They include gypsum, dolomite, conglomerates, phyllites, and a basic series of eruptive rocks (gabbros, peridotites, serpentines).
A few hornblendeschists are metamorphosed gabbros; others have developed from dikes or sills of lamprophyre.
Syenites, gabbros, norites and volcanic rocks are also represented.
Archean gneiss, Cambrian sandstone, Silurian quartzite, limestone and schist, Jurassic sandstone and limestone, Cretaceous sandstone, and Tertiary basalts, gabbros, and granitic rocks all enter into the composition of the islands.
The axis along which they have been elevated runs north-east and south-west, and on either flank a series of " green rocks " appears, consisting of altered amygdaloidal andesitic lavas, intrusive dolerites, coarse gabbros and diorites, and at Beagh-beg and Creggan in central Tyrone ancient rhyolitic tuffs.