County Dublin, Past and Present. 93 



The beds are stratified, as if laid out in water ; and the rain, 

 in cutting numerous gullies, has left rough pinnacles standing 

 out, where the larger boulders protect the beds below. 



We have reserved one of the most important characters 

 until last ; the limestone blocks, and many of those of other 

 materials, are conspicuously marked by glacial striae ; and at 

 Howth, Portraine, and elsewhere, these sands with scratched 

 blocks are seen to rest on a similarly scratched floor of solid 

 rock." lyand-ice, despite all other theories, seems alone com- 

 petent to account for this wide-spread glaciation. 



Hence here, as elsewhere, geologists have discarded the 

 agency of huge flood- waves^ in accounting for the "drift" 

 accumulations ; but they find themselves facing the difficulties 

 presented by the theories of the " Glacial Period." Mr. 

 Maxwell Close,^ from a prolonged study of the forms of the 

 roches moutonnees and the direction of the boulder-drift, has 

 concluded that vast ice-sheets spread outwards from the north- 

 central plain of Ireland, one of the ice-streams splitting on the 

 Dublin Mountains ; the S.B. branch of this would be some 

 1 1 20 feet thick at Bray, and capable of riding up slopes of 14° 

 to 28°, so as to striate the surface at a height of 653 feet above 

 the sea. Unocal glaciers, descending from the mountains, and 

 remaining long after the great ice sheets, would complicate 

 the deposits finally left behind. The clays with huge granite 

 boulders, well seen in the new cuttings of the proposed 

 railway near Knniskerrj^ are regarded as the product of land- 

 ice ; while the stratified sands and gravels are generally held 

 to be marine deposits, formed during a period of subsidence, 

 when icebergs carried abundant scratched blocks and dis- 

 tributed them on the sea-floor and in the inlets of the coast. 

 But the fragmentary nature of the shells found in the sands 

 indicates that they were moved from one place to another 

 during these changes ; and hence certain authors have argued 

 that in the Dublin Mountains, and the correspondingly high 

 area of Moel-3^-Tryfaen in Caernarv^onshire, the marine shells 

 have been thrust up to their present positions by advancing 

 ice-sheets, or have been gradually raised from the sea-floor to 

 the surface of the vast glaciers by internal movements of the 

 ice. The controversy is still proceeding; but the most ardent 

 supporters of the ice-sheets ad^lit that a considerable sub- 

 sidence has occurred within the limits of the "glacial period." 

 Considering the magnitude of past movements, a subsidence 

 of some 1300 feet in recent times cannot be looked on as in 

 any way more improbable than the passage of ice-sheets over 

 our hills and dales, as maintained by the majority of geologists. 



1 This was insisted upon, before its significance was known, by Dr. 

 Oldham [G.S.D., iii, p. 132). ^ 



2 See Kelly, G.S.D., vi, 148. 



8 "The General Glaciation of Rocks near DubHn." G. S. /., i. (1864), 3; 

 and "The General Glaciation of Ireland," ibid. (1865), 207. 



