THEORIES EXPLAINING TRANSPORT OF BOWLDERS, 



537 



of the existence of dry land anterior to the deposition of this drift, hut that it contains 

 marine shells of existing species, and has all the characters of submarine or shore de- 

 posits ; in short, that it must have been spread out beneath the sea. 



Another theory is, that these blocks, in common with the associated drifted materials, 

 were transported by powerful currents, set in motion during the elevation of mountain 

 chains. The geologists who have taken this view (which I also once adopted) have not 

 sufficiently placed before us the probable condition of the physical geography of the 

 country when these currents were in action. Without showing what portions of the 

 present land were above and what below the sea, or what was the power of water under 

 such conditions, they have only spoken generally of great debacles and abrading floods, 

 thrown off upon the flanks of the upraised masses. Such causes may afford a true ex- 

 planation in Alpine or mountainous regions where there are evidences of repeated 

 violent action. The grooves on certain rocks of Scotland and other parts of the world 

 appear also to prove that blocks have been driven along their surface in various direc- 

 tions by powerful local currents 1 . But although this theory may be applicable in such 

 cases, it will not explain, in the region under review, the occurrence of distantly trans- 

 ported blocks, imbedded in local debris ; for if currents sufficiently powerful to transport 

 the blocks had prevailed, the local detritus and shells would have also been removed by 

 the same action. 



In our case, however, the blocks having certainly been deposited under the sea, and 

 not transported over previously dry land, we get rid of much difficulty ; but can we 

 explain the phenomenon by the modern analogies of deltas and tidal currents ? If we 

 suppose a great delta, extending 60 or 80 miles southwards from the shores of the an- 

 cient Cumberland, and in which blocks of granite as well as other ancient rocks were 

 commingled with the marine shells at the bottom of the sea, those who argue for the 

 power of tidal currents may contend that, if a strait existed between England and Wales, 

 the tide stream might have exercised considerable power in carrying materials, and 

 that by comparison with straits in which tidal streams now flow, there might have been 

 a preponderance of transporting force in one direction • — that as there can be no doubt, 

 that all tidal streams have a stronger tendency to carry loose materials out to sea than 

 they have to bring them back again, so the ancient tide streams, which from the dis- 

 tribution of materials have been supposed to now and ebb in the straits of Malvern, 

 may have had an effective transporting power from north to south ; and lastly, that if 

 the shore of the ancient Cumbria which skirted the trumpet-shaped arm of the sea 

 extending into the straits of Malvern, formed a long inclined plane, a large portion 



1 For the details concerning the grooved and scratched surface of rocks see Colonel Imrie's account of the 

 Campsey Hills, Trans. Wernerian Society, vol. ii. p. 35 ; Sir James Hall's memoir on similar phenomena in 

 the neighbourhood of Edinburgh, Trans. Royal Soc. Ed., vol. vii. ; also Dr. Buckland's commentary thereon, 

 Reliquiae Dil., p. 201 et seq. ;. and my notice of the scored surface of the sandstone of the oolitic series of Braam. 

 bury Hill, Brora, Sutherlandshire, Trans. Geol. Soc, vol. ii. p. 257, 



