174 
THE GEOLOGIST. 
grits and purple slates, in which are the great slate-quarries of 
Llanberis and Penrhyn. 
Part of the foregoing remarks show that, in the lower Silurian epoch 
in what is now Wales, there have been two periods of volcanic activity. 
The oldest of these, during the deposition of the Llandeilo flags, is 
marked by the interbedded porphyries and ashes of Cader Idris, the 
Arans, the Arenigs, the Manods, and the Moelwyns. The second, 
which occurred during the Caradoc or Bala period, is separated from the 
older porphyries by from 4,000 to 6,000 feet of intervening strata of the 
Bala beds, in the higher parts of which lie the porphyries and ashes 
that range from Moel Hebog to Conway, and form the great Snow- 
donian chain. In all these areas no trace of the original craters 
remain from which the volcanic products found vent. These slates 
and sandstones, with all their interbedded igneous rocks, are so old, 
and have been so often disturbed and contorted in wide sweeping 
curvatures after the close of the Lower Silurian period — besides which, 
the whole country has suffered from such frequent denudations — that 
all traces of the original forms of the volcanos have long since 
perished. But, though the craters have disappeared, there are, among 
the rocks that underlie the lava and ashes, many bosses of intrusive 
felspathic porphyries, syenites, &c., and doubtless some of these are 
the deep-seated nuclei of melted matter, which, underground, were 
connected with volcanic vents from whence the lavas poured. 
Examples occur in the syenite of Ffestiniog, the quartz-porphyry of 
Llyn Padarn, and in the Rivals and other intrusive bosses that give 
such a beautiful outline to the long promontory that forms the north 
horn of Cardigan Bay. 
