12 CORUNDUM IN THE UNITED STATES. [bull. 180. 
particles of chmite. These peridotite rocks have been shown to be of 
igneous origin. 1 The blunt lenticular form in which they are found 
would be difficult to associate with any origin but that of an intruded 
igneous mass, which would also account for the apophyses that have 
been observed extending into the inclosing gneiss. At Webster, 
Jackson County, N. C, a large block of gneiss is completely inclosed 
by the peridotites in such a manner as could be attributed only to the 
intrusion of the latter while in a molten condition. The line of sepa- 
ration of the peridotites and the gneisses is always sharp, and there 
is no transitional zone from the acid gneiss to the basic peridotite. 
Under the microscope the latter rock shows the granular structure 
characteristic of plu tonic origin, the grains fitting perfectly into one 
another without cementing material. 
Associated with all these peridotites is the mineral chromite, which 
occurs as disseminated particles near the borders of the lenticular 
masses of the peridotites. There is very little carbon found associated 
with these rocks, and what has been observed is unquestionably of 
secondary origin. 
Until recently the common occurrence of corundum (not including 
emery), and the occurrence in which the mineral had been found in 
commercial quantity, was in association with these basic magnesian 
rocks, peridotites, principally dunite. The country rock that is 
commonly in contact with the peridotite is a hornblende-gneiss, but 
these peridotite formations have also been found in contact with a 
biotite-gneiss and with a mica-schist. Where the country rock is a 
gneiss it is usually considerably decomposed near the contact, and, 
while retaining the appearance of the unaltered rock, it readily crum- 
bles to pieces when handled. The peridotite is also more or less 
altered, the change being usually to serpentine. 
The corundum is not an accessory mineral in these peridotites, but 
is either concentrated near the contact of the peridotite with the other 
country rocks or in pockets within the peridotite formation. A series 
of alteration products has been developed in this contact zone, so 
that the corundum is not found in direct contact with the peridotite, 
but is separated by intermediate zones of chlorite, enstatite, etc. 
Chlorite and vermiculite are usually developed between the corundum 
and the gneiss. For convenience, the occurrences of the corundum 
in these alteration products between the peridotite and the gneiss are 
designated contact veins, and those wholly within the peridotite 
dunite veins. 
In a cross section of a contact vein extending from the gneiss to the 
peridotite (dunite) the following sequence is often observed : 
a. Gneiss, hornblendic or micaceous, apparently unaltered. 
b. Gneiss with same general appearance as a, but so decayed that the particles 
readily separate from one another. 
1 Elisha Mitchell Sci. Soc. Jour., Part II, 18!t."), p. 35, and Am. Jour. Sci., 4th series, Vol. VI, 
1898, p. 50. 
