OBIGIN IN IGNEOUS ROCKS. 79 
The report « of Mr. J. X. Lewis on ^' Coninduiu and the basic mag- 
nesian rock in North Carolina " is mainly a report of field observa- 
tions and does not take up any discussion of facts bearing on the 
origin of the corundum, though many points of interest and impor 
tance are brought out regarding the alterations of the peridotite and 
of the character of the vein and vein materials, and also concerning 
the alteration of corundum. 
Mr. F. P. King,^ in his report on the corundum deposits of Georgia, 
which is principally devoted to the description of localities, occur- 
rences, and varieties of corundum, etc., comes to the conclusion that 
corundum is an accessory mineral and that its presence is occasioned 
by an excess of alumina present in the rock masses, "chrysolite, 
gneiss, and hornblende-gneiss." This is explained by the alteration 
of these rocks into magnesium silicates, alkaline salts, and ferro- 
silicates, which, in conjunction with the carbonic acid of percolating 
waters, would dissolve the combined alumina and produce on crystal- 
lization all the minerals associated with corundum, and when the 
alumina is in excess would produce corundum itself. 
As has been shown, a number of theories concerning the origin of 
tlie corundum have been advanced. With one exception, liowever. 
they were all prior to the numerous and elaborate experiments that 
have been made by different investigators on the solubility of alumin 
in a molten basic glass and the separating out of corundum 
spinel crystals from this molten glass on cooling. They were also 
prior to Vogt's ^ very important investigations into the igneous origin 
of certain ore deposits. All these experiments have aided very mate- 
rially in solving the numerous problems in relation to the origin of 
many of the ores; and in the present investigation the facts proved 
by these experiments have been of exceptional assistance in the com- 
piling of evidence to substantiate the theory proposed. 
I 
md 
RELATIONS OF CORUNDUM TO PERIDOTITE. 
In all the field observations a careful search was made to find the 
I corundum directly in contact with the peridotite, and this contact was 
I observed at one locality. At the Egypt mine, on the western slopes 
j of Sampson Mountain, about 10 miles west of Burnsville, in Yancey 
County, N. C, several specimens have been found Avith the corundum 
crystals entirely surrounded by granular peridotite (dunite), and 
without any of the chloritic minerals usually present Avith the corun- 
dum. (See fig. 8, p. 33.) Both the peridotite and the corundum 
I have altered someAvhat, the corundum having a little muscovite 
dcA^eloped on its basal termination, while the peridotite is stained 
a yelloAvish broAvn and is rather friable. The specimens that w^ere 
" Geol. Survey North CaroUna, Bulletin No. 11 ; see also Bulletin No. 19. 
* Geol. Survey Georgia, Bulletin 2. 
" Zeitschr. fur prakt. Geol., Nos. 1, 4, and 7, 1893. 
