CORUNDUM IN IGNEOUS ROCKS. 29 
olivine. These peridotite rocks have Ikhmi shown to be of igneous 
origin/* The bhint lenticular fcu'ui in which they are found would 
he difficult to associate with any origin l)iit that of an intruded igne- 
ous mass, which Avould also account for f\\i\ ap()])hyses that have been 
observed extending into the inclosing gneiss. At AVebster, Jackson 
County, N. C, a large block of gneiss is completely inclosed by the 
])eridotites in such a mannc^r as could be attributed only to the intru- 
sion of the latter Avhile in a molten condition. The line of separation 
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 shoAvs the granular structure character- 
istic of plutonic origin, the grains fitting 2)erfectly 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 
nuisses of the peridotites. There is very little calcite found associated 
with these rocks, and what has been observed is unquestionably of 
secondary origin. 
Five years ago 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 dunite variety of peri- 
dotite is the more common one throughout the southern area, and it 
is Avith this variety that most of the corundum is associated. PI. II 
is a reproduction of a photograph of an outcrop of peridotite (dunite) 
at Buck CVeek, Clay County, N. C. 
The peridotites and associated basic rocks occur as oval, lenticular, 
and irregular masses and sheets in a region of metamorphic rocks 
composed chiefly of biotite-gneiss. As subordinate facies of this 
normal gneiss, however, more or less extensive areas of hornblende- 
gneiss, mica-schist, and quartz-schist are developed. Peridotites are 
found inclosed by or in contact Avith all of these A^arious types. On 
account of greater resistance to Aveathering it often happens that 
hornblende-g-neiss is most conspicuous in outcrops, even w^here rela- 
tiA'ely iniimportant in extent. Plence it has often been reported that 
the peridotites are always, or at least in most cases, associated Avith 
hornblende-gneiss. 
The gneiss is usually considerably decomposed near the contact, 
and, while retaining the appearance of the unaltered rock, it readily 
crumbles Avhen handled. The peridotite is also generally altered 
near the contact, but in quite a different manner from the gneiss. In 
fact, it usually sIioavs a strong tendency to undergo complete decom- 
position, in which only a residue of quartz or chalcedony and an 
" Lewis, J. A'., Elisha Mitchell Sci. Soc. Jour., pt. 2, 1895, p. 35. Pratt, J. H., Am. Jour. 
Sci., 4th ser., vol. 6, 1898, p. 50. 
