Lewis is the largest area in the chain of
islands which constitute the Outer Hebrides, some-
-times termed the Long Island, and which extend from
Barra Head to the Butt of Lewis, a distance of 130
miles. The long tortuous shore of Lewis is mainly a
series of high precipitous cliffs penetrated by sea
lochs, or breaks, where stretches of sand or shingle
and boulder beaches add to the natural attractions of
the island. Many islets begird the shores and afford
sanctuary to birds and seals. The surface of mid
and north Lewis, for over 300 square miles, is an ex-
-tensive stretch of bog, moorland, and loch, leaving
a cultivated fringe bordering the sea, inhabited by
crofting communities. A prospect from the central
ridge of the northern moor does not disclose a rock
vestige for many miles, and even the streams flow
between deep banks of peat, barely disclosing the
glacial till. Hilly country in Lewis is found in
the high plateau ground of the Park district to the
south -east, a central tract from the Oleisham koun-
-tain northwards to Roineval, west of iialallan, and
the Uig Hills to the south -west; while a few groups
and isolated hills rise from surrounding bogs.
The old crystalline Lewisian gneiss, of
which the island is principally composed, shows a primary north -west strike, but upon this has been
superimposed a later terrestrial crust movement which
produced a north -north -east strike with dips to the
south -east. This latter foliation is predominant
from east to west of Lewis. The main Flinty Crush
belt, so prominent a feature in the Loñg Island, is
chiefly developed in a line trending north -north -east.
Obscure foliation planes are observed on high levels
of the Park hill -tops and the South Uist Hills, which
show north -west strikes. Local movements are evidenced on the north -west shores of Lewis, the structural lines changing to north -west and north -east or
east and west with acute folding. Faulting is more
prevalent in the crush belts, especially on the east
side of the Eye Peninsula and the north -east coast of
Lewis. The conglomerate*of Lewis is much faulted
within the synclinal trough in which it lies.
While the Long Island may be described as
essentially a gneissic ridge, there occur outstanding
rock types which have a direct bearing on the physical
aspect of Lewis. These are: -
I. The Granitic Igneous Rocks and Pegmatites.
II. The Basic Igneous Rocks of lviaaruig and associated types.
III. Series of Graphitic and Siliceous Schist types.
IV. The Flinty Crush Belts and related Metamorphic
a Hebridean Main Belt.
b minor Belts.
V. Lewis Conglomerates.