are small masses of peridotite, 
but no corundum has been found associated with them. 
CORUNDUM IN BASIC MINETTE. 
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 
followed for over a mile in a nearly east-west course. Their gen- 
eral 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, decided 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 and 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, Hat, tabular crystals, some of which 
are half an inch in diameter. 
Professor Pirsson, of the Sheffield Scieni ilic School, has made a pet- 
rographical examination of this rock, and describes it as follows: 1 
In the section the rock at once shows its character as a dark, basic lamprophyre, 
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 colorless and a strong, 
clear, brown tint. It occurs in ragged masses, rarely showing crystal outline, and 
it contains a large amount of small apatite crystals. The pyroxene is of a pale- 
green tint, with the habit of diopside, and is filled with many inclusions, now 
altered, but probably originally of glass; in some crystals these inclusions are so 
abundant as to render the mineral quite spongy. The grains sometimes show crys 
tal form, but are mostly anhedral and vary in size, though the evidence is not suf- 
ficient to show two distinct generations. 
These two minerals lie closely crowded together, and no feldspars are seen in the 
rock. The interstices between them consist of a small amount of a clouded, brown- 
ish, kaolin-like aggregate, which appears to represent some former feidspathoid 
Am. Jour. Sci., 4th series. Vol. IV, 1897, p. 421. 
