66 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
dominant
mica, as in the gneissic granite. The texture is wholly irregular and
typically pegmatitic because of the great range in size exhibited by
crystal grains of the same mineral species. Quartz is the most abundant
mineral with microcline>oligoclase>biotite >or =
orthoclase>muscovite. The rock differs from the associated gneissic
granite mainly in its texture and in the fact that microcline dominates
over orthoclase instead of bearing the reverse relation to it.
The
close association of the granite and pegmatite and the fact that the
same minerals are present in the same order of abundance in both rocks
is highly suggestive of a genetic connection between the two.
At
a point on the east shore of Boothbay Harbor the fine-grained pegmatite
was observed to be traversed by a vein of white quartz 2 to 3 inches in
width. The borders of this vein are not sharp; feldspar crystals of the
bordering pegmatite project into it, and in some instances their inner
borders (next the quartz) show well-developed crystal faces. Isolated
crystals of feldspar up to 3 inches in length also occur, apparently
wholly surrounded by the quartz of the vein. The feldspathic character
of this vein and the absence of a sharp or straight boundary between it
and the pegmatite indicate that it was not deposited as a fissure
filling along a fracture plane traversing solid pegmatite, but rather
that it was genetically a part of the pegmatite magma and was formed
before the complete solidification of its host. Apparently it
represents an end product of the pegmatite crystallization. The
sheetlike form of the vein indicates presumably that the pegmatite was
sufficiently rigid to permit the formation of a rift of some sort along
which the more quartzose magma could penetrate, but that coarsely
interlocking crystallization between vein and wall was still possible.
Similar relationships have been observed by the writer on a larger
scale in some of the feldspar quarries of Connecticut. (See PL XVI, B, p.
18.) They are of importance as showing without much question that many
at least of the quartz veins associated with the pegmatites may be
regarded as an end product of the crystallization of the pegmatite
magma.
Southward
along the east shore of Boothbay Harbor to Spruce Point abundant dikes
of pegmatite are found traversing the schists: they vary from
one-fourth to one-half an inch to 10 feet or even 50 feet across.
Xearly all of the dikes and particularly the smaller ones assume the
form of a succession of connecting lenses, indicating a very uneven
penetration of the pegmatite magma between the schist folia. The
schists usually exhibit a thickening of their lamina? opposite the
"nodes" of these irregularly bulging dikes, indicating a
crystallographic rearrangement of the schist constituents as an
accompaniment of the pegmatite intrusion.
