ORIGIN IN IGNEOUS ROCKS. 87 
analysis showed a variable per cent of alumina and magnesia), but 
no corundum or spinel. 
3. Peridotite rocks containing chromite, corundum, and spinel. 
4. Peridotite rocks containing chromite and corundum, but no 
spinel. 
5. Peridotite rocks containing corundum and feldspar. 
A brief review of these phenomena in the order stated gives the 
following conclusions : 
1 and 2. Of the analyses of the chromites that have come under the 
writer's notice, all but two show the presence of alumina and mag- 
nesia in varying quantities. In some of them the percentage of alu- 
mina and magnesia is very large, and whereas in the ordinary chromite 
the formula may be represented by 9(FeO,Cr20.5), (MgO,Cr20;,), 
2(MgO,Alo03), at one locality near Webster, Jackson County, N. C., 
a chromite (mitchellite) was found which had the formula 
(FeO,Cr,03), (MgO,Cr203), 2(MgO,AL,03). Thus, when there is 
considerable chromite found in these rocks there is usually no corun- 
dum, or at most but a trace of corundum present ; and from what has 
been said above regarding the analyses of chromite, it would seem to 
be the case that when there is but little alumina and only a small 
excess of magnesia in peridotite magmas which contain considerable 
chromic oxide they unite to form the spinel molecule, and that this 
molecule, instead of separating out independently as the mineral 
spinel, enters into combination with the FeO,CroO:{ molecule in the 
mineral chromite. 
3. At a few of the peridotite localities both corundum and spinel 
have been found, as at the Corundum Hill mine, Macon County, N. C., 
where a fine-grained, almost black spinel occurs, through which are 
scattered particles and masses of corundum ; and at the Carter mine, 
near Democrat, N. C., where there is a great abundance of corundum 
and a green-black spinel. In peridotite magmas of this type it would 
seem that there has been an excess of magnesia present, which has 
united with a portion of the alumina present to form the molecule 
MgO,AL03, which separated out as spinel. The remaining portion 
of the alumina formed corundum. Undoubtedly there is a strong 
affinity between magnesia and alumina, which tends to form the 
spinel molecule; but in these peridotite magmas it is only the excess 
of magnesia over that required for the normal magnesian silicates that 
has united with the alumina. Except alteration products, there are 
no alumina-magnesia silicates found in these rocks, which shows that 
in magmas of this type there is no tendency for the alumina to unite 
with the magnesia in forming double silicates. 
4. At most of the peridotite localities there has been no spinel at all 
observed, but there is often a considerable quantity of corundum 
