micas,
quartz, chlorite, hornblende, staurolite, andalusite (chiastolite),
cyanite, garnet, epidote, titanite, fibrolite, calcite, dolomite,
tremolite, tourmaline, serpentine, talc, pyroxene, hematite, magnetite,
pyrite, while in notable instances coarse granites (pegmatites)
intruded among them supply less usual minerals. On Manhattan Island Mr.
W. Niven found Mon-azite and Xenotime (see List of N. Y. Is. Minerals,
p. 139) in a granite matrix, and the granite areas have contributed the
larger number of well-developed minerals.
Metamorphic schists are
universally folded, crumpled, in wavy plications like undulating paper
sheets, evidence of those squeezings and thrusts which have played an
extreme part in their formation. Such titanic force has been engaged,
that fracture and flowage of parts has taken place, crystals have been
pulverized, and nodular stones in conglomerates stretched or elongated.
In this connexion it may be recalled that experiments in the
geo-physical laboratory of the Carnegie Institute have shown that
crystallization can be made to proceed in silicate glasses at a
temperature much below the melting point of the individual minerals,
but while viscous. Thus crystallized under stress, a parallel
arrangement of the crystals ensued along planes, as shown in gneisses
and schists.
MINERALS AND ROCKS
A
brief reference to these topics, which is not in any sense to be
regarded as a substitute for a study of the larger geological manuals,
is here inserted as helpful to teacher and student.
A
mineral is the ultimate geological unit. All the rocks of the earth,
its entire crust, are mineral aggregates, and such aggregates of
minerals form! in three ways: by sedimentation, deposition from water
(including organic accumulations) ; by igneous effusions, the
outpouring of liquid or pasty lavas, the blowing out of volcanic ash,
lapilli, dust, from volcanic vents; by metamorphism, recrystallizations
through heat and pressure of sedimentary and igneous rocks. Sedimentary
rocks are
