File:The Quarterly journal of the Geological Society of London (12646223504).jpg

206 PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
is again succeeded by fine-grained, grey gneiss much contorted, but
dipping south-east, probably at a high angle. Still higher, beds of
quartzite appear, dipping at 10° to S. 47° E., and thus, though the
immediate junction is not seen, apparently unconformable to the
gneiss. In others of the surrounding hills the quartzite, readily
recognized by its white colour, also appeared to rest unconformably
on the grey gneiss, but I was prevented from determining this point
by further examination.
About ten miles from Fort William, Glen Nevis is crossed by a
great ridge of rock, through which the river forces its way in a series
of narrow gorges and rugged cascades. This ridge must have been
cut through by the river in some very different condition of the
country, as the valley beyond it lies at a much lower level than its
summit, and opens out to the upper part of Glen Treig, by what
would be its natural drainage. Above this ridge a large branch of
the river enters from the south, forming a fine waterfall over an
overhanging cliff. In this place there is another interesting section,
represented generally in fig. 12, though from the incessant rain and
mist I was again prevented following out all its details.
Fig. 12. — Section of the upper part of Glen Nevis.
n. s.
Red Granite. Mica-slate.
On the north-west is the granite dome of Ben Nevis, pierced by
the porphyry prism that forms the summit of the mountain. To this
succeeds a ridge of mica-slate, dipping at 70° to N. 47° W., and
thus apparently below Ben Nevis. Both in mineral character and
dip these beds correspond with the mica- slate seen in Glen Spean,
above the opening of Glen Roy. The valley forms an anticlinal, as
the strata on the other side dip about 45° to the south, and in the
higher part of the mountains appear to be again covered by the
quartzite.
22. Glen Spean. — In Glen Spean, on the north side of Ben Nevis,
a similar series of rocks is seen, but with some diversity in the
details. In the lower part of this glen the mica- slate dips generally
to S. 50° E., and contains one or more beds of limestone, which are
quarried in some places, and is also intersected by veins of red
felspar-porphyry and granite. Near the mouth of Glen Hoy the
dip changes, and the mica-slate further up dips bb° or 60° to N.
50°-55° W. These beds appear to correspond to the mica-slate
seen in the upper part of Glen Nevis. Some of the lofty mountains,