It may be conjectured that the coal measures underlie the greenstone through the entire range of Fairhead, since they again make their appearance beneath it on the east side of the promontory; but the whole of its base is too much encumbered with debris to allow of our ascertaining this point. On the west side, at a lower level than the coal measures, we noticed a ledge of clay porphyry running out to sea; this rock is probably analogous to the porphyries of Killnadore.
The columnar greenstone of Fairhead, and the coal measures which underlie that rock in Gobb cliff, have been already described, and I have to mention but very few additional circumstances, principally relating to the whin dykes which traverse that cliff`; of these dykes, the first in advancing from the east, is Carrick Mawr, the “ great crag,” a name well deserved by its dimensions: it forms a broad causeway, traversing the beach and terminating in a nearly insulated mass of rocks rising about thirty feet; of this mass only the central line consists of the dyke itself, the sides being evidently composed of portions of the strata traversed by it, but much altered in their character and degree of induration by its contact. These beds appear to have been chiefly derived from the slate clay of the coal measures, which has become so compact as to assume the character of flinty slate. In one point this rock maybe seen on one side of the dyke, and on the other the sandstone grit, which usually accompanies the coal beds, also in a highly indurated state: its colour changed from red to white, and its mass penetrated by
- It should be noticed, that in the section all the coal measures are coloured with a dark tint, though this in fact belongs only to the coal itself and to the slate clay; the thick beds of red sandstone which alternate with these strata form the predominating features of the cliff.
This system of colouring has been adopted, because it seemed necessary to distinguish the coal measures from the red sandstone not containing coal.