266 Proceedings of the Royal Physical Society. 



Both drew the inference that the terraces of the coasts and 

 the rivers were due to the same cause. Darwin's terraces, 

 on entering the mouths of the valleys, assumed the sea- 

 ward slope of the rivers; he found them to be continuous 

 with " terrace-like fringes " among the Cordilleras in the 

 interior, at 7000 to 9000 feet above the sea-level. He ac- 

 counts for the whole series by " the arrestment of river-borne 

 detritus at successive levels " by the sea ; in other words, by 

 the formation in each valley of a sloping delta lengthened 

 out by a process of gradual elevation, and passing into 

 beach-lines around bays — the terrace-steps being sea-cliffs 

 drawn gradually away from the power of the sea.* Eobert 

 Chambers gave a similar explanation of the few sloping 

 terraces which he observed in Scotland, but he seldom 

 noticed their slope, and includes the terraces generally as 

 level sea-margins. This latter view has been abandoned. 

 The terrace-gravels are not of marine origin; they contain 

 shells of the land and fresh water, but not of the sea, and the 

 alternation of conditions — fresh water and marine — so sharply 

 marked in terrace-deposits towards the mouth of some valleys, 

 e.g. the Somme, only serves to give point to the difference. 

 His explanation of the sloped terraces has fared better. He 

 supposed them to be the vnngs — as he phrased it — of suc- 

 cessive deltas raised and intersected one after another as the 

 land, stage by stage, arose from the sea. This view has been 

 widely accepted. It differs from Darwin's only in assuming 

 intermittent elevation. 



With some modification it was this view that was taken 

 by the uniformitarian school during the controversy respecting 

 the palseolithic terrace-gravels of the Somme Valley and 

 Southern England. " According," says Prestwich, " to any 

 variability in the rate of elevation, to intervals of repose, or 

 to deflections in the flow and velocity of the rivers, so there 

 may exist intermediate terraces or levels, sudden variations 

 in the slopes, and gravels lodged at various levels." f 



* Loc. ciL, pp. 258, 291. 



t Phil. Trans., 1864, vol. cliv., p. 298. It is right to remark that the 

 phrase "deflections in the flow and velocity of the rivers" also covers the 

 ground occupied by Hitchcock. See postca. 



