IV.—On the Albian, or Gault, of Folkestone

From the proposition of Professor Forbes, that a species once extinct never reappears, it follows that when a species recurs, it must have existed elsewhere during the whole of the time occupied by the deposition of the strata between the deposits containing it. In viewing the distribution of species through the cheif stages of the Lower Cretaceous system, it appears that the same species reappear when there is a recurrence of the same or similar physical conditions,—the Neocomian and Albian clays having more species in common than the intervening Aptian; and the Aptian and Cenomanian sands, being more closely allied than the intervening Albian clay. An examination of the latter, at Folkestone, appears to allow of its being divided into eleven lithological stages or beds which have been more or less recognized by all geologists and fossil collectors who have visited the district. To these beds provisional names have been assigned expressive either of their colour, position, or characteristic fossils. But in tracing all the recurring species from their genesis in one stratum to their extinction in another, these beds are found to have no great palæontological value, but to resolve themselves into two groups divided by a junction bed, in the same way as the “junction bed” separates the Albian from the Upper Aptian. Beds I. to III. forming an Upper, beds V. to X. a Lower Albian, beds IV. and XI. being the two phosphate junction beds,