It appears to me, therefore, that the relations between the two groups of rock in this area are similar to those between the granophyres and bedded basalts on the south side of Loch na Keal in Mull (p. 396). In other words, the rhyolites have risen through the basalts, and are therefore younger than these lavas. This conclusion is corroborated by the actual proofs of the intrusion of rhyolite into the basalts at Templepatrick.
All the known rhyolitic masses in Antrim are confined to the Lower group of basalts.[429] And as they traverse some of its highest members, they may be regarded as certainly younger than that group. Mr. M'Henry, who first indicated this relation, suggested that the rhyolites were erupted in the interval between the two basaltic series, and he connected with their eruption the rhyolitic detritus found in association with the iron-ore at so many places in Antrim. It appears to me that this suggestion carries with it much probability. The rhyolitic conglomerate of Glenarm proves that, in the long period represented by the iron-ore and its associated group of sedimentary deposits, there were masses of rhyolite at the surface, the waste of which could supply such detritus. The resemblance between the material of that conglomerate and the rhyolites now visible at Tardree and elsewhere is so close that we cannot doubt that, if not derived from some of the known rhyolitic protrusions, this material certainly came from exposed masses that had the same general petrographic characters.
[429] The only exception to this rule was believed to be that of the mass at Eslerstown, four miles east of Ballymena, which, as originally mapped, was shown as crossing from the Lower into the Upper basalts. Mr. M'Henry, however, has recently ascertained that the acid rock is entirely restricted to the area of the older group.
While the rhyolite pebbles in the Glenarm conglomerate are distinctly rounded and water-worn, showing that some prominences of acid rock were undergoing active denudation at the time when this conglomerate was laid down, the finer rhyolitic detritus in the tuffs of Ballypallidy rather suggests the actual discharge of rhyolitic ashes during the same period. But it would appear that the superficial outbursts of rhyolitic material, whether in the form of lava or of tuff, were only of trifling extent, or else that the interval between the eruption of the two basalt-groups was so prolonged that any such superficial material was then removed by denudation. The varieties of lithological character to be met with among the acid protrusions of Antrim suggests a succession of uprises of rhyolites differing from each other more or less in composition and structure. Unfortunately the ground is generally so covered with superficial accumulations, and the exposures of rock are so poor and limited, that no sequence has yet been determined among the several kinds of acid rock. The only locality where I have observed clear evidence of such a sequence is on the old quarries half a mile west of Shankerburn Bridge, and three miles north-west of Dromore, County Down. A small boss of rhyolite there rises through the Silurian strata. It consists partly of a coarse-grained lithoidal rhyolite, with large smoky quartzes and felspars, and partly of a much finer textured variety. The latter, on the south side of the small brook which separates the quarries, can be seen to ascend vertically through the coarse-grained rock into which it sends a projecting vein. Its margin shows a streaky flow-structure parallel with its vertical wall and is in places spherulitic. Here the closer-grained rock is certainly later than the rest of the mass.
CHAPTER XLVIII
THE ACID SILLS, DYKES AND VEINS
i. THE SILLS
Not only have the acid rocks been protruded in small and large bosses, they have also been injected as sills between the bedding-planes of stratified rocks, between the surfaces of the basalt-beds, and between the bottom of the plateau-basalts or of the gabbros and the platform of older rock on which the volcanic series has been piled up. Every gradation of size may be observed, from mere partings not more than an inch or two in thickness, up to massive sheets, which now, owing to the removal of their original covering of rock by denudation, form minor groups and ranges of hills. Where the sheets are numerous, they are usually small in size; where, on the other hand, they are few in number, they reach their greatest dimensions.
It is not always possible to discriminate between bosses and large irregular sills. A good illustration of the connection between these two forms of intrusion will be cited from the island of Raasay, where a widespread intrusive sheet is in part connected with a true boss.
In Mull, sills of acid eruptive rocks are profusely abundant throughout the central mountainous tract between Loch na Keal and Loch Spelve. If we ascend the slopes from the Sound of Mull, for instance, we have not gone far before some of these sheets make their appearance. They are usually dull granular quartz-porphyries, or granophyres, often only two or three feet in thickness, and interposed between the beds of basalt that form the mass of the hills. Along the crest of the ridge that stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in Mull is probably that which has thrust itself between the base of the basalts and the underlying Jurassic strata and crystalline-schists on the shore of the Sound of Mull at Craignure. The porphyry of this sheet is referred to by Professor Zirkel as only a finer-grained variety of the same quartziferous rock, with hornblende and orthoclase crystals, which in Skye breaks through the Lias.[430] On the south coast also, at the base of the thick basalt series, similar porphyries have been injected into the underlying strata; and under the great gabbro mass of Ben Buy similar protrusions occur. But as we retire from the mountainous tract into the undisturbed basalts of the plateau, these acid intercalations gradually disappear.
[430] Zeitsch. Deutsch. Geol. Gesellsch. xxiii. p. 54.