Unconformity. 77 



known as the district of Lleyn. These, ere exposure 

 by denudation, probably were the roots of the volcanos, 

 or in other words the deep-seated centres from whence 

 the explosive force of steam drove out the lavas and 

 showers of ashes, which, during successive eruptions, with 

 minor periods of repose, got interstratified with the 

 mud and sand beds that were deposited in the sea of 

 the Llandeilo and Bala or Caradoc period. 



On a smaller scale similar volcanic rocks are inter- 

 stratified with the Llandeilo and Bala beds of the 

 Berwyn Hills, also of the Breidden Hills, and the hills 

 west of the Longmynd and Stiper Stones towards 

 Chirbury and Church Stoke, of the country between 

 Builth and Llandegley in Eadnorshire, and in North 

 Pembrokeshire from the ground round St. David's, ex- 

 tending for many miles to the east, by Mathry, Fish- 

 guard, St. Dogmells and Mynydd Preselley. 



The next question that occurs to me is, what was 

 the nature of the physical geography of this area 

 during the deposition of the Arenig slates, and also at 

 a later epoch when the Llandeilo and Caradoc or Bala 

 beds were being deposited. 



With regard to the Arenig slates in Pembrokeshire 

 and Merionethshire, I know of no signs of unconformity, 

 that is to say, of a lapse of time unrepresented by the 

 deposition of marine strata either in Pembrokeshire 

 or in Merionethshire, unless there be some symp- 

 toms of it in the latter county. But when we go 

 further north into Carnarvonshire, the case is different. 

 There, at the widening of the Passes of Llanberis and 

 Nant Ffrancon, the Lingula flags are not more than 

 2,000 feet thick, whereas further south, between Ffes- 

 tiniog and Portmadoc, they are at least 4,000 feet in 

 thickness. Furthermore, in those valleys in Caernar- 



