Notes 07i Glacial Deposits in Ireland. 197 



observed, particularly in its upper portion ; it is well seen in 

 the cliff at the sluice where a mill-race leaves the river two- 

 thirds of a mile above Bray Bridge. Here the lower part is 

 horizontally banded somewhat coarsely, and contains large 

 boulders ; in the upper portion it becomes much more finery, 

 but still horizontally laminated, and the included stones are 

 smaller ; in this portion blackish layers are separated by white 

 ones of granite sand, two inches in thickness. Shell fragments 

 are abundant here ; the species identified were Cardiumedule, 

 Cyprina Islandica, Astarte sulcata, A. borcalis, Tellina balthica, 

 Mya truncata. The boulder clay is here overlaid b}^ a bed of 

 boulders, remarkable for their size, being frequently three feet 

 long by two feet high. One of them was of I^ambay porphyry, 

 rounded, polished, and grooved. If we ascend the cliff and 

 the succeeding slope to the S.E., we reach a road leading to 

 Bray, which runs above the valley parallel to the stream, and 

 in a cutting here is seen a blue sandy clay, containing striated 

 boulders chiefly of Carboniferous limestone, together with 

 granite, and also fragments of marine shells. This is evidently 

 a boulder clay overlying the boulder gravels, and separated 

 by them from the thick deposit of boulder clay which we have 

 described ; numerous other sections terminate upward in a 

 somewhat similar manner. 



It is an admitted generalisation, to which we have found no 

 exception, that the surface of the rocks underlying the boulder 

 clay have been denuded by glaciation ; and the district which 

 we have described, like other districts in Ireland, has evidently 

 been overridden by ice. This denuded surface is overlaid by 

 a thick deposit of boulder clay, which points clearly to a 

 period of deposition. The conditions under which the 

 boulder clay was deposited cannot have been, as appears 

 so generally assumed, the same as those under which the 

 district was denuded. Fine clay is generally deposited in 

 quiet waters some distance from its source, and our boulder 

 clay has much the aspect of a tranquil marine deposit. On 

 the other hand, the boulder clay contains large stones, but 

 these are of an exceptional character, they show signs of 

 glaciation, and are suggestive of a different mode of transport. 

 Judging from the thickness of the boulder clay, the conditions 

 under which it was deposited must have persisted for a con- 

 siderable period. The intercalation of occasional beds of 



