340 PROP. J. PRESTWICH ON THE RIISED 



the several forms assumed by this Eubble-drift, the organic 

 remains are invariably confined to those of land-shells and land- 

 animals (see list, p. 300). No subaerial action could have en- 

 tombed those fragile shells entire. Ice or running surface-water 

 that could have moved the surface-debris would have shattered 

 the shells, or they would have perished if exposed to atmospheric 

 agencies on the surface. The shells seem also to have been living 

 at the time of the submergence, for had they been dead shells they 

 would have been floated off and lost, whereas they remained on 

 the spot where drowned, until swept down with the general mass 

 of debris. When the currents were strong and the debris coarse 

 the shells have been destroyed, while they have been preserved when 

 the currents were gentle and the sediment fine and suitable in 

 character. 



The animals, on the other hand, had, as their carcasses decayed 

 and sank, their detached members scattered over the submerged 

 land-surface, whence, on the re-elevation of that land, they were 

 swept by the efflaent currents and entombed in the mass of rubble. 

 That the action was from time to time short and rapid, and the 

 debris swept along tumultuously is evident from the condition of the 

 fractured and shattered bones, as well as from that of the broken 

 rubble. Both are free from wear by long erosion in water, and 

 their fractured fragments retain their sharp angles uninjured. ^ 

 Kor is there any appearance of wear by exposure to atmospheric 

 influences, although the action of water filtering underground is 

 sometimes apparent. Many of the animals may have been carried 

 out to sea, and it is not unlikely that some of the teeth and bones 

 which are now dredged up from time to time in the JS'orth Sea 

 and English Channel may have this origin. 



Pew vestiges of the land-vegetation are preserved. Here and 

 there in the rubble there are traces of carbonaceous matter, and 

 fragments of wood have been found in the Elephant Bed at 

 Brighton, and plants, nuts, and insects in the Brixton (I. of W.) 

 drift and elsewhere, but it is readily conceivable that either the 

 wood may have floated to a distance or have been lost by decay. 

 There is a remarkable mass of driftwood, probably of this age, 

 in the N'orth of Erance. 



"We have no accurate measure of the depth of the submergence. 

 It apparently exceeded 900 feet, and may have been not less than 

 1000 feet.^ That it was temporary and not of long duration is 

 indicated by the absence of marine moUusca on the submerged 

 land ; and that the emergence was continuous, by the absence of 

 terraces within the area, as well as by the absence of any break in 



^ The Rev. Gr. N. Smith, speaking of the Ossiferous Breccia in Caldy Island, 

 remarks that " the whole seemed to have been forcibly carried into the cave by 

 the action of vfater Some of the bones were wedged into the fissures of the 

 rock at the cave's ends, just as pieces of driftwood and wreck are observed to 

 be on the shore beneath." See ' The Bone-Oaves of Tenby,' p. 6. 



2 A map showing the extent of land left dry, supposing the British Islands 

 were submerged to the depth of 1000 feet, is given by De la Beche in the 

 ' Geological Observer,' 1st ed. p. 301. 



