break I find to be greater and more complete than I once thought. It appears also, as shown by the list before 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 edulus, 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 b, 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 debris, 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 defective at present to justify any conjectures as to what results might in this respect be expected to ensue on the purple-clay submergence. Our knowledge, moreover, of the mollusca of the purpleclay without chalk is very restricted. Mr. Leckenby enumerates[1] 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 (ć and c͏̄ of the vertical section). The species so enumerated are as follows: — Cardium edule, Mytilus edulus, 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 satisfactorily with the newer age of the purple clay without chalk
- ↑ Geol. Mag. vol. ii. p. 348.
