CHAP. XI 
NORTH WALES 
167 
feet thick, and pass under the dark shales and grits of tlie Lower Silurian 
(Arenig) series. Some of the most persistent liands • among them are con- 
glomerates, which differ from each otlier in composition, but most of which 
consist largely of fragments of various igneous rocks. Some of the coarser 
masses might be termed agglomerates, for they show little or no trace of 
bedding, and are essentially made up of blocks of volcanic material. Ihere 
are abundant beds of grit, sometimes pelildy or finely conglomeratic, alter- 
nating with tuffs and with bands of more ordinary sediment. Courses of 
purple shale and sandstone, green shale and dark grey sandy shale occasion- 
ally occur to mark pauses in the volcanic explosions. Perhaps tlie most 
striking feature in the pyroclastic materials is the great abundance of very 
fine co°npact pale tuffs (hallefiintas of some writers), sometimes thinly 
laminated, sometimes occurring in ribbon-like bands, eacli of which presents 
internally a close-grained, almost felsitic or flinty texture.^ 
A cursory examination of the contents of the conglomerates, breccias 
and grits shows them to consist largely of different felsites, with fragments 
of more basic lavas. Some of these might obviously have been derived 
from the rock of the porphyry ridge, but, as at Llyn I’adarn, there is a far 
greater variety of material than can be found in that ridge. Some of the 
fragments show perfect flow-structure. Professor Bonney has described the 
microscopic characters of some of these fragments, and has especially 
remarked upon their glassy cliaracter. Among the slides prepared from 
specimens collected by myself, jiesides the abundant fragments of felsite 
(rhyolite), there are also numerous pieces of different andesitic lavas and fine 
tuffs, as well as grains of quartz and felspar, and sometimes a good deal of 
granular iron-ore. 
That a large proportion of the material of the so-called “Bangor beds 
was directly derived from volcanic explosions can hardly be doulited. 
There appears to have been a prolonged succession of eruptions, \arying in 
intensity, and somewhat also in the nature as well as in the relative fine- 
ness of the material discharged. On the one hand, coiirse massive agglomer- 
ates were probably accumulated not far from the active vents, as the result 
of more violent or transient explosions ; on the other hand, exceedingly fine 
and well-stratified tufts, which attain a great thickness, serve to indicate 
a phase of eruptivity marked by the long-continued discharge of fine 
volcanic dust. Ordinary sediment was doubtless drifted over the sea- 
bottom in this district during the volcanic episode, but the comparative 
infrecjuence of distinct interstratifications of shale or sandstone may be taken 
to imply that as a rule the pauses between the eruptions were not long 
enough to allow any considerable accumirlation of sand or mud to take 
place. 
No satisfactory proof has yet been obtained of any interstratified lavas 
among the tuffs of the Bangor district. Some rocks, indeed, can be seen 
^ The ooeurreuce of fliuty or cherty deposits, in association with volcanic rocks of Lower 
Silurian age, is well established in Britain, and will be more particularly referred to in the 
sequel. 
