SPEETON CLAY, 
811 
tive strata. Whenever similar conditions, he says, are re¬ 
peated, the same species reappear, provided too great a lapse 
of time has not intervened 5 whereas if the length of the in¬ 
terval has been geologically great, the same genera will re¬ 
appear represented by distinct species. Changes of depth, 
or of the mineral nature of the sea-bottom, the presence or 
absence of lime or of peroxide of iron, the occurrence of a 
muddy, or a sandy, or a gravelly bottom, are marked by the 
banishment of certain species and the predominance of 
others. But these differences of conditions being mineral, 
chemical, and local in their nature, have no necessary con¬ 
nection with the extinction, throughout a large area, of cer¬ 
tain animals or plants. When the forms proper to loose 
sand or soft clay, or to perfectly clear water, or to a sea of 
moderate or great depth, recur with all the same species, we 
may infer that the interval of time has been, geologically 
speaking, small, however dense the mass of matter accumu¬ 
lated. But if, the genera remaining the same, the species are 
changed, we have entered upon a new period; and no similari¬ 
ty of climate, or of geographical and local conditions, can then 
recall the old species which a long series of destructive causes 
in the animate and inanimate world has gradually annihilated. 
Speeton Clay, Upper Division. —On the coast, beneath the 
white chalk of Flamborough Head, in Yorkshire, an argilla¬ 
ceous formation crops out, called the Speeton clay, several 
hundred feet in thickness, the palaeontological relations of 
which have been ably worked out by Mr. John W. Judd,^ 
and he has shown that it is separable into three divisions, 
the uppermost of which, 150 feet thick, and containing 87 
species of mollusca, decidedly belongs to the Atherfield clay 
and associated strata of Hythe and Folkestone, already de¬ 
scribed. It is characterized by the 
Pevna Mulleti (Fig. 283) and Tere- 
hratula sella (Fig. 281), and by Am- p n 
monites Deshayesii (Fig. 2 84), a well- p 0 
known Hythe fossil. Fine skeletons 
of reptiles of the genera Pliosaurus 
and Teleosaurus have been obtained fcr 
from this clay. At the base of this K 
upper division of the Speeton clay ^ 
there occurs a layer of large Septa- Ammonites Desliayesii,Jueym.^ 
ria, formerly worked for the manu- Upper Neocomian. 
facture of cement. This bed is crowded with fossils, espe¬ 
cially Ammonites, one species of which, three feet in diame¬ 
ter, was observed by Mr. Judd. 
* Judd, Speeton Clay, Quart. GeoL Journ., vol. xxiv., 1868, p. 218. 
Fig. 284. 
