CHAP. XIII 
BALA ERUPTIONS OF CAERNARVONSHIRE 
217 
extent, with the great line of western vents. It is remarkable that the older 
strata which emerge from under the volcanic group on its western outcrop are, 
on the whole, singularly free from sills, though some conspicuous examples 
are shown opposite to Mynydd-mawr, while a few more occur further north 
along the same line. Their lenticular forms, their short outcrops, and their 
appearance 011 different horizons at widely separated points seem to indicate 
that the sills probably proceeded from many distinct subterranean pipes. 
Their greater abundance along the eastern part of the district may be taken 
to indicate that the ducts lay for the most part considerably to the eastward 
of the line of western vents. They may have risen in minor funnels, like 
that of Capel Curig. 
It is noteworthy that so abundant an extravasation of basic material 
should have taken place without the formation of numerous dykes. We 
have here a repetition of the phenomena that distinguished the preceding 
Arenig volcanic period in Merionethshire, and it will be remembered that 
the Llandeilo eruptions of Builth were likewise followed by the injection of 
large bodies of basic rock. As an enormous amount of igneous magma may 
thus be impelled into the Earth’s crust without the formation of dykes, it is 
evident that the conditions for the production of sills must be in some 
important respects different from those required for dykes. 
No evidence has yet been obtained that any one of these sills established 
a connection with the surface. Not a trace can be found of the outpouring 
of any such basic lava-streams, nor have fragments of such materials been 
met with in any of the tuffs. On the other hand, there is abundant proof 
of the usual contact-metamorphism. Though the sills conform on the whole 
to the bedding, they frequently break across it. They swell into thick 
irregular masses, and thin out rapidly. In short, they behave as true 
intrusive sheets, and not as bedded lavas. 
In regard to their internal character, they show the customary uniformity 
of texture throughout each mass. They are mapped under the general 
name of “ greenstones ” by the Geological Survey, and are described in the 
Memoir as hornblendic.^ The more precise modern methods of examination, 
however, prove them to he true diabases, in which the felspar has, as a rule, 
consolidated before the augite, giving as a result the various types of diabasic 
structure.^ 
Tlie date of the intrusion of these basic sills can be fixed by the same 
process of reasoning as was applied to those of the Arenig volcanic group. 
Their connection with the other igneous rocks of Caernarvonshire is so 
obvious that they must be included as part of the volcanic history of the Bala 
period. But they clearly belong to a late stage, perhaps the very latest 
stage, of that history. They probably could not have been injected into their 
present positions, unless a considerable mass of rocky material had overlain 
them. Some of them are certainly younger than the tuffs of Snowdon and 
Moel Hebog, which l ielong to a late part of the volcanic period. On the other 
hand, they had been intruded before the curvature and compression of the 
' Op- dt. 11 . 156. “ ilr. Hai'ker, Bala Volcanic Series, p. 83. 
