41 6 Submergence. 



ground where the circumstances can be best studied 

 are as follows. 



On the north-western flank of the Caernarvonshire 

 mountains, which looks towards the Menai Straits, there 

 are certain high moorland tracts, the surfaces of which, 

 more or less strewn with boulders, have very gentle 

 slopes, and when the sections are exposed, caused by the 

 cutting action of brooks, the subsoil is found to be 

 Boulder-clay, full of ice-scratched stones. The slopes of 

 Moel Tryfan are surrounded by such material, which 

 stretches from thence north-east towards the valley of 

 Llyn Cwellyn or Cwm Seiont, and on the opposite side 

 of that valley, beyond Bettws Grarmon, comes on again 

 in the higher ground. Its continuity is again inter- 

 rupted by the valley of Llanberis at Llyn Padarn, on 

 the north-east side of which, from 800 to 1,200 feet 

 above the sea, the same inclined plains of drift are con- 

 tinued to Nant-ffrancon, north-west of the great Penrhyn 

 Slate quarries, while on the north-east side of that valley 

 the same plain stretches still further north. In one part 

 of these glacial drift deposits, on the moor of Ffridd 

 Bryn-mawr, I found sea-shells at a height of about 

 1,COO feet above the level of the sea, and I was in- 

 formed by Mr. Trimmer that shells had also been found 

 in corresponding clays, on corresponding heights, on 

 the east side of the Ogwen, beyond Bethesda. The 

 shells which I found were examined by Edward Forbes, 

 but unfortunately they have since disappeared. Similar 

 deposits in the same region seem to attain a height of 

 at least 1,500 to 1,800 feet, but without insisting on this 

 it is something to be assured that marine strata-bear- 

 ing shells attain a height on these Welsh mountains of 

 1,000 feet and more. 



Further proof of this is to be found in an upper 



