or exposed during certain states of the tides till the surface became dried and fissured so as to form wedge-shaped cavities filled with the materials of the beds which were accumulated above them (fig. 8). In beds deposited under such circumstances, subject to such constant removal and reconstruction, it is not surprising that it is difficult to find one in which the shells of the period exist in situ. Possibly some of the beds at "Walton were less frequently disturbed, or may have been deposited under rather more tranquil circumstances than elsewhere ; still most of those beds are full of comminuted shells, and many of them show oblique lamination, so that they also were subject to the action of tides and currents. With a sea of this character, and with shoals and reefs of the soft friable Coralline Crag exposed to its action, a large destruction of the latter was inevitable ; and the shells of that deposit must consequently have been transferred in great numbers to the newer beds of the Red Crag formed around it. To what extent this may have taken place it is not possible yet to say. I quite agree with Mr. Searles Wood, that a large number of the shells found in the Red Crag have had this origin ; but although we can feel little doubt on the subject, yet as the shells are mostly single valves, and the staining given by the Red Crag assimilates them to its own fossils, it is difficult to distinguish those which are derived from those which belong to the deposit. When we meet with the entire or the rolled fragments of Bryozoa or some of the rare corals, evidently derived from the Coralline-Crag beds, there can be no doubt of their origin. Still it is clear that a considerable number of shells lived in the Red- Crag sea, and that a certain number of new species were introduced. Many species of bivalves are found double ; but this is not always a proof that they are not extraneous.
I believe also that a large proportion, if not all the coprolites (so called) of the Red Crag were derived from the Coralline Crag, as detached ones are found not unfrequently in its mass, and a bed of them was found to underlie it at Sutton, at the only place where its base has been exposed (ante, p. 117).
The direction of the currents, judging from the nature of the foreign materials found in the Red Crag, seems to have been from the east to south-east. Though the sea was open to the north, the communication with the south was probably cut off. Innumerable Balani covered the blocks of Septaria and flint scattered on the London Clay and Chalk-shores of this Red-Crag sea. Pholades abounded on its water-line ; and we have seen at Sutton and elsewhere that Mytili were common in its shallow waters, while other genera found shelter in bays and any tranquil places. I can see no distinction in the organic remains, from the base of the Red Crag to the top of the lower division. The like abundance of species of Pectunculus, Mactra, Tellina, Cardium, Fusus, Natica, Purpura, &c. occurs almost throughout, modified by situation, and having a distribution sufficiently independent in character to show that it could scarcely have been dependent upon one source alone, and that a derivative one, for its fossils, but that there were banks and feeding-grounds for the various species of Mollusca, which served as centres from
