2i6 
THE SILURIAN VOLCANOES 
BOOK IV 
lavas and tuffs attain their greatest thickness, and whence they thin away 
in all directions. Tlie Myuydd-inawr boss may be presumed to have been 
one of the main vents. But there were not improbably others, now 
concealed under the deep cover of their own ejections. 
More diligent search, with a special eye to the discovery of such vents 
might indeed be rewarded, even in the midst of the volcanic district itself 
To the north-east of Capel Curig, for example, there is a prominent knob of 
agglomerate, which T visited with Mr. B. N. Peach, and which we regarded 
as probably marking one of tlie minor vents. The material of this eminence 
has a base which by itself would probably be regarded by the field-geologist 
as a felsite. But through this compact matrix are dispersed abundant 
stones of all sizes up to six inches or more in diameter. They are mostlv 
subangular or somewhat rounded-off at the edges, and generally markedly 
cellular. Among them may be observed pieces of trachyte, felsite, and a 
rock that is probably a devitrified pitchstone or obsidian. The vesicles in 
these stones are sometimes lined with an acicular zeolite. Traces of rude 
bedding can be detected, dipping at high angles. On the north-east side of 
t le lull finer agglomerate is seen to alternate with ashy grits and orey 
shales, which, dipping E.N.E. at 20°-30°, pass under a group of felsites,'’one 
at least of which retains a very fine perlitie structure and evidently fiowed 
as a true glass. Some of these lavas are full of enclosed pieces of various 
inty cellular and porphyritie felsites and andesites or trachyte.s, like the 
stones which occur abundantly in the agglomerate. The connection of these 
bedded lavas and tufts with the agglomerate-neck seems obvious. 
The Caernarvonshire volcanic area furni.shes another admirable example 
of the intrusion of basic sills as a final phase of eruptivity. These masses 
have been carefully separated out on the maps of the Geological Survey, 
which jiresent a striking picture of their distribution and their relation to 
the other igneous rocks. An examination of the maps shows at once that 
the basic sheets tend to lie parallel with the bedding along certain horizons. 
In the southern and western portions of the area they have forced themselves 
among the Lower Silurian sedimentary strata that underlie the Bala volcanic 
group a position analogous to that taken by the corresponding sills of the 
Arenig series. But they likewise invade the volcanic group itself. Alono' 
the eastern borders of the district they abound, especially in the higher parts 
of the volcanic pile, where they have been injected between the fiows, and 
have subsequently participated in the abundant plication of the rocks between 
the monntaiirs and the line of the Iliver Conway. 
Ihe curvatures into which the rocks of the region have been thrown, 
and the consequent breadth of country over which the volcanic sheets can 
now be examined, furnish a much better field than Merionethshire for the 
attempt to trace the probable centre or centres from which the basic macma 
of the sills was protruded. A study of the Survey maps soon leads to a con- 
viction that the intrusions were not connected, except perhaps to a trifling 
• 1 his rock IS referred to m tlie iI*!;i02V as “ a short thick band of 
meratic ash, -which strikes northwards about half a mile and then disapiiears ” (p. 134). 
conglo- 
