76 CORUNDUM^ ITS OCCURRENCE AND DISTRIBUTION. 
ORIGIN OF CORUNDUM IN IGNEOUS ROCKS. 
That there are several modes of origin of corundum would seem 
clear from the various ways in which it occurs, as shown in its asso- 
ciation with plutonic and metamorphic rocks, such as granites, 
gneisses, and crystalline schists, and with the basic magnesia rocks, 
v/hich have been described above. Alumina is now known to be 
soluble in many basic magmas, and as these cool corundum separates 
out. Furthermore, not only can the manufactured hydrous oxide of 
aluminum be converted into corundum by means of heat and pressure, 
but also the mineral bauxite, the natural hydrous oxide of aluminum. 
From what has been stated under the section on modes of occur- 
rence of corundum it is seen that there are many widely separated 
localities which contain rocks in which corundum is one of the 
essential constituents and not simpl}^ an accessory mineral. Thus 
there have been described corundum-syenites from India,'' Russia,' 
(janada,^ and Montana,^' corundum-pegmatite from Russia,'' and plu- 
masite, an oligoclase-corundum rock, from California.'' 
It is very evident, then, that there are magmas that contain an 
excess of alumina, just as there are magmas with an excess of silica, 
and that the alumina separates out as corundum in the same mannei 
that silica separates as quartz in granitic rocks. 
The different methods of formation of the corundum associated 
with the various rocks in the United States are discussed in the fol- 
lowing paragraphs. 
ORIGIN OF CORITNDTTM IN PERIDOTITE. 
REVIEW OF LITERATITRE. 
Although many writers in their papers on the occurrence of corun 
dum and the peridotites mention what they consider might be th( 
probable origin of this mineral, they do not take up an}^ discussioi 
of the problem. 
In 1872, Mr. C. IT. Shepard,^ in an extended article on the corun 
dum of North Carolina and Georgia, describes the occurrences oi 
corundum and chrysolite, the associated minerals, and what he calh 
the development of the " strata,'' which " exhibit the following ordei 
of formations: (1) Chrysolitic rock somewhat mixed with anthophyl 
lite; (2) a layer of micaceous rock; (3) a seam of chalcedony; (4) j 
" Man. of Geol. India, Economic Geo!., pt. 1, 1898, and Mem. Geol. Survey India, vol 
30, pt. 3, 1901. 
" Experimentelle Untersnchunsen iiber die Bildung der Minerale im Magmas: Tscher 
malis mineral, und petrog. Mittheil., 1898, pp. 1-90, 105-240. 
<' Rept. Bureau of Mines, vol. 8, 1899, pt. 2, p. 207. 
<^ See p. 48. 
« Bull. Dept. Geol., Univ. California, vol. 5, No. 8, pp. 219-229. 
f Am. ,Tour. Sci., 3d ser., vol. 4, pp. 109-114, 175-177, 1872. 
