The Alaska citizen., 1914-10-12

This assemblage may be in part
the rocks of an older group of strata
than the typical unaltered Mesozoic
beds Chat are generally without
exposed by erosion in local anticlines
produced by the intrusion Of the
GRANTEES. Or it may be that the
granite intrusions conceded in posi-
btion with locally developed volcanic
members in the Mesozoic succession.
However, as many Of the rocks more
I,or less characteristic of the whole
assemblage are known to have been
derived from the Mesozoic sedi-
evidence of extensive igneous meta
morphism is conclusive, 1t seems
to the writer that these rocks are
best interpreted as having developed
from the Mesozoic sedimentary rocks
through the metamorphc influences
that attended their invasion by the
/ 2ranltlc magma.
The metamorphism attending the
granite intrusions is illustrated in
the present condition of fine even
grained quartz conglomerate bed
that abuts against the granite area
west of the head of Pocahontas creek
Near the contact the conglomerate is
reduced to a solid mass Of quartz
in which structure and the original
form of pebbles have been effaced
This condition holds for over hun
dred feet Beyond this the pebble
forms may be more or less clearly
distinguished A pronounced indura
ton and silicification extends for
more than a mile from the contact
Flow structure is developed in places
and the addition Of magmatic ma
terial is seen near the contact The
impure sedimentary beds associated
with the conglomerate are profound
their contact with the GRANTEES, and
for the most part have taken on ar
igneous appearance generally charac
teristic of the rocks of this group
PAS the rocks of the whole assemblage
are considered to be analogous to
the metamorphosed beds associated
with the conglomerate, they are
grouped together in mapping under
the head of contact rocks, and no
attempt is made to distinguish be-
tween various members of the group
that may differ considerably as to
the details of their genesis.
The contact rocks are for the most
part dark or greenish in color, en-
tirely dense or porphyritic, with a
dense groundmass, and are very
hard and resistant to weatherins.
In the least altered phases of the
typical arkose of the sedimentary
series there is partial isolation of
the coarser quartz and feldspar grain,
| by the development of CLASSY or
finely crystalline matrix. In more
altered phases the matrix is develop
ed in relatively greater volume. the
quartz and feldspar grains are com.
pletely isolated and the quartz grains
approach complete resorption and
the recrystallized or CLASSY matrix
is developed in progressively greater
quantities. In still more advanced
phases of alteration the more acidjc
of the feldspars are more or less
resorbed and secondary basic field
spars are developed. The porphy-
due in part to the resistance OF
basic feldspars original in the ark
ose. in part to the development of
secondary feldspars. and in part to
masses of epidote and chlorite that
have replaced some Of the original
grains, the principal mass of the
rock having been reduced to
CLASSY or finely crystalline condition.
Near the granite areas the magma
probably contributed much of the
material of the contact rocks. Here
femic minerals are abundant and
there are phases that differ from
the typical GRANTEES only in the
relative abundance Of their common
constituents. Apparently there are
gradations of the contact rocks into
the GRANTEES on the one hand and