Chapter xlix

.]

Nowhere else among the Tertiary basalt-plateaux of Britain has any trace been found of so marked and prolonged a pause in the volcanic

## activity as is indicated by the Antrim zone of tuffs and clays.

Throughout the Inner Hebrides, indeed, numerous intercalations of sedimentary material occur among the basalts, but these consist mainly of tuffs and volcanic conglomerates with less frequent shales and coal-seams, and they never suggest so distinct and lengthened an interval as is indicated by the Antrim deposit.

It is not improbable that this interval was marked by the outbreak of rhyolitic eruptions somewhere in the region. The abundance of rhyolite fragments in some of the tuffs is striking evidence that acid rocks were in one way or other brought to the surface at this time. At Glenarm one of the members of the stratified series is a marked rhyolitic conglomerate, composed of rounded pebbles of a rock not unlike the well-known rhyolite of Tardree and Carnearny. These fragments, obviously of local origin, must either have been derived from a surface of acid rock laid bare by denudation, or from rhyolite ejected in lapilli or poured out in streams. I formerly believed that all the Antrim rhyolites had been injected into the basalts after the close of the plateau-period. But the proved abundance and wide extent of the rhyolitic detritus among the sediments associated with the iron-ore point to a possible outflow of acid lavas with accompanying tuffs during the sedimentary interval between the two groups of basalt. The characters of the Antrim rhyolites, however, will be more

## particularly discussed in