152 
THE CAMBRIAN VOLCANOES 
BOOK III 
would appear from the evidence at present known, that while the basic 
lavas were most abundant in the vents during the volcanic period 
recorded by the rocks of St. David’s, furnishing the material for most of 
the fragmental eruptions, and issuing in occasional superficial streams of 
molten rock, the siliceous lavas did not flow forth at the surface, though 
their debris was copiously discharged in the form of dust and lapilli. 
The rise of both basic and acid lavas at different periods in the same or 
adjoining vents, so familiar in recent volcanic phenomena, thus appears to 
have also characterized some of the oldest examples of volcanic actiion. An 
interesting parallel may be traced between the succession of events at Sf. 
David’s and that which occurred in the volcanic group of the Lower Old 
Eed Sandstone of the Pentland Hills, near Edinburgh, of which a detailed 
account will be given in Chapter xx. of this volume. It is also worthy 
of remark that in the latest of the volcanic episodes in British geology a 
remarkable similarity to the St. David’s volcanic group may be observed. 
Some of the older Tertiary agglomerates are full of pieces of acid rocks 
(felsites, rhyolites or granophyres), while the lavas poured out at the surface 
were mainly basalts. 
In the volcanic group of St. David’s the tuffs contain evidence that 
ordinary sedimentation was not entirely interrupted by the volcanic dis- 
charges. Thus, in the Allan valley, west from the Cathedral, one of the 
schistose tuffs is full of well-rounded pebbles of white quartz. Occasional 
shaly bands indicate the deposit of mud with the tuffs. 
Excluding the granites and porphyries (which are described at p. 155), 
two kinds of eruptive rocks are associated with the volcanic group. One 
of these is certainly intrusive and of late date, viz. dykes and veins of 
diabase, to be afterwards referred to. The other kind occurs in long parallel 
sheets, some of which, if not all, are true contemporaneous lava-streams, 
erupted at intervals during the accumulation of the volcanic group. They 
form pi'ominent crags to the west of St. David’s, such as Clegyr Eoig, Ehos- 
son, and the rocky ground rising from the eastern shores of Eamsey Sound. 
Their dip and strike coincide with those of the tuffs above and below them. 
It is possible that some of these sheets may be intrusive sills intercalated 
along the bedding of the tuffs ; and in one or two cases I have observed 
indications of what, on further and more careful exploration, may prove to 
be disruption across the bedding. 
But it is the interbedded sheets that possess the chief interest as 
superficial lava-streams of such venerable antiquity. They present many 
of the ordinary features of true lava-flows. In particular a slaggy structure 
may be detected at the bottom of a sheet, the vesicles being here and there 
lengthened in the direction of flow'. Some of the sheets are in part 
amygdaloidal. The alternation of these sheets with tuffs, evidently derived 
from lavas of similar character, is another argument in favour of their 
contemporaneous date. One of the best localities for studying these features 
lies between Clegyr Eoig and the coast, west of Eliosson. 
The eruptive rocks thicken towards the south-w'est, as if the main vents 
