INTRODUCTION 11
is
changed to biotite and chlorite, and again, secondarily, to zoisite and
epidote. The mineral augite changes to hornblende, and it is thought
that by addition of needed elements a dolomite (the carbonate of
calcium and magnesium) can become hornblende, a variable silicate of
aluminum, iron, calcium, magnesium, and the alkalies. Throughout these
metamorphoses the mica minerals retain permanency or are an ultimate term in the transitions.
The
pressure, almost inevitable as an agent in these changes, gives
flatness and parallelism to the resultant minerals, and the schists and
gneisses which contain them are banded, fissile, laminated, splitting
into rudely smooth leaves or cakes or exhibiting schistosity, which in the very compact slates becomes fissility, whereby the slate rock cleaves into thin and useful plates.
The
minerals that play the most conspicuous part in the structure of the
crystalline rocks are the feldspars (silicates of aluminum, calcium,
potassium, and sodium), quartz (oxide of silicon, silica), the micas
(silicates of aluminum, magnesium, potassium), and amphibole
(hornblende), with pyroxene, the last two related minerals having
composite compositions (for the most part silicates of aluminum,
calcium, magnesium, and iron).
Among
the crystalline rocks granite takes a prominent place. It is quite
noticeably contrasted with the gneisses and layered rocks from its
massive and heterogeneous texture. Its component minerals are not
arranged in sheets, but are irregularly intercrystallized and
interlocked, though, under pressure again, granites become
granite-gneisses and assume schistosity. Granites are contrasted with
the gneisses as massive rocks, made up of feldspar, quartz, and mica,
which are mixed together and intercrystallized with accessory
minerals. They are regarded as eruptive, the cooled and crystallized
magmas which have been forced upward from underlying sources into the
areas above heated, pasty reservoirs, or pushed out and injected as
dikes, apophyses or arms into cracks or openings of
