The acid rocks of the west of Rhum, previously considered
to be one petrological unit designated 'granophyre', have
been divided into two petrographically and geographically
distinct units; a core of microgranite is surrounded by an
outer zone of graphophyre. Along rost of its northern
boundary the graphophyre has been thrust over Torridonian
Sandstone. Along a short length of the contact, however,
no faulting has occurred and the graphophyre grades into the
Torridonian Sandstone through a belt of transitional rocks.
These transitional rocks are divisible into five zones which
are interpreted as marking successive stages in the conversion
of the Torridonian into graphophyre. The graphophyre grades
inwards into the microgranite which is believed to represent
a still more advanced stage of metasomatism. The production
of the microgranite from the Torridonian has been achieved
by the enrichment of the latter In Al, Na, Fe, Ca, K, Ti, 1
and P, at the expense of Si. The excess Si has been
driven outwards and fixed as an acid front in the bleached
and indurated Torridonian Sandstone to the north. Seven
small dolerite sills and several basalt dykes had been
intruded into this part of the Torridonian before Its conversion
to microgranite and graphophyre. These basic intrusions have suffered considerable metamorphism involving
the addition of various constituents, principally Na.
The petrography and internal structures of the basic
plutonic rocks of the west of Rhum are described; the
harrisite mass is divided into six zones and the gabbro into
two. The lower zone of the gabbro conistc of an earlier
coarse gabbro and a later fine gabbro. The fine gabbro was
injected from below into almost horizontal parting planes
in the coarse gabbro and forms a series of sheets the members
of which are a few inches thick and a few inches apart.
This alteration of fine and coarse layers gives the lower
zone of the gabbro a highly banded appearance.
The contact between the grahophrre and the basic plutonic
rocks to the east is everywhere marked by a narrow zone of
hybrid rocks against which both the basic lutonic rocks and
the graphophyre have been metamorphosed. These hybrids are
interpreted as highly metamorphosed fragmental rocks
occurring along a faulted juntion up which streams of hot
gases were passing. The hybrid rocks are believed to
have been of both cataclastic and pyroclastic derivation;
although many fragments were derived prom the wall rocks
locally, some were transported for considerable distances.
The southern and central parts of the narrow zone of hybrid
are believed to mark the continuation of the ring
fault which is well known in eastern Rhum. The northern hybrid
rocks were probably developed along a fault which intersected
the western continuation of the ring-fault tangentially.
A small Tertiary vent, which cuts the ring-fault in
the east of Rhurm, is described. The hyrbrid rocks which
occupy this vent are very similar to those found between the
graphothyre and the basic plutonic rocks in the west of the
island but they are somewhat finer in grain. The hybrids
in the vent are fragmental rocks highly metamorphosed by
rising gases at temperatures in excess of 1,0000C. These
gases have enriched the vent rocks in K, Na, C, Al, Si, and
P. and the contiguous Torridonian Sandstone in the same elements
togeher with Fe, Mg and water.
The rocks of the four Tertiary volcanic outliers in the
west of Rhum have been correlated and a succession, nearly
1,700 feet in thickness, has been synthesised from the combined
data. The volcanic rocks have been shown to lie
unconformably upon the grathophyre.
The post-granitic minor intrusions of the west of Rhum
have been divided into Six distinct rock types. Two of these,
viz. mugearite-tachylyte and augite-andesite, have not previously
been recognised in this part of the island.