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ORIGIN.
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commonly
lens-shaped pegmatite body. Within this zone pockets arc also apt to be
particularly abundant. Pockets are found in some of the normal
pegmatites also, but their disposition seems to be wholly sporadic and
most of them are barren of gem minerals, though some contain groups of
fine quartz crystals or of small feldspar crystals on their walls.
In
the richly gem-bearing pegmatites the zone of sodium and lithium
minerals is generally separated from the normal pegmatite which borders
it by a highly garnetiferous zone from an inch to several inches wide,
the garnet being associated with a granular aggregate of quartz and
feldspar. A few such bande are paralleled by a second one of similar
character 1 or 2 inches away. These garnet bands are frequently of
practical service in tracing the gem-bearing zone. The pegmatite
outside them is invariably barren of gem minerals.
As
explained in the detailed descriptions of Mount Mica and other gem
localities, the gem tourmalines are usually found in pockets, paving
deveioped on their walls. The pockets, though confined mainly to the
zone rich in soda and lithium minerals, may be wholly absent from
considerable portions of such zones and are distributed with great
irregularity through the remaining portions. The character and
distribution of pockets is best illustrated at the Mount Mica
tourmaline mine. Plate XII is from an early photograph of the workings
at Mount Mica, taken at a time when only the outcropping portion of the
gem-bearing zone had been worked. A stake with card attached marks the
position of each pocket.
Most
of the pockets are somewhat spherical in form, but others are oval or
elongate and others exceedingly irregular. Their size ranges from a few
inches across to a magnitude such as is shown in Plate XIII, which
represents the largest pocket ever found at Mount Mica.
Minor details of structure of the gem-bearing pegmatites are discussed in the locality descriptions (p. 18).
ORIGIN OF THE MAINE PEGMATITES.
The
writer does not purpose in this report to discuss the voluminous
literature on pegmatites except in so far as it bears closely on those
of the region under discussion. Previous writings and theories have
been well summarized by George II. Williams a and especially by Brigger.6
RELATIONS TO GRANITES.
The
ireologic relations of the Maine pegmatites, as already pointed tt (pp.
13-15), show beyond reasonable doubt that they are geneti-
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