D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



361 



district, where they consist of Carboniferous grit or sandstone 

 (and partly of quartzose conglomerate), many of them must have 

 been floated in an easterly direction from near the summit of the 

 mountain-range after the submergence had reached that altitude ; 

 for how otherwise can their position on the surface of the drift- 

 deposits be explained ? 



4. Junction of the Arenig -felstone and Eshdale-granite Dispersions. 

 — West of Frondeg, on the summit-level of the mountain-range, about 

 1500 feet above the sea, and extending a short distance eastwards, 

 the stones, with the exception of those of local derivation, consist of 

 Arenig felstone ; and large boulders of the same felstone are found 

 on or near to the summit of the range further south, as well as 

 strewn over the whole district between there and the Great Arenig 

 mountain. I was surprised to find so great a number of Arenig 

 pebbles on the ridge west of Frondeg, and equally surprised to dis- 

 cover that they abruptly terminated in an easterly direction not far 

 from the water-parting, where their place (with very little dove- 

 tailing) was taken by Eskdale granite, reaching up to about 

 1400 feet. I arrived at the conclusion that there the northern-drift 

 current which floated the Eskdale erratics was sufficiently powerful 

 to turn aside the current by which the Arenig erratics were floated 

 from the west. I cannot believe that the latter could have been 

 brought by land-ice, because it appears improbable, if not impos- 

 sible, that land-ice proceeding from the Arenig mountain could 

 have attained a surface-level of 1500 feet (above the present sea) 

 by the time it reached the ridge west of Frondeg ; and it may like- 

 wise be remarked that as Arenig erratics have been found, not far 

 from this ridge, on Cyrn-y-brain up to a height of 1830 feet, the 

 supposed land-ice which brought them must have reached as high as 

 that level, while further S.W., on Moel Gamelin, it must have 

 reached to about 1900 feet*. This is about the limit reached by 

 the glacial submergence in the N.W. part of Wales, where (as well 

 as in the Frondeg district) the upward termination of the zone of 

 rounded gravel-and-sand may be explained by an increased rapidity 

 in the rate of submergence having deprived the sea of the time 

 required to round stones by a process of combined rolling and 

 attrition f . 



5. Frondeg Erratic Stones and Shells not transported by Land- 



* See my paper on the Boulders of North Wales in Quart. Journ. Greol. Soc. 

 vol. xxx. (1874), p. 711. 



t Mr. S. V. Wood, F.Gr.S , has written to me to the effect that he regards 

 the discoveries I made in the Frondeg district as corroborating the conclusion 

 at which he arrived after reading my paper on boulders (Quart. Journ. Geol. 

 Soc. vol. xxxv. (1879), p. 425), namely that land-ice extended from the Arenig 

 mountain to the top of the ridge west of Frondeg, on which it left the Arenig 

 erratics ; while floating ice from the Lake District brought the Eskdale errat ics 

 to the edge of the land-ice which prevented them from reaching further west, at 

 the time when the submergence culminated at about 1400 feet. Mr. Wood 

 likewise believes that the land-ice of the central part of North Wales spread 

 out towards the N.E. and north, so as to prevent the erratics brought by floating 

 ice from the north from getting into the interior of the country. 



