in the Dalradian sediments of Loch Arne in Scotland and the archsean nucleus of Saxony.
The justification of assigning the Manhattan Island schists to metamorphosed Hudson River slates is doubted.
Again,
Albert Heim has indicated the extreme physical changes brought about in
sediments by mountain uplifts and compression: " Enormous zones, for
instance, in the interior of the Finsteraa massif, that were
formerly held to be true crystalline schists, prove to be originally
plastic rocks of the carboniferous period that have been squeezed into
schists and pervaded by secondary, mica. Conglomeratic rocks of the
Verrucano group and clay-slates nipped into the central massif have
become crystalline schistose, and even gneissose. They can scarcely be
distinguished, in the field and in the hand specimens, from crushed
gneisses pervaded by sericites. Granites can be proved locally, and
perhaps also regionally, to have been compressed into gneisses.
Gneisses, having a different position relatively to the pressure, have
locally become granitoid. Massive eruptive felsite-porphyries have
become felsite-schists. Mica-schists have been dragged out, their
quartz grains ground down, and the whole converted into a rock that one
would be inclined to describe as a sandy clay-slate. Even Liassic
slates, with fossils, have been converted into garnetiferous
mica-schists, staurolite-schists, etc. The boundary between the old
crystalline schists and real sediments in the Alps has, by such
processes of dynamic meta-morphism, been obliterated."
Metamorphism
must not be associated exclusively, or even predominantly, with the
alteration of soft or unconsolidated beds of sediments. Its effects are
quite as evident in rocks which have become completely solidified and
which have already advanced to a crystallized or semi-crystallized
stage, as in slates, limestones, and schists, sandstones, even
granites, diorites, and dolerites.
A. C. Spencer has described the metamorphism of a conglomerate in the Encampment District of Wyoming, which
