CHAP. XLVII 
THE RHYOLITES OF ANTRIM 
429 
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. 
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 witli 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. 
