424 
THE TERTIARY VOLCANOES 
BOOK VIII 
flanks of the hill called Tievecrom. The Upper Silurian grits and shales, 
in a much indurated and shattered condition, are there traceable for several 
hundred feet up the slope, until they are abruptly cut off by the agglomerate. 
The line of separation appears to be nearly vertical, the truncated ends of 
the strata being wrapped round by the mass of fragmental material. 
The most remarkable features of this agglomerate, which has been well 
described by Mr. Nolan, are the notable absence of truly volcanic stones in 
it, and the derivation of its materials from the rocks around it. I found 
only one piece of amygdaloid, but not a single lump of slag, no bombs, no 
broken fragments of lava-crusts, and no fine volcanic dust or enclosed lapilli. 
The rock may be said to consist entirely of fragments of Silurian grits and 
shales where it lies among these strata, and of granite where it comes 
through that rock. Blocks of these materials, of all sizes up to two feet in 
breadth, are confusedly piled together in a matrix made of comminuted 
debris of the same ingredients. 
The agglomerate on the ridge of Carrickbroad has no definite boundary, 
but seems to graduate into an andesitic rock, and then into a quartz-felsite 
or rhyolite. This apparent gradation is one of the most singular features 
of the ridge. The andesite resembles some of the “ porphyrites ” of the 
Old Bed Sandstone. It is close-grained, with abundant minute felspar- 
laths, and numerous large porphyritie felspars, which latter are sometimes 
aggregated in patches, as in the old porphyries of Portraine, Lambay Island 
and the Chair of Kildare. This rock has undoubtedly been erupted at the 
time of the formation of the agglomerate, or at least before the loose materials 
were compacted together ; for it is full of separate stones of the same materials, 
and becomes so charged with them as to become itself a kind of agglomerate, 
with a small proportion of andesitic matrix cementing the blocks. 
A thin slice prepared from one of the specimens obtained by me from 
this hill has been studied by Mr. Watts, who reports that the fine-grained 
andesitic matrix in which the stones are imbedded has often been injected 
into their minute fissures, and that the minute fragments enclosed in this 
matrix consist here of a trachyte-like porphyry, felsite, andesites, basalts of 
various degrees of fineness and olivine-basalt, together with isolated grains 
of felspar, such as might have been derived from the breaking up of some 
of these fragments. 
Westward from Carrickbroad, the chief eruptive rock is a dark, sometimes 
nearly velvet -black, flinty, occasionally almost resinous, quartz -porphyry 
or rhyolite, with abundant quartz and large felspars and occasional well- 
marked flow-structure. This material, near the much smaller protrusion 
of andesite, is curiously mixed up with that rock, as if the two 
had come up together. Sometimes they seem to pass into each other, 
at least the separation' between them cannot be sharply drawn. 
There can be little doubt, however, that the acid magma continued to 
ascend after the other, for it sends veins and strings into the more basic 
material, and encloses blocks of it. This thoroughly acid porphyry plays 
the same part as the andesite in regard to the stones of the agglomerate. 
