Fig. 295.—Section of Volcanic Vent at Carnmony Hill (E. Hull).
T, Lower basalt; C, Cretaceous strata; L, Lower Lias; M, Triassic marls; V, Vent.
At the nearest point to which the two rocks are traceable, the basalts appear somewhat indurated, break with a peculiar splintery fracture, and weather with a white crust. These characters are still better shown on abundant fragments which may be picked up among the debris further up the slope. There can be no doubt, I think, that a ring of flinty basalt, differing considerably in texture from the usual aspect of that rock in the district, surrounds the neck. The meaning of this ring will be more clearly seen from the description of another example in Mull. About four miles to the north-east of Slemish, a smaller and less conspicuous neck rises out of the plateau-basalts. The rock of which it consists is less coarsely crystalline than that of Slemish, but its relations to the surrounding volcanic rocks are obviously the same. On the west side of Belfast Lough a boss of similar rock, about 1200 feet in diameter, rises at the very edge of the basalt escarpment into the eminence known as Carnmony Hill (Fig. 295). On its northern side it presents along its wall a mass of interposed volcanic agglomerate.[295] On visiting with Mr. M'Henry the quarry opened on the eastern face of this vent, I was much struck with the remarkable cellular structure of some parts of the dolerite. Many of the vesicles are lined with a thin pellicle of black glass, and the same substance occurs in minute patches in the body of the rock. A thin slice exhibiting this structure was found by Mr. Watts to possess the following characters:—"The rock is an ophitic dolerite consisting of plagioclase, augite, and iron ores, without olivine, enclosing one or two patches of finer basalt. The vesicles in the latter, and certain angular spaces between the crystals of the former, have been wholly or partially filled with brown glass, the outer part of which has been converted into radiating crystals of a brown mineral." The occurrence of patches of glass which seem to have been squeezed into vesicles or cracks in the body of a dolerite or andesite has been noticed in some of the Tertiary dykes. But in the present case the glass occurs as a mere coating on the walls of the larger spheroidal vesicles, the interior of which generally remains empty.
[295] This neck was recognised by Du Noyer in 1868 as "one of the great pipes or feeders of the basaltic flows." See Prof. Hull, Explanation of Sheets 21, 28 and 29, Geol. Survey of Ireland (1876), p. 30.
Fig. 296.—Section of the east side of Scawt Hill, near Glenarm.
a, bedded basalt; b, mass of chalk; c, basalt neck.
Of the other doleritic necks scattered over the surface of the Antrim plateau, I will refer to only one which occurs on the hillslopes between Glenarm and Larne. It forms a prominence known as the Scawt Hill, and consists of a boss of basalt, which, in rising through a vent in the plateau-sheets, has carried up with it and converted into marble a large mass of chalk which is now exposed along its eastern wall ([Fig. 296]).
Fig. 297.—Section of Neck of Basalt, Bendoo, Ballintoy.
a a, Chalk; b, neck.
As examples of similar necks which have been exposed by denudation outside the present limits of the same plateau, I may allude to those which rise through the Cretaceous and other Secondary strata on the northern coast near Ballintoy. One of the most striking of these may be seen at Bendoo, where a plug of basalt, measuring about 1400 feet in one diameter and 800 feet in another, rises through the Chalk, and alters it around the line of contact ([Fig. 297]). Another remarkably picturesque example is to be seen near Cushendall, where a prominent doleritic cone rises out of the platform of Old Red Sandstone, some distance to the north of the present edge of the volcanic escarpment ([Fig. 298]).