Meyer— Lower Greensand Brachiopods. 255 
deposit, its presence should rather be regarded as affording 
evidence of some slight elevation or depression of the sea-bed 
or adjacent land-surface at a particular period, affecting more 
or less the whole of our Greensand area.* Viewed thus, the 
pebble-bed becomes at once serviceable as marking a distinct 
horizon or boundary-line in the Cretaceous series, and one 
which might possibly be found to afford a more correct boundary 
between the faunas of the Lower and Upper Greensand forma- 
tions than the Gault itself; few of the characteristic fossils of 
the Lower Greensand passing above this line, while it is the 
starting-point of many of the common forms of Testacea which 
range upwards through the Gault and Upper Greensand. 
In the present state of our knowledge, however, it is still 
difficult to determine the exact relation of the pebble-beds of 
Kent and Surrey to those somewhat similar deposits to the 
west of the Wealden area, namely the Sponge-gravels of 
Farringdon, &c., excepting upon paleontological evidence ; 
and even this admits of much difference of opinion. Thus we 
find, for instance, that those species of Brachiopoda which are 
common at Farringdon (7. Tornacensis, var., T. depressa, &c.), 
are of such rare occurrence within the Wealden area as to 
have been overlooked by nearly all collectors; and their sup- 
posed absence in typical Lower Greensand deposits has led 
many Geologists to consider the Farringdon Sponge-gravels as 
wholly distinct in age from the Lower Greensand. It is a 
question, however, how far the abundance of a particular 
species in one locality and its scarceness in some other should 
be admitted as a proof of difference in the age of the deposits 
in which such species occurs; for one cannot but notice in the 
case of living Mollusca how much irregularity exists in the 
range and abundance of almost every known species. 
The occurrence, then, though ever so rarely, in a well defined 
position in the Lower Greensand of such forms as Terebratella 
Menardi and Terebratula Tornacensis, at Godalming, or 7’. de- 
pressa at Shanklin,—species which may be regarded as highly 
characteristic of the Farringdon Sponge-gravels, is very in- 
teresting; and the more so as tending to confirm the opinion 
now generally entertained with regard to the age of these last- 
mentioned deposits; an opinion which the natural distribution 
of our British Cretaceous Brachiopoda surely tends to uphold. 
For in granting the Sponge-gravels to be of Lower Greensand 
age we restore and restrict to that formation such characteristic 
* Possibly the commencement of the Wealden axis of elevation; for there it 
evidence at Godalming, and I imagine also at Folkestone, of slight unconformity 
between the lower and the upper beds of the Lower Greensand. 
