1)4 CORUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
Yogi." In that case the pyrrhotite is concentrated toward the border 
of the norite, and there is at some places a gradual transition from 
the pure pyrrhotite through pyrrhotite-norite to the pure norite, 
while at others there is a sharp contact between them. 
ORIGIN or CORUNDUM IN PLUMASITE. 
Pliimasite has been described on page 43 as an oligoclase-corunduju 
rock Avhich is either a separate dike rock cutting through the perido- 
tite, or is a differentiated zone that has separated out from the 
original peridotite nuignia. Whichever of these tAVo may be the exact 
origin of the oligoclase-corundiun rock, it is very evident that the 
corundum has separated from a molten magma which contained the 
chemical compounds of plagioclase feldspar with an excess of aluuuna. 
Upon the cooling of this magma a sufficient portion of the alumina 
would be utilized in forming the oligoclase molecule, and only the 
excess would separate out as corundum. This would take place 
whether the magma had simply the composition of a medium acid 
plagioclase with an excess of alumina, or whether the magma had 
principally the composition of olivine Avith a certain amount of 
alkalies, lime, and alumina, the three latter in sufficient quantity to 
make the oligoclase molecule with an excess of alumina. This forma- 
tion of the corundum is in accord with the experimental work of 
Logorio. 
ORIGIN or CORUNDUM IN MONCHIQUITE. 
The sapphires which are found in the monchiquite of Yogo, Mont., 
are all of some • shade of blue, and they occur rather sparingly 
in the rock; but, from their sharp, distinct crystals and their general 
distribution in this rock, it is just as evident that they have crystal- 
lized out of a molten magma as that the Avell-formed phenocrysts of 
feldspar in porphyry have crystallized out from a molten magma. 
As stated by Prof. L. V. Pirsson,'' however, the general character of 
the rock shows that it could not originally have been sufficiently rich 
in alumina to have allowed a general separating out of corundum. 
The molten magma during its intrusion undoubtedly took up great 
quantities of inclusions from the sediment through which it passed. 
The Belt formation, consisting of clay shales of great but unknown 
thickness, probably underlies the limestones of this district, and the 
included fragment of these shales would be the source of the alumina 
of tlie sapphires. 
It may be, as some believe, that the Belt formation is absent, but 
it is known that the Cambrian beds, which contain almost every 
« Zeitschr. fiir prakt. Geol., Nos. 1, 4, and 7, 1898. 
^ Pirsson, L. V., Geology of Little Belt Mountains, Montana : Twentieth Ann. Rept. 
U. S. Geol. Survey, pt. 3, 1898-99, p. 554. 
