84

quartz and feldspar. The purer siliceous sediments interbedded with these
rocks are little more than highly indurated sandstones. The limestones
show in places a sheared, cataclastic texture, and all those examined have
been recrystallized. Many of the rocks contain carbonaceous material.
Some carry considerable plrite, probably of diagenetric origin.

With slightly higher metamorphic grade, the main changes observed
are the disappearance of the cloudy clay-like material from the ground-
mass, and a corresponding increase in size and coherence of the flakes of
chlorite and sericite. The beginning of recrystallization of the detrital
quartz may be observed in these rocks, manifested by the development of
sutured borders on the larger grains and a crystalloblastic texture of some
parts of the groundmass. A few lenticles of quartz, lying parallel with
the poorly developed schistosity, may be entirely crystalloblastic. At this
stage the detrital plagioclase feldspar grains appear to remain completely
unaffected. Among the minor detrital constituents, grains of garnet may
be observed at various stages in the process of alteration to an aggregate
of chlorite, sericite, quartz, and magnetite; originally relatively large
grains are in places represented by an outer ring of chlorite grading inward
to a core of sericite and quartz, with only a few fragments of the original
garnet. In many of the rocks of this and all higher metamorphic grades,
tourmaline is conspicuous in well-developed, euhedral, prismatic crystals;
although many of the grains themselves seem to have been originally
detrital, tourmaline may have begun to undergo recrystallization at this
early stage. At this stage, also, carbonaceous matter has disappeared,
and the pyrite has apparently been completely converted to magnetite or,
in some cases, to limonitic material. The rocks of this stage represent true
slates, phyllites, quartzites, and quartzitic conglomerates. The require-
ments for the development of smooth platy slates, as contrasted with
minutely crenulated phyllites, are not clear, for the two rock types, although
very fine grained, appear in thin section to be alike in mineral composition.
In the field, slate and phyllite are intimately interbedded, so that some
sedimentary beds have produced slate and others have developed into
corrugated phyllites in the same structural and deformational environment.
In such cases the controlling factors were probably the grain size and
porosity of the original sediment, with the slate representing denser and
more compact beds, in which circulation of recrystallizing solutions would
be impeded.

The metamorphic changes throughout the middle and most of the
lower stratigraphic levels of the Ingenika group consist chiefly of an
enlargement of chlorite and sericite grains, and further recrystallization
of detrital quartz and feldspar. Few good slates or phyllites are found
in the stratigraphically lower half of the group; their place, in beds of
equivalent bulk composition, has been taken by chlorite-sericite schists.
The schistosity of these rocks is due primarily to the sericite, which has
in a few specimens grown into discrete flakes of muscovite; from an early
stage in its development this mica occurs in individual lath-like grains,
commonly smaller but much more perfectly oriented than the radiating
erystals and irregular strings and patches of chlorite. The chlorite grains
large enough to be optically identified are the variety clinochlore. The
detrital quartz grains are more completely recrystallized, and in the lower
horizons the purer quartzites have a uniform crystalloblastic texture. The