184-6.] PRESTWiCfi ON THE ISLE OF WIGHT TERTIARIES. 237 



evident ; first, froin the considerably increased transporting power 

 of the waters for a short time, the result of which has been to cover 

 large areas of the London and Hampshire basins with a layer of 

 from six inches to three or four feet thick of rolled flint pebbles, 

 varying in size from a pea to an egg ; secondly, from the sudden 

 appearance (without transition or alternation) of a lithological struc- 

 ture not exhibited in any of the underlying tertiary strata, and evin- 

 cing by its continuance the permanent nature of the change ; and 

 thirdly, by the presence of a distinct group of organic remains dif- 

 fering considerably from that of the beds below. 



On examining the fossils of the Bognor beds at Alum Bay, it will 

 be noticed that the deep-sea mollusca are absent ; and that, on the 

 contrary, their most characteristic and widely-spread genera form a 

 group indicating marine waters of very moderate and tolerably uni- 

 form depths (e. g. Pinna, Vermetus, Cali/ptrcea, Panopcsa, Phola- 

 domya, Turritella, &c.). 



Sir Henry De la Beche, in his ' Theoretical Geology,' assigns, on 

 the authority of Mr. Broderip, a depth not exceeding about twelve 

 to sixteen fathoms to the genera constituting this group ; and Prof. 

 E. Forbes, in his Report on the Testacea of the i^gean Sea, gives 

 further evidence to the same point ; whence we may assume the 

 predominating littoral and shallow-sea character of the testacea of 

 this period. At the same time all the genera are essentially marine, 

 unless we except the Ostrea and Cerithium, which also inhabit estu- 

 aries ; but as the species of these genera occurring in this division 

 are found so constantly associated with well-known and essentially 

 marine families, their presence does not invalidate the argument. 

 The weight of evidence is, I think, so far as the Hampshire basin 

 is concerned, in favour of these beds having been deposited in an 

 open and not a deep sea; whence, as these beds are here from 

 200 to 300 feet thick, and they are further overlaid by a great thick- 

 ness of other marine strata, it follows that to have retained a con- 

 stant fauna throughout so considerable a vertical range, the con- 

 ditions under which they were formed must have changed progres- 

 sively in adaptation to the prolonged existence of the same animal 

 life. If therefore these Bognor beds exhibit throughout nearly their 

 entire thickness a group of testacea, indicating a sea-bottom not 

 often attaining a depth of thirty to forty fathoms, and averaging 

 more probably not more than ten to twenty fathoms, this would 

 clearly be insufficient for the accumulation of the mass of detritus 

 necessary to form strata, which when desiccated are 200 to 300 feet 

 thick. At the same time their lithological character denotes a tran- 

 quil and uniform deposit during some length of time : and as the 

 evidence of fossils proves that the condition of animal life was simi- 

 lar at the end to that which existed at the commencement of this 

 period, it follows that there must have been throughout its duration 

 a quiet and gradual subsidence of the bed of the sea, immediately 

 preceded, as before mentioned, by a disturbance suflficiently powerful 

 and sudden partially to interrupt the continuity of deposit with the 

 Paris basin, a circumstance rendering the recognition of the foreign 



