This historic book may have numerous typos and missing text.
Purchasers can download a free scanned copy of the original book
(without typos) from the publisher. Not indexed. Not illustrated.
1905 Excerpt: ... with accessory quartz, biotite, hornblende,
magnetite, apatite, titanite, pyrite. pyrrhotite, garnet, and
orthoclase. Exclusively decomposition minerals are actinolite,
chlorite, and serpentine. Quartz is very variable in amount,
ranging from 0 to 30 per cent. The pyroxenic constituent may be
exclusively hypersthene, chiefly hypersthene, or less frequently,
chiefly augite or exclusively augite or diallage. It constitutes
from 10 to 40 per cent of the rock. The plagioclastic constituent
is labradorite or labradorite-bytownite and varies from 5 to 50 per
cent of the rock. Between the pyroxene, whether hypersthene or
augite, and the labradorite there occur reactionary peripheral
bands of garnets. Between the garnets and the pyroxene there is a
narrow zone of quartz and hornblende. Hypersthene and the anorthite
molecule will produce hornblende, quartz, and common garnet (an
isomorphous mixture of grossular, almadine, and pyrope). In the
case of the monoclinic pyroxene the feldspar molecule is not needed
and lime is liberated. These garnet rims are the most striking
petrographic features of the gabbro (see plates 57, 58, and 59).
Wherever the gabbro has been subjected to pressure, as along the
periphery of the intrusive mass, pyroxene is replaced, chiefly by
green hornblende and subordinately by biotite. Such a margin of
hornblendegabbro is so marked a feature of the Cecil County gabbro
mass that it can je mapped as a separate formation. A more or less
schistose structure accompanies the development of these two
minerals. Along the contacts hornblende also accompanies biotite as
a constituent of the gneiss. This contact phenomenon increases the
difficulty of separating the two formations. The gabbro is
associated with and, through decrease in feldspar, grades i...
|Country of origin:
Geological Society of America
||246 x 189 x 15mm (L x W x T)
||Paperback - Trade
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