334 



SCIENCE 



[N. S. Vol. XXXII. No. 819 



land on the eastern side, and on the more 

 western is defined by a small valley which 

 enlarges gradually as it descends towards 

 the Severn. If the country were gradu- 

 ally depressed for nearly 1,200 feet, this 

 upland would become, first a promontory, 

 then an island, and finally a shoal. 



The third instance, on Moel Tryfaen in 

 Carnarvonshire, was carefully investigated 

 and described by a committee of this asso- 

 ciation^o about ten years ago. The shells 

 occur in an irregularly stratified sand and 

 gravel, resting on slate, and overlain by a 

 boulder clay, no great distance from and a 

 few dozen feet below the rocky summit of 

 the hill, being about 1,300 feet above the 

 level of the sea and at least five miles from 

 its margin. About fifty-five species of mol- 

 luscs and twenty-three of foraminifers have 

 been identified. According to the late Dr. 

 J. Gwyn Jeffreys,^'' the majority of the 

 molluscs are littoral in habit, the rest such 

 as live in from ten to twenty fathoms of 

 water. Most of the erratics have been 

 derived from the "Welsh mountains, but 

 some rocks from Anglesey have also been 

 obtained, and a few pebbles of Lake Dis- 

 trict and Scotch rocks. If the sea were 

 about 1,300 feet above its present level, 

 Moel Tryfaen would become a small rocky 

 island, open to the storms from the west 

 and north, and nearly a mile and a half 

 away from the nearest land. 



I must pass more rapidly over Ireland. 

 The signs of vanished glaciers — ice-worn 

 rocks and characteristic boulder-clays — are 

 numerous, and may be traced in places 

 down to the sea-level, but the principal out- 

 flow of the ice, according to some competent 

 observers, was from a comparatively low 

 district, extending diagonally across the 

 island from the south of Lough Neagh to 



^Brit. Assoc. Report, 1899 (1900), pp. 414-423. 

 ~' Quart. Journ. Geol. Soc, XXXVI., 1880, p. 

 355. 



north of Galway Bay. Glaciers, however, 

 must have first begun to form in the moun- 

 tains on the northern and southern side of 

 this zone, and we should have expected 

 that, whatever might happen on the low- 

 lands, they would continue to assert them- 

 selves. In no other part of the British 

 Islands are eskers, which some geologists 

 think were formed when a glacier reached 

 the sea, so strikingly developed. Here also 

 an upper and a lower boulder clay, the 

 former being the more sparsely distributed, 

 are often divided by a widespread group of 

 sands and gravels, which locally, as in 

 Great Britain, contains, sometimes abun- 

 dantly, shells and other marine organisms ; 

 more than twenty species of molluscs, with 

 foraminifers, a barnacle, and perforations- 

 of annelids, having been described. These 

 are found in counties Dublin and Wicklow, 

 at various altitudes,-^ from a little above 

 sea-level to a height of 1,300 feet. 



Not the least perplexing of the glacial 

 phenomena in the British Isles is the dis- 

 tribution of erratics, which has been al- 

 ready mentioned in passing. On the Nor- 

 folk coast, masses of chalk, often thousands 

 of cubic feet in volume, occur in the lowest 

 member of the glacial series, with occa- 

 sional great blocks of sand and gravel, 

 which must have once been frozen. But 

 these, or at any rate the larger of them, 

 have no doubt been derived from the imme- 

 diate neighborhood. Huge erratics also 

 occasionally occur in the upper boulder 

 clay — sometimes of chalk, as at Roslyn HiU 

 near Ely and at Ridlington in Rutland, of 

 Jurassic limestone, near Great Ponton, to 

 the south of Grantham, and of Lower 

 Kimeridge clay near Biggleswade.-" These 



^ See T. M. Reade, Proc. Liverpool Geol. Soc, 

 1893-94, p. 183, for some weighty argumenta in 

 favor of a marine origin for these deposits. 



"^ H. Home, Quart. Journ. Geol. Soc, LIX., 1903, 

 p. 375. 



