PRATT.] MODES OF OCCURRENCE. 11 
containing as much as 5 per cent of one or the other. The purest 
forms of corundum that have been analyzed arc the sapphire <>r gem 
varieties, which Sometimes show over 99.5 per cent of alumina. 
All corundums do no1 behave alike when heated to the high tem- 
perature necessary for the manufacture of vitrified wheels. While 
most corundums can, if properly cleaned, be used in the manufacture 
of these wheels, some will, when heated, crumble to a powder. It is, 
therefore, very essent Lai, before beginning to mine a corundum deposit, 
to thoroughly tesl the ore and ascertain its adaptability to the manu- 
facture of vitrified and other wheels. 
MODES OF OCCURRENCE OF CORUNDUM. 
Corundum was formerly regarded as occurring sparingly in nature, 
and in only a few types of rocks, Inil il is now known to occur rather 
widely, and instead of being in quantity in the basic raagnesian or 
peridotite rocks only, il has been found in abundance in syenites, 
in gneisses, and in schists. Although occurring in many of the crys- 
talline rocks, it has been observed as a rock constituent in only a few 
of them. In some cases il is an original constituent of the rock, and 
in other cases it has been formed Later, during the process of metamor- 
phism. 
In the United Slates corundum is known to occur in the following 
rocks: In peridotite (dunite ami saxonite), in biotite contact on sax- 
onite, in enstatite, in serpentine, in chlorite-schist, in amphibolite, 
in norite, in basie minette, in andesite, in syenite, in amphibole- 
schist, in gneiss, in mica-schist, in Limestone, and in cyanite. These 
modes of occurrence are. described below in the order in which the} 7 
are incut ioned. 
CORUNDUM IN PERIDOTITES. 
Extending from Tallapoosa County in east-central Alabama to 
Trenton, N. J., there is a narrow belt which contains disconnected 
outcrops of peridotite rocks; north of New Jersey (in New York, 
Connecticut, Massachusetts, New Hampshire, and Maine) thje out- 
crops are fewer and do not form a continuous belt, it is in the 
southern portion of this belt that these rocks have reached their 
greatest development, in some localities outcropping over an area of 
several hundred acres. In North Carolina and in the more southern 
portion of the belt the prevailing type of the rock is dunite, while in 
the northern portion the secondary rocks, serpentine and talc, are 
prominent. PI. I is a reproduction of a photograph of an outcrop of 
dunite at Buck Creek, Clay County, N. C. 
Throughout nearly the entire southern portion of the belt the peri- 
dotite rocks show a freshness to almost the surface of the exposures, 
and there are few localities where there is any considerable area of 
peridotite entirely altered to serpentine. Under the microscope thin 
sections of the dunite show an alteration to serpentine between the 
