106 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 8, 
should, however, mention that the formation of this clay must, I think, 
have commenced when much of the country to the south of the sea- 
limit, represented in the sketch map (p. 102), was uncovered by sea, 
because the junction of the chalky clay with the Middle Glacial sands 
indicates an uninterrupted succession of deposit, the change, abrupt 
as it is, being only in the material deposited, viz. from sand to Boul- 
der-clay. Coupling this with the indications everywhere pointing 
to the circumstance that, during the Middle Glacial period, the sea 
does not appear to have stood more than from 400 to 450 feet above 
the present sea-level*, we must suppose the area of sea repre- 
sented in the map, to the south of the ice-sheet, to have been con- 
siderably less at the commencement of the chalky-clay deposit than 
at the stage selected for illustration. Moreover I hope, when de- 
scribing the Middle Glacial structure, to be able to show that land 
much below this elevation was, during the Middle Glacial deposit, 
occupied with ice which blocked out the sea of that formation, pre- 
cisely as I have represented it (in the triple section) as doing in 
Yorkshire during the chalky-clay deposit. It seems to me also to 
follow, from the facts touched upon, that it was during this interval 
of pause in the subsidence, when intense cold prevailed, that the 
truly Arctic fauna of Bridlington, which so contrasts with that of the 
Middle and Lower Glacial formations, gradually became established, 
and that thus, the Arctic forms haying become denizens of this part 
of England by the close of this interval, we find it in full force in 
the Bridlington bedt, at an horizon in the deposit which indicates 
a very early stage in this renewed subsidence—that is to say, in the 
lowest or chalky part ofthe purple clay. On the other hand, so far 
as the mollusca yet obtained afford an indication, there is ground for 
supposing that during the period in which the whole of that thick 
body of deposits forming the Cromer cliff was accumulated, and 
during that of the Middle Glacial sands which succeed and overlie it, 
no arctic mollusk, beyond such as occur in the Cragt, except Tellina 
balthica, established itself in these parts. The introduction of Tellina 
balthica constitutes a very clear paleeontological horizon, marking the 
commencement of the Glacial formation, which (as I hope on a future 
occasion to show) exactly coincides with a physical break and un- 
conformity with the Chillesford beds and Crag that takes place at 
the horizon where this shell first makes its appearace; and that 
* The highest point I know of at which this deposit occurs is 420 feet, which 
is on the north side of Rugby. 
+ Many of the peculiar shells of Bridlington have been found at depths 
equal to, and even exceeding, that supposed to have been the depth of 
Bridlington at the time of the setting in of the renewed submergence. 
It is, however, highly probable that molluscan life extends to much greater 
depths than we have been in the habit of supposing. The ice-foot is a favourite 
habitat: of molluscan life (see Watson, Trans. Royal Soc. of Edin. vol. xxiii. 
p- 538, note) ; and Dr. Sutherland speaks of the extreme depths (1800 feet) at 
which the Esquimaux fish for halibut, whose food doubtless consists of mollusca 
living at those depths. 
t One other new shell besides has been obtained from the Lower Glacial, 
but if seems to belong to a genus of freshwater mollusca, and is therefore 
omitted from the list. 
