364 



D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



So far as I can recollect, the western slopes of the Penine hills 

 between the above-mentioned levels and the plain of Cheshire are 

 comparatively, if not entirely, free from rounded gravel ; but I 

 hope to have an early opportunity of speaking with certainty on 

 this subject. 



VII. — Arrangement op the Drift-deposits of North Wales 

 into Vertical Zones, showing probable Variations in the 

 Hate of Submergence. 



The existence of the above rounded gravel- and sand-deposits, 

 with shells, at about the same level in different parts of North 

 Wales, and likewise in England and Ireland, could scarcely have 

 been the result of accident. At a somewhat lower level than these 

 deposits the drifts are angular, with a few exceptions, which may 

 have arisen from local conditions having been unusually favourable 

 to the rounding of stones by rolling and attrition. Above the level 

 of the shelly deposits rounded gravel and sand with shells would 

 appear to be everywhere absent. If the time required for rounding 

 stones be viewed in connexion with that necessary for the migration 

 and multiplication of Mollusca (as already remarked), it will not 

 appear too fanciful to suppose that the rate of submergence was 

 slower (probably much slower) in what may be called the rounded 

 gravel and shelly zone than in the zones above and below, especially 

 as it, is a priori improbable that the submergence (up to at least 

 1350 feet above the present sea-level) progressed at a uniform rate. 

 Many believe that the submergence terminated at the upper limit 

 of this zone ; but, as long ago advocated by Professor Eamsay, and 

 recently repeated in his * Physical Geography and Geology of Great 

 Britain,' a clay similar to that in which he found shells west of 

 Llanberis may be seen rising to a height of from 1500 feet to 

 1800 feet about the Turbary, and east of the river Ogwen. I noticed 

 that the drift between the Turbary and the final ascent to 

 Marchllyn Mawr (about 1700 feet) consisted either of clay or small 

 chips and fragments arranged in a manner which could not possibly 

 be explained by the action of fresh water ; and in a continuation of 

 this drift at a lower level the late Mr. Griffith Ellis, of Llanberis, 

 told me he had discovered sea-shells. In the basins of the Llafar 

 and Caseg (S.E. of Bethesda) the general surface- configuration of 

 the ground up to about 1900 feet can be more easily explained by 

 the former action of the sea than by freshwater agency ; and this 

 accords with the height reached by the Arenig boulder-dispersion 

 in the nighbourhood of Llangollen (1897 feet on Moel Gamelin) 

 and on the neighbouring hill-sides. In the present state of dis- 

 covery it may therefore be said that the glacial submergence 

 certainly reached an altitude of about 1350 feet (the extreme 

 height at which sea-shells have been found), and very probably 

 culminated at about 1900 feet. Below the middle (or Moel- 

 Tryfan, Frondeg, and Macclesfield-Eorest) zone of rounded and 



