ORTOTN TN TONEOUS ROCKS. 85 
corundum would bo found surrounded hy the peridotite, but, as was 
stated on page 80. there is but one locality known to the author where 
the corundum is found thus directly surrounded. Tlie absence of any 
general occurrence of the corundum in peridotite is readily explained 
by the ease with which the peridotites themselves sufl'er decompo- 
sition. 
The corundum concentrated in the peridotite near its contact with 
the gneiss, where the thermal waters coming in contact with the heated 
masses would have the best chance to act, would furnish ahnnina 
for reacting with the magnesian minerals to form the aluminum- 
magnesium silicates found surrounding the corundum. That the 
peridotites decompose very readily is apparent from the numerous 
specimens of these rocks found completely surrounded by foreign 
material that must have been formed at their expense. The corundum 
lias been found in serpentine, l)ut this is often surrounded by chlorite. 
The descriptions of the cross sections of the corundum veins, given 
on pages 30-32, are similar to those described by Shepard," Chatard,'' 
and Lewis,^ and show practically the same sequence. 
As the peridotite and the corundum began to alter and decompose 
under the influence of atmospheric agencies and thermal solutions, 
there would be formed a series of decomposition minerals on the 
peridotite side of tlie vein and onl}^ a few on the gneiss side. The 
decomposed material that would be found on the gneiss side of the 
vein would vary according to the amount of the peridotite that had 
been held between the cornndum and the gneiss (fig. 13). The com- 
mon decomposition product surrounding the corundum is chlorite 
(clinochlore) or a further alteration of the mineral to vermiculite. 
Between the corundum embedded in chlorite and the gneiss there 
is often very little and at times no chlorite or vermiculite developed, 
the corundum-bearing porticm of the zone being apparently in direct 
contact with the gneiss {h of cross section, fig. (>, p. 31). 
In the interior veins the alteration ])roducts developed are the 
same on both sides of the corundum-bearing zone and are in most 
cases nearly an exact reproduction of the peridotite side of a cross 
section of a border vein (figs. (> and 7). This demonstrates that the 
gneiss had little or no influence in the formation of the alteration 
products of the border veins. 
The analysesof Chatard,'' which show the chemical ciiara(;ter of the 
vein to increase in magnesia and decrease in alumina as the peridotite 
is approached, are in accord with the present (lieory regarding the 
formation of the alteration products. 
" Am. Jour. Sci., ;M ser., vol. 4. 1872, p. 111. 
"BuU. No. 42, U. S. Oeol. Survey, 1887, p. 41). 
<• Bull. No. 11, (Jeol. Survey North Carolina, 18J>G, p. 93. 
" Bull. No. 42, TJ. S. Geol. Survey, pp. 50-.56. 
