214 
THE SILURIAN VOLCANOES 
BOOK IV 
felsite fragments smaller scattered pieces of andesitic rocks may be fonnd. 
This mixture of more basic materials appears to increase upwards, the 
highest ashes containing detritus of andesitic lavas like those which occur 
among them as flows. 
The tuffs ill the upper part of Snowdon are well-bedded deposits made 
up partly of volcanic detritus and partly of ordinary muddy sediment.' 
Layers of blue shale or slate interstratified among them indicate that the 
enfeebled volcanic activity marked by the fine tuffs passed occasionally into 
a state of quiescence. As is well known, numerous fossils characteristic of 
the Bala rocks occur in these tuffs. The volcanic discliarges are thus 
pioved to have been submarine and to have occurred during Bala time. 
I have already alluded to some of the probable vents from which the 
lavas and tuffs were discharged, and to their position along a line drawn 
from Penmaen-mawr into the peninsula of Lleyn. It will be observed that 
they lie outside the area of the bedded volcanic rocks and rise through 
parts of the Silurian system older than these rocks. The largest and most 
important of them is unquestionably that formed by Y-foel-fras and its 
neighbouring heights. As mapped by the Geological Survey, this mass of 
Igneous rock is irregularly elliptical, measures about six square miles in 
area, and consists mainly of intrusive “ felstone-porpliyry ” passiim into 
“ hornblenclic greenstone.” Mr. Harker, however, has made an important 
correction of this petrography, by showing that a large part of the area 
consists of augitic granophyre, while the so-called “greenstone” is partly 
diabase and partly andesitic ashes and agglomerates. He suggests that an 
older vent lias here been destroyed by a later and larger protrusion of 
Igneous matter.® This high and somewhat inaccessible tract of ground is 
still in need of detailed mapping and closer study, for undoubtedly it is the 
most important volcanic vent now visible in Horth Wales My former 
colleague in the Geological Survey, Mr. E. Greenly, spent a week upon it 
some years ago, and kindly supplied me with the following notes of his 
observations “ The central and largest area of tlie neck is mainly occupied 
with diabases and andesites, wliile the ashes and agglomerates, wliich are 
intimately connected with them, seem to run as a belt or ring round them, 
and to occur in one or more patches in the midst of them. Portions of 
green amygdaloid run through the pyroclastic masses. Outside the rim^ of 
agglomerate and ashes an interrupted border of felsite can be traced, which 
may be presumed to be older than they, for a block of it was observed in 
them. The granophyre, on the other hand, which is interposed between 
the fragmental masses and the surrounding rocks on the western wall of the 
vent, seems to be of later date. Dykes or small bosses of diabase, like the 
material of the sills, pierce both the agglomerates and the rocks of the centre.” ' 
m Siven by Sir A. Ramsay, Mem. Gcal. Survey, vol. 
^ V /■ iii- 2nd edit. pp. 137, 139. 
Bdloj r olccinic So'ies, pp. 41, 71 72 123 * 
■* Mr. (Ireenly has made a skct’cli map of this interesting locality. As he lias uoiv established 
iis home m North Wales, I trust he may find an opportunity of returning to Y-foel-Ms and 
completing his investigations. 
