206 
THE TERTIARY VOLCANOES 
BOOK VIII 
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, com- 
posed 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 denuda- 
tion, or from rhyolite ejected in lapilli or poured out in streams. I formerly 
believed tliat 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 dis- 
cussed in Chapter xlvii., in connection with the acid rocks of the Tertiary 
volcanic series. 
Immediately above the iron-ore of Antrim, or separated from it in 
places by only a few inches of tuff, conies the group of Upper Basalts, 
which varies up to (100 feet in thickness, though as the upper portion has 
been everywhere removed by denudation, no measure remains of what may 
have been the original depth of the group. The general character of these 
basalts is more frequently columnar, black and compact, and with fewer 
examples of a strongly amygdaloidal structure than in the lower group. 
But this distinction is less marked in the south than in the north 
of Antrim, so that where the intervening zone of tuffs and iron-ore dis- 
appears, no satisfactory line of division can be traced between the two 
groups ot basalt. The occurrence of that zone, however, by giving rise to a 
hollow or slope, from which the upper basalts rise as a steep bank or cliff, 
furnishes a convenient topographical feature for mapping the boundary of 
these rocks. Among the upper basalts, also, there is perhaps a less fre- 
quent occurrence of those thin red partings of bole between successive 
flows, so conspicuous in the lower group. But the flows are not less dis- 
tinctly marked off from each other. Nowhere can their characteristic 
features be better seen than along the magnificent range of cliffs from the 
Giant’s Causeway eastwards. The columnar bed that forms the Causeway 
is the lowest sheet of the upper group, and may be seen resting directly on 
the zone of grey and red tuffs. It is about 60 or 70 feet thick; and, 
while perfectly regular in its columnar structure at the Causeway and the 
“ Organ,” assumes further eastward the confusedly starch-like arrangement 
of prisms already referred to. But in the great cliff section of the “ Amphi- 
theatre,” the more regular structure is resumed, the bed swells out to about 
80 feet in thickness, and columns of that length run up the face of the 
precipice, weathering out at the top into separate pillars, which, perched on 
the crest of an outstanding ridge, are known as the “ Chimneys.” The 
basalt-beds that succeed the lowest one are each only about 10 to 15 feet 
thick (Fig. 265). 
Between the successive sheets of the Upper Basalts thin seams of red 
