Ch. xil] marine shells of moel tryfaex. 159 



vocal marks of prolonged glaciation. Mr. E. D. Darbishire, after a 

 diligent search in 1863, formed a collection from this same drift of 

 Moel Trvlaen of no less than 54 species of mollusca, besides three 

 characteristic arctic varieties — in all 57 forms. They belong without 

 exception to species still living in British or more northern seas ; eleven 

 of them being exclusively arctic, four common to the arctic and Brit- 

 ish seas, and a large proportion of the remainder having a northward 

 range, or, if found at all in the southern seas of Britain, being com- 

 paratively less abundant. 



The whole deposit has much the appearance of an accumulation in 

 shallow water or on a beach, and it probably acquired its thickness 

 during the gradual subsidence of the coast — an hypothesis which would 

 require us to ascribe to it a high antiquity, since we must allow time, 

 first for its sinking, and then for its re-elevation. As the layers of 

 shell-bearing sand and gravel are so porous, we may naturally feel sur- 

 prise that they have escaped decomposition. To account for this, Mr. 

 Darbishire suggests that a bed of overlying clay, nearly two feet thick, 

 may, by its impermeable nature, have prevented the fossils from being 

 dissolved by the percolation of rain-water. 



The elevation reached by these fossil shells on Moel Tryfaen is no 

 less than 1360 feet — a most important fact when we consider that we 

 have scarcely a well-authenticated case as yet on record beyond the 

 limits of Wales, whether in Europe or North America, of marine 

 shells having been found in glacial drift at half the height above in- 

 dicated. 



Mr. Darwin, after studying the Welsh glacial drift previously 

 shown by Mr. Trimmer to have been of submarine origin, came to 

 the opinion that the land, when it was re-upheaved to its present 

 height, was covered a second time, at least in the higher valleys, by 

 glaciers which swept the surface clean of all the rubbish left by the 

 sea.* 



Professor Ramsay, also, in a " Memoir on the Welsh Glaciers," in 

 1851,f announced his conviction that there had been, first, an intensely 

 cold period when the land was more elevated than now, then a sub- 

 mergence beneath the sea, and lastly, a re-elevation attended by a second 

 period of glaciers. Although he had not been able to trace marine 

 shells in the drift to a level exceeding 1300 feet above the sea, he es- 

 timated the probable amount of submergence during some part of the 

 glacial period at about 2300 feet ; for he was unable to distinguish the 

 superficial sands and gravel which reached that high elevation from the 

 drift which, at Moel Tryfaen and at lower points, contains shells of 

 living species. 



The evidence of the marine origin of the highest drift is no doubt 

 inconclusive in the absence of shells, so great is the resemblance of the 



* Philosophical Magazine, ser. 3, vol. xxi. p. 180. 

 f Quart. Geol. Journ., 1852, vol. viii. p. 3*72. 



