248 
THE SILURIAN VOLCANOES 
BOOK IV 
of making himself acquainted with some of the conditions of volcaiiism 
during older Palreozoie time. 
At the east end of the section, IJack shales containing Llandeilo 
graptolites, and calcareous bauds full of Bala fossils, dip westward below a 
group of soda-felsites and felsitic tuffs, which seem to lie quite conformablj^ 
on these strata. Here, then, we start with proof that the volcanic eruptions 
of this locality began during some part of the Bala period. But immedi- 
ately to the west, these bedded igneous rocks are broken through by a neck 
of coarse agglomerate stuck full of chips and blocks of shale, some of them 
a foot long, with abundant fragments of seoriform and flinty felsites. Some 
columnar ilykes of dolerite cut through the neck, and a larger intrusion 
seems to have risen up the same funnel. The bedded tufts appear again for 
a short distance, but they are soon replaced by a tumultuous mass of 
^§8'lomerate. And from this part of the coast onwards for some distance 
all is disorder. 
The agglomerates are crowded with blocks of various felsites and micro- 
granites sometimes 18 inches in diameter, many of them presenting the 
most exquisite streaky flow-structure. The angularity of these stones and 
the abrupt truncation of their lines of flow prove that they were derived 
from the shattering of already consolidated rocks. In other places the 
ejected materials consist almost wholly of black shale fragments, but with 
an intermixture of felsite-lapilli. 
It is difficult to convey an adequate idea of the way in which the 
agglomerates are traversed by dykes, veins and bosses of various felsites, 
and of how these break in endless confusion through each other. Some 
of the intrusive rocks are compact and amorphous, others are vesicular, 
others close-grained and columnar. Again and again they present the 
most perfect flow-structure, and it is noticeable that the lines” of flow follow 
the inequalities of the walls of the fissure up which the rock has ascended, 
and not only so, but even of the surfaces of detached blocks of shale or 
felsite which have been caught up and enclosed in the still rnovino- 
mass. " 
A few of these intrusive rocks were examined in thin slices by Dr. 
Hatch. Most of them appear to be soda-felsites, but they include also 
rather decomposed rocks, some of which are probably diorites and cpiartz- 
diorites. Occasionally, thoroughly basic dykes (dolerite) may be observed. 
In the midst of this tumultuous assemblage of volcanic masses, repre- 
senting the roots of a group of ancient vents, there occur occasional 
interspaces occupied liy ordinary stratified rocks. In the eastern part of 
the section these consist mainly of black shale, sometimes with calcai’eous 
bands, from which a series of Bala fossils has been obtained.' A very 
cursory examination suffices to show that these intercalations do not mark 
pauses in the volcanic eruptions. They are, in fact, portions of the marine 
accumulations under the sea-floor through which the vents were blown ; 
1 But see tlie Geol. Sumy Memoir on Sheets 167, 168, 178 and 179, Ireland (I860), ji, 28, for 
a description of the association of Bala and Llandeilo fossils on that coast-line. 
