12 HISTORY OF THE GEMS FOUND IN NORTH CAROLINA.
origin,
and the gem corundums of Montana are derived from intrusive dikes. The
occurrence in crystalline limestone, in northern New Jersey, like that
in Burma, is probably of the other type, a result of contact
metamorphism, although Messrs. Brown and Judd have advanced a theory
for the Burman mines, that attributes even these to an original igneous
source.
The
whole question of the geology of corundum,—its origin, mode of
formation, etc., has been obscure and uncertain for a long time. Many
theories have been advanced, only to be modified by subsequent
discoveries. Within a few years past, however, important progress has
been made; and though much remains to be ascertained, a number of
points have gradually been established.
Among
these is the fact that corundum, long regarded as a somewhat rare
mineral, is really of more frequent occurrence than was formerly
supposed; and also that it has been formed under various conditions and
in several distinct ways. As already stated above, it is now known to
have been produced (1) by crystallizing directly out of igneous rocks;
and (2) by various forms of alteration and metamorphism, in both
igneous and sedimentary rocks. The first head is further divided into
occurrences in basic and in acidic rocks, and again into cases when the
alumina was present in excess in the igneous rock itself, as an
original constituent (autogenic), and those when it was introduced in
pieces of an aluminous shale traversed by the igneous rock and taken up
by it in its ascent (allothigenic). All these cases of occurrence have
now been fairly identified in the corundum localities in the United
States.
The
earlier writers generally held that pure alumina (corundum )was a
secondary or derivative mineral, formed by the alteration of other
species in which it had previously existed in combination, as a
silicate. Its close association with the altered peridotite or
chrysolite (dunite) belt of the South Atlantic States, has already been
referred to, and the belief of some geologists that the corundum was
derived from the chrysolite, by various processes of alteration. The
late eminent Dr. F. A. Genth, while not committing himself to any
positive statement as to the origin of the corundum, developed a
remarkable body of facts as to the alteration of corundum itself into
various other and associated minerals.7 There is not space
here to go into any full outline of the course of observation and
opinion. This has been very well done by Dr. J. H. Pratt, of the North
Carolina Geological Survey, in his recent paper " On the Origin of the
Corundum associated with the Peridotites in North Carolina." 8 In this
