CHAP, XIII 
BALA ERUPTIONS OF CAERNARVONSHIRE 
215 
No agglomerate appears to have been noticed by any observer among 
the other supposed vents along the line that runs south-westwards from 
Penmaen-raawr, to the promontory of Lleyn. These bosses are rudely 
circular in ground -plan and rise vertically out of the Lower Silurian or 
Cambrian strata, or partake more of the nature of lenticular sheets or 
laccolites which have been thrust between the planes of bedding. There is 
usually an observable alteration of the surrounding rocks along the line of 
contact. 
The material of these bosses is sometimes thoroughly acid, as is the 
granophyre of Y-foel-fras, tlie microgranite of Mynydd-mawr with its 
riebeckite crystals, the augite-granite-porphyry of Clynog-fawr, and the 
granophyric and rhyolitic quartz-porphyries of the Eivals. In other cases 
the rock is of an intermediate grade, as in the enstatite-diorite of Penmaen- 
inawr, the pyroxene-andesite of Cam Boduan, and the quartz-augite-syenite 
of Llanfoglen.^ A few bosses of still more basic material occur in the Sarn 
district, including hornblende-diabase and hornblende-picrite. Sometimes 
both the acid and the more basic rocks are found in the same boss, as in the 
large mass of Y-foel-fras. 
It must be confessed that there is no absolute proof that any of these 
masses mark tlie actual sites of eruptive vents, except probably the boss of 
Y-foel-fras. Some of them may have been intruded without establishing 
any outlet to the surface.^ But that a few of them really represent orifices 
from which the Bala volcanic group was erupted may be plausibly inferred 
from their neck-like form, from their positions with reference to the volcanic 
district, from tlie obvious thickening of the lavas and tuffs in the direction 
of these bosses, and from the petrographical relation that exists between 
their component materials and rocks that were discharged at the surface. 
This last-named feature has been well pointed out by Mr. Harker, who has 
established, by a study of microscopic slides, a gradation from the grano- 
phyric material of the bosses into structures greatly resembling those of the 
bedded felsites, and likewise a close similarity between the intermediate 
rocks of the other bosses and the andesites which have elsewhere been 
poured out at the surface.^* But perhaps the most impressive evidence 
as to the sites of the chief centres of eruption is supplied by the lavas and 
tuffs themselves as they thicken in certain directions and thin away in 
others. This feature of their distribution has been well expressed in the 
maps and sections of the Survey, and has been clearly summarized by Mr. 
Marker.^ The oldest lavas now visible lie at the northern end of the 
district, and the vents from which they proceeded may, with considerable 
probability, be placed somewhere in the tract which includes the chain 
of bosses of I*enmaen-mawr, Y-foel-frfis, and Y Drosgl. The chief centre 
of eruption no doubt lay somewhere in the Snowdon tract, where the 
1 The geological relations and petrographical characters of these various rocks are given by 
Jlr. Harker in the fourth and fifth sections of his Essay. 
- Mr. Harker speaks of some of them as laccolites. ^ Op. cit. pp. 57, 72. 
^ See especially pp. 9, 120 el seq., and fig. 6 of his Essay. 
