SOUKCES TN UNITED STATES. 27 
iy considered by many to excel in (Quality the best Naxos emery 
obtained from the (irecian ishmds. The Chester emery, whicli was 
at one time considered not to be a true emery, has taken its pUice with 
the Naxos and Turkish emery, and is considered by many to be supe- 
rior to these. For a description of tlie foi-eion (Mueries, see ])a^es 155- 
157. 
CORITN^DITM-BKARTNG ROCKS 1^ THE TrN^TIF.T) STATF.S. 
A generation ago corundum was regarded as a comparatively rare 
mineral, whereas now it is known to be widely distributed in nature. 
With the exception of emery, it was thought to occur in quantity 
only in the basic magnesian rocks, such as the peridotites and their 
serpentine derivatives. Now it is known to occur abundantly in 
syenites and other igneous rocks, as well as in various gneisses and 
schists. It has been found in many crystalline rocks, and in some of 
them as an original mineral, but only within the last few years has 
it come to be regarded as an essential rock constituent, as in the 
syenites of Ontario, Canada, and of Montana, and in similar rocks 
of California and Colorado. In some cases it is und()u})tedy a 
product of metamorphism. 
Corundum has been found in igneous and metamorphic rocks and 
in alluvial deposits. AVhere it occurs in the igneous rocks it is either 
as a direct constituent of the more acid intrusive rocks, or it occurs 
as segregated masses near the periphery of the basic intrusive rock, 
or is associated with inclusions that have been picked up by the intru- 
sive rocks. In the metamorphosed rocks the corundum is either one 
of the original constituents of the igneous rock that has been sub- 
jected to metamorphism or the result of regional metamorphism by 
which shales, bauxite, or other rich aluminous minerals have been 
converted into corundum, or it may be due to contact metamorphism, 
the corundum occurring simply in the narrow zone adjoining the 
intrusive rock. In the alluvial deposits the corundum is found 
either in the recent alluvium composed of soil, or in sands, or in 
gravels. 
There a large number of rock types in which corundum has been 
found, and although they do not represent entirely different modes 
of occurrence of corundum, as perhaps the classification given above 
indicates, still in the present paper the descriptions of the modes of 
occurrence of corundum are given accoi'ding to the type of rock in 
which the corundum is found, and no attempt has been made to 
classify them directly according to the outline given above. They 
are, hoAvever, divided into those Avhich occur in igneous rocks, meta- 
morphic rocks, and alluvial deposits. 
Descriptions of these various modes of occurrence are given below, 
some of which are discussed in connection with th(» question of the 
