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.
Subsequently, in the Tertiary period, there were two enormous outpourings of volcanic material - first andesitic lava, and later, after a long interval of quiet, rhyolitic - which nearly half filled the basin, converted it into a plateau and broke up the mountain rim.
Rhyolitic lavas frequently are more or less vitreous, and when the glassy matter greatly predominates and the; crystals are few and inconspicuous the rock becomes an obsidian; the chemical composition is essentially the same as that of granite; the difference in the physical condition of the two rocks is due to the fact that one consolidated at the surface, rapidly and under low pressures, while the other cooled slowly at great depths and under such pressures that the escape of the steam and other gases it contained was greatly impeded.
It is certain, however, that most obsidians are very acid or rhyolitic. The dark, semiopaque glassy forms of the basic igneous rocks are known as tachylytes.
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