CHAP. Xin 
BALA ERUPTIONS OF CAERNARVONSHIRE 
209 
Glyder-Fach, which, according to the Survey measurements, are about 1500 
feet thick, while still lower lie the ancient couUes of Carnedd Dafydd and 
those that run north from the vent of Y-foel-fras, which must reach a united 
thickness of many hundred feet. We can thus hardly put the total depth 
of volcanic material at a maximum of less than 6000 to 8000 feet. The 
pile is, of course, thickest round the vents of discharge, so that no measure- 
ment, however carefully made at one locality, would be found to hold good 
for more than a short distance. 
Though little is said in the Survey Memoir of the vents from which this 
vast amount of volcanic material was erupted, the probable positions of a 
number of these oriheos may be inferred from the maps. From the shore 
west of Conway a series of remarkable eminences may be traced south-west- 
wards for a distance of nearly forty miles into the peninsula of Lleyn. At 
the northern extremity of this line stands the prominent boss of Penmaen- 
mawr, while southward beyond the large mass of Y-foel-Ms, with the smaller 
knobs west of ISTant Francon, and the great dome of Mynydd-mawr, the 
eye ranges as far as the striking group of imy-lika cones that rise from the 
sea around Yr Fill and Nevin. Some of these hills, particularly Y-foel-fras, 
were recognized by the Survey as vents.^ But the first connected account 
of them and of their probable relation to the volcanic district in which they 
occur has been given by Mr. Harker in his exceedingly able essay on “ The 
Bala Volcanic Series of Caernarvonshire,” — the most important contribution 
to the volcanic history of Wales which has been made since the publications 
of the Geological Survey appeared. I shall refer to these vents more specially 
in the sequel. I allude to them here for the purpose of showing at the 
outset the marvellous completeness of the volcanic records of Caernarvon- 
shire. So great has been the denudation of the region that the pile of lavas 
and tuffs which accumulated immediately around and above these orifices has 
been swept away. Xo trace of any portion of that pile has survived to the 
west of the line of bosses ; while to the east, owing to curvature and subsequent 
denudation, the rocks have been dissected from top to bottom, until almost 
every phase of the volcanic activity is revealed. 
The volcanic products discharged from these vents consist of a succession 
of lava-streams separated by bands of slate, tuff, conglomerate and breccia. 
These fragmental intercalations, which vary from a few yards to many 
hundred feet in thickness, are important not only as marking pauses in the 
emission of lava or in the activity of the volcanoes, but as affording a means 
of tracing the several lavas to their respective vents. Essentially, however, 
the volcanic materials consist of lava-flows, the intercalations of fragmentary 
matei'ials, though numerous, being comparatively thin. The thickest accumu- 
lation of tuffs is that forming much of the upper part of Snowdon. It is 
set down by my predecessor at 1200 feet in thickness, but I should be 
inclined to reduce this estimate. T shall have occasion to show that the 
summit and upper shoulders of Snowdon are capped with andesites inter- 
1 Op. cit. pp. 137, 220. 
2 This was the Sedgwick ITize Essay for 1888, and was publislied in 1889. 
V OL. I i> 
