174 THE GEOLOGIST. 



grits and purple slates, in which are the great slate-quarries of 

 Llanberis and Pecrhyn. 



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 Pfestiniog, 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. 



