1869. | ’ SEARLES WOOD——BOULDER-CLAY. 107 
break I find to be greater and more complete than I once thought. 
It appears also, as shown by the list betore given, that during the 
Middle Glacial formation several Crag shells (the greater part of 
which are non-Arctic) of which we get no trace in the Lower Glacial 
returned to their former habitat. This may perhaps be due, in 
part (though it is far from being probable that it is so in the 
whole, looking at the non-Arctic character of the returned group, 
and the abundance of some of the shells in both the Crag and 
Middle Glacial), to the incompleteness of the series of mollusca 
obtained; but it is very significant that amongst this group of 
apparent returns are two shells (Pectunculus glycimeris and Ostrea 
edulis, neither of them Arctic) which disappeared even during the 
deposition of the upper beds of the Crag itself. 
The recommencement of subsidence I conceive so far altered the 
movements of the ice that a material of which we find no trace in 
the great chalky clay became extruded over Holderness, viz. that 
reddish-brown or brownish-purple sediment in which some chalk 
occurs. The beds of sand and bands of blue and blackish chalky clay, 
alternating with bands of purple clay, which mark the junction of 
the purple with the great chalky clay along the Holderness coast, 
and which are indicated in the sections of Mr. Rome and myself as well 
as in the accompanying vertical section by the letter 6, seem to me 
to indicate the termination of this long stationary interval, or pause 
in the subsidence during which the great volume of chalk was shed 
out, and the setting in of the renewed depression, the first result 
of which was the limited deposit of purple clay, wherein the chalk 
débris, tolerably abundant at first, rapidly diminishes in quantity 
upwards, and the eventual result the passing over of the Shap 
boulders and deposit of the purple clay without any chalk. 
The transition from a depression of 600 or 700 feet to one of 1500 
and upwards would, we may infer, produce a great change in the 
conditions under which the mollusca existed. Our ideas about the 
depths which mollusca inhabit are undergoing much change from the 
dredging expeditions, and our knowledge on the subject is too de- 
fective at present to justify any conjectures as to what results 
might in this respect be expected to ensue on the purple-clay sub- 
mergence. Our knowledge, moreover, of the mollusca of the pur- 
ple clay without chalk is very restricted. Mr. Leckenby enume- 
rates* ten species as obtained by himself and Mr. Jeffreys from 
the Glacial beds of the cliff about Scarborough and Whitby, which 
belong to the purple clay without chalk and with Shap boulders 
and with gravel intercalations (c’ and é of the vertical section). 
The species so enumerated are as follows :—Cardium edule, My- 
tilus edulis, Cyprina islandica, Venus lincta, Astarte borealis, Astarte 
sulcata, var, elliptica, Tellina lata, Tellina balthica, Mya truncata, 
var. uddevallensis, and Pholas crispata. 
These are too few in number to afford any reliable comparison 
with the Bridlington fauna; but, as far as they go, they agree satis- 
* factorily with the newer age of the purple clay without chalk 
* Geol. Mag. vol. ii. p. 348. 
