Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/218

 completely broken through. The ruins of an ancient tower rise above this chasm, on the sides of which the course of an enormous whin dyke may be traced; and the whole promontory is shattered in every direction, masses of basalt (sometimes mingled with chalky debris and flints) protruding through numerous fissures. The chalk, where it comes into contact with these dykes, is often converted into a compact and crystalline marble.

The mass of chalk just described is covered by thick beds of basalt; but near the top of the cliffs, which here rise about 300 feet, are seen two other beds, which viewed from a small distance appear to be chalk, one rising to the surface above the western, and the other above the eastern extremity of the inferior chalky mass. These, on nearer examination, appeared to be a breccia composed of fragments of chalk, of various sizes, intermixed with flints and basaltic concretions. The outer surfaces of these fragments are much altered, and they are penetrated by small nests of a greenish substance appearing to be steatite, the interior presenting the chalk in its usual state.

The opposite coast of the island of Rathlin exhibits, as will be seen by the section of it in Plate 10. an exact analogy to that of the main land; and on that point of it lying directly over against Kenbaan head, a singular combination of dykes occurs, seeming to be continuations of those which at the latter place appear to have been attended with such extraordinary disturbances. Here, within the distance of 90 feet, three dykes may be seen traversing the chalk, which is converted into a finely granular marble, where contiguous to the two outer dykes, and through the whole of the masses included between these and the central one: these dykes are situated a little to the west of Church bay, they are marked in the section Plate 10; and a ground plan, on a larger scale, is added in Pl. 11. fig. 2. representing their appearance as traced upon the beach.