7. DOLERITIC DYKES
Dolerite dykes must be relatively rare in this area as only six were observed. Their overall trend is approximately north-south (though it is variable since they often follow curving trends). The largest dolerite dyke observed crops out between Loch na Beiste and Loch na h-Uidhe and can be followed for 350 yards. At its maximum it is 35 feet thick. Other dolerite dykes are exposed on the shore of Loch Rodel; Ard Druim an Tolla; two between Ard Mor and Sron Ghaoithe (one is shown on figure 5); and one 100 yards south of Loch na Cachlaidh. A specimen from the latter was studied in thin section and consists of phenocrysts of olivine, clinopyroxene, orthopyroxene and andesine with minor magnetite-ilmenite. The matrix consists primarily of plagioclase and pyroxene. The plagioclase phenocrysts are, on average, 2mm long. This is an olivine dolerite composition, and is probably typical of this rook unit.
The dolerites, though unaltered, weather to a brownish colour and are usually preferentially weathered with respect to other rock types, especially on the coast. All show chilled margins and since they show no signs of migmatisation they must be considered to be younger than the migmatites. They are probably of Tertiary (?Eooene) age, since they are similar to the Tertiary olivine doleritea of the Inner Hebrides and the mainland of Scotland.