peridotite from about three-fourths of a mile east of north of the emerald mine carried 0.19 per cent Cr203; and diorite from near the emerald mine carried 0.04 per cent Cr203.
Pale-greenish,
yellowish, and gray beryl crystals have been found in pegmatite cutting
granite and mica schist and gneiss in this region, but the only emerald
deposit discovered is where such pegmatite cuts a chromium-bearing
rock. This association of emerald-bearing pegmatite with a chromiferous
basic rock seems a most natural one. A theory of origin of the emeralds
at the Turner mine may be thus briefly stated:
Chromium-bearing
basic rock was intruded by granite magma either before or after final
solidification of the basic rock. There was a partial absorption of the
latter by the granite magma and some of the constituents of the basic
rock were taken into solution. Pegmatite veins, in which distinct
crystals were only partly developed, formed during the cooling of the
rocks. The elements necessary for the formation of beryl were present
in the magmatic solutions producing pegmatite, as in some other places
in the region. Chromium oxide necessary to impart a color to the
emerald was obtained by the action of solutions on the basic rock. A
curious coincidence is the fact that the per cent of Cr2Os
in the basic rocks at the Turner mine is about the same as that found
in emerald from Colombia, by T. Wohler, as mentioned above.
Should the theory of origin outlined above be essentially correct,
p
rospecting in areas
where similar rock associations occur might locate other deposits of
emeralds. Of course, it should not be expected that every pegmatite
associated with the rocks mentioned will carry emerald, for beryl has
been found with only a very small proportion of the large number of
pegmatites occurring in the region. Besides chromium-bearing basic rock
and pegmatite, the presence of the elements of beryl in the original
pegmatite-producing solutions would be necessary. Outcrops of basic
rocks similar to those at the Turner mine occur in a number of
localities in the region south of Shelby, and some of these are near if
not associated with granite. Pegmatites cut many of these basic rock
masses, and over some of the outcrops crystals of black tourmaline,
quartz, and quartz-inclosing actinolite are plentiful as at the Turner
mine. In some of these areas the associations are very similar to the
emerald locality. The occurrence of the basic rocks is not always
easily recognized, since weathering has been extensive. In such places
a study of the soil will often give the clue desired.
GARNET.
NEW HAMPSHIRE.
The
occurrence of a quantity of deep-red garnets on a hill near Chesham, in
Cheshire County, is reported by Mr. Leon Allen, of Keene, N. H. The
garnets are small but would yield clear gems ranging from one-eighth to
one-half of a carat. Further prospecting will be carried on during 1912.
COLORADO.
The
discovery of small bright-yellow grossularite garnets is reported by
Mr. J. D. Endicott, of Canon City, Colo., in the basin of North
