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The Small isles of Inverness-shire, comprising Rum, Eigg, Muck and Canna, are largely formed of Palaeogene volcanic and subvolcanic rocks which rest on or intrude a basement of Lewisian gneisses overlain by Torridonian and Mesozoic sedimentary rocks.
The Lewisian gneisses are confined to central Rum. The late Precambrian Torridon Group on Rum comprises river-deposited sandstones with subordinate shales and silty sandstones. Triassic sandstones, breccias and cornstones on Rum mark the eastern edge
of the deposits of the Sea of the Hebrides Basin, while Jurassic and Cretaceous sandstones, shales and limestones on Rum, Eigg and Muck are deposits of the lnner Hebrides Basin.
Palaeocene Iavas on Eigg and Muck predate the development of the Rum Central Complex which began with caldera-controlled rhyodacitic ash-flows and intrusions, together with collapse and explosion breccias and later granitic intrusions. Subsequent movements on ring faults account for the presence of enclaves of Lewislan, Torridonian and Lower Jurassic rocks and some Palaeocene lavas within the central complex. The whole area was injected by a swarm of NW-trending basaltic dykes, and suites of basaltic radial dykes and cone sheets focused on Rum. ln the final phase of development of the central complex, pulses of Mg-rich basaltic magma were intruded to build the sequence of layered ultrabasic and gabbroic rocks. Rapid erosion in the Palaeocene exposed the roots of the complex; the products were partly interleaved with further basaltic eruptions on north-west Rum and Canna. Eocene ash and lava flows of the Sgurr of Eigg Pitchstone ended the igneous activity in the district.
The major Camasunary—Skerryvore fault system passes between Rum and Eigg. On Rum, the Long Loch Fault probably controlled emplacement of the ultrabasic rocks. Glaciation during the Quaternary produced corries, moraines and U-shaped valleys on Rum, and generally sparse till deposits on all the islands. Landslips and rock falls are extensive in northern Eigg. Marine placer deposits rich in chromite and olivine, derived from the ultrabasic rocks, occur offshore around Rum.