the more viscous by the less viscous portions of the same magma when both were under lateral compression.
At
an old road-metal quarry in the city of Brunswick, schists of probable
igneous origin are also well exposed in association with pegmatite. The
schists show very even and regular foliation and an alternation of
broad light-gray layers with narrower ones which are dark gray to
nearly black. The lighter bands are seen under the microscope, to be a
hornblende granite of interlocking granular texture and without
cataclastic structures. The foliated structure is due to a greater
abundance of hornblende along certain planes than along others and to
subparallel elongation of the hornblende grains. The dark-gray phases
of the schist have the mineral composition of quartz diorite, the
feldspar being largely andesine. A few bands up to one-eighth inch or
so across are a more coarsely crystalline association of quartz with a
little feldspar. These schists cany none of the minerals, such as
staurolite and andalusite, frequently observed in metamorphosed
sediments, and though their derivation by metamorphism from arkoses or
graywackes is conceivable it is not probable. The pegmatite associated
with these schists locally cuts across their foliation, but in other
places grades into them so completely as to suggest that the schist was
not completely solidified at the time the pegmatite was intruded.
Here
and at a number of other localities a slight foliation parallel to the
schist folia is visible in some of the pegmatites; it suggests a Blight
flowing movement in the schist subsequent to the intrusion of the
pegmatite. The thickening of the schist folia opposite the nodes of
pegmatite dikes and their thinning opposite the bulges (see fig. 1) is
also indicative of flowing movements in the schists at the time the
pegmatite was intruded. Many of the pegmatite bodies associated with
the primary flow foliates are probably to be regarded as intrusions
under high pressure of a less viscous into a more viscous magma.
PEGMATITES IN MASSIVE GRANITES.
The
relationships exhibited at a number of localities between the
pegmatites and the granites throw much light- on the origin of the
former. Of broad significance is the fact that granite is present in
all of the districts in which pegmatite occurs. The reverse relation
also holds, though to a lesser degree. The similarity in mineral
composition between the granites and the pegmatites will be considered
later.
The detailed relationships existing between the two rocks are various. At the Woodside quarry in the town of Brunswick, 2-1/2 miles
southeast of Hillside station, a rather fine-grained muscovite-biotite
granite has been quarried for flagging and underpinning. In it the
pegmatite often forms lens-shaped or wholly irregular bodies,
