30 COKUNDUM, ITS OCCURRENCE AND DISTRIBUTION. 
oclierous clay-like substance remain. This contact decomposition is 
altogether difl'erent from and independent of the alteration to ser- 
pentine Avliich almost luiiversally characterizes the peridotites of this 
region. Only a moderate degree of serpentinization, hoAvever, is 
usually observed, not enough in the majority of cases to be visible to 
the naked eye; and in only a few localities has it proceeded to the 
extent of entirely replacing the original rock. 
The corundum found in thes^ peridotites does not occur as acces- 
sory mineral or as rock constituent, but is concentrated either near 
the contact of the peridotite and the inclosing gneissic rocks or in 
pockets Avithin the mass of the peridotite. A series of secondary 
minerals, hoAvever, has been developed both along the contacts and 
Avith the corundum masses Avithin the peridotite, so that the corundum 
is not found in direct contact Avith either the peridotite or the gneiss, 
nor are these rocks in contact Avith each other. The secondary 
minerals are chiefly chlorites, vermiculites, enstatite, and talc, and 
are not in any sense the results of contact metamorphism. It is cus- 
tomary to refer to these corinidum-bearing zones as '' A^eins,'' and 
that term is used here merely for convenience, Avithout implying any 
particular character or origin. Those occurrences about tile borders 
of the peridotites are designated as " border veins," and those Avholly 
Avithin the peridotite as '' interior veins." " 
The appearance and general character of the veins difi'er someAvhat 
in these tAvo classes. The interior veins trend approximately toAvard 
the center of the peridotite mass, generally groAving gradually thinner 
in this direction until they pinch out altogether. This character Avas 
distinctly observed at Buck Creek, Clay County, N. C., and at Laurel 
Creek, Rabun County, Ga. (PL XIU.B, and PI. XVII, B). In the 
border veins, hoAvever, the corundmii seems to extend doAvuAvard 
indefinitely along the plane of contact, varying irregularly and some- 
times very greatly, both vertically and horizontally, but exhibiting 
no marked tendency to thin out in any particndar direction. The 
border veins often give off branches into the peridotite, Avhich grad- 
ually thin out toAvard the center of the mass and exhibit the usual 
characteristics of interior veins. These distinctions betAveen the two 
types of A^eins haA^e been observed by many of those Avho have mined 
and prospected for corundum in this region. 
In a cross section of a border A^ein extending from the gneiss to the 
peridotite (dunite) the folloAving sequence is often observed: 
a. Gneiss, hornbleiidic or inieaceous, apparently unaltered. 
h. Gneiss with same general appearance as a, but so decayed that the particles 
readily separate from one another. 
« In Bulletin 180 on The occurrence and distribution of corundum in the United States. 
" contact vein " was used instead of " border vein," and " dunite vein " instead of 
" interior vein." Bull. U. S. Geol. Survey, No. 180, 1901, p. 12. 
