The chalk is frequently traversed by basaltic dykes, and often undergoes a remarkable alteration near the point of contact; where this is the case the change sometimes extends eight or ten feet from the wall of the dyke, being at that point greatest, and thence gradually decreasing till it becomes evanescent. The extreme effect presents a dark-brown crystalline limestone, the crystals running in flakes as large as those of coarse primitive limestone; the next state is saccharine, then fine grained and arenaceous; a compact variety having a porcellaneous aspect and a bluish-grey colour succeeds: this towards the outer edge becomes yellowish white, and insensibly graduates into the unaltered chalk. The flints in the altered chalk usually assume a grey yellowish colour; the altered chalk is highly phosphorescent when subjected to heat.
Examples of this conversion of the chalk into granular marble may be seen on the east slope of Divis mountain, near Belfast, in a ravine to which Dr. Macdonald has given the name of Allan's Ravine, in honour of a mineralogical friend.
They are again exhibited in the neighbourhood of Glenarm, where a singular compound dyke, consisting of three branches, traverses the chalk: the included masses of which are altered in the manner above described.
A dyke, very similar to the preceding, occurs in the isle of Rathlin, near Church bay, and produces the same effect on the chalk. This dyke appears to be resumed on the opposite point of the Antrim coast at Kenbaan head, where the granular marble is likewise found. (See section, plate 10. and the explanatory notes.) A dyke near Ballintoy affects the chalk in the same manner.
In the south-west extremity of Antrim the same marble is said to occur near a whin dyke; the spot assigned is Bamersglen, near Trummery, about one mile north-east from Moira.
