224 
THE SILURIAN VOLCANOES 
BOOK IV 
of the north of Anglesey because of the diversity of opinion respecting 
them, and because of their great interest in regard to the history of 
volcanic action in Wales. These rocks contain a record of volcanic 
eruptions, probably contemporaneous on the whole with those of the Bala 
period in Caernarvonshire, yet independent 
of them and belonging to a different type 
of volcanic energy. Some of the vents 
probably lay in the north-western part of 
Anglesey. The materials ejected from 
them were, so far as we know, entirely of 
a fragmentary kind. Vast quantities of 
detritus, largely in the form of line dust, 
were thrown out ; but no trace has yet 
been found of the outflow of any lava. 
The lower part of this volcanic series con- 
sists of bedded breccias which are some- 
times remarkably coarse. Their included 
stones, ranging up to six inches or more 
in diameter, are usually more or less 
angular, and consist mainly of various fel- 
sites. Layers of more rounded pebbles occasionally occur, while the bedding 
is still further indicated by finer and coarser bands, and even by intercala- 
tions of fine tuffs and ashy shales. Towards their upper limits some of 
these volcanic bands shade off into pale 
grey or greenish ashy shale, followed by 
black sandy shale of the usual kind. The 
relation of the peculiar greenish shale of 
the Amlwch type to these tuffs and 
breccias is well shown east of Carmel 
Point. This shale is interleaved with 
tuff and contains frequent repetitions of 
finer or coarser volcanic breccia, as well 
as occa.sional seams of black shale. An 
illustration of this structure is given in 
Fig. 59, where some yellow decomposing 
breccias (1), cut by a fault (/), are over- 
lain by about 40 or 50 feet of black shale 
(2) , above which lies a flinty felsitic rock 
(3) that appears to run in bands or dykes 
through the agglomerate. At Carmel 
iOlUt (ing. 00) a similar structure breccia, Carmel Point, Anglesey, 
may be observed to that at Llyn Padarn 
already referred to (p. 163). The cleavage, which is well developed in the 
green slates (a), is much more faintly marked in the overlying breccia (6), 
but the bedding can still be detected in both rocks running parallel to 
their mutual boundary -line. Beyond Porth Padrig, which lies east from 
Fig. 59. — Section of intercalated black sliale 
in the volcanic series at Poidh Iiwcli, 
south of Carmel Point. 
