xxvi SUPPLEMENT TO THE CRAG MOLLUSCA. 



by a few feet of sand. This thickness, added to what appears above the beach line, does 

 not differ much from the maximum thickness of the clay in Suffolk. Over this clay, and 

 separated from it in places by the beds b, there appears another clay, c, differing from it 

 in colour (being of a purplish brown), but more particularly differing from it in its 

 contents ; for while in the clay, a, chalk debris is the principal, and debris of beds older 

 than the chalk the subordinate ingredient, in the overlying purple clay, c, these proportions 

 are reversed. Further, the purple clay, c, is not only less abundantly supplied with chalk, 

 but this ingredient diminishes so much upwards, that where the uppermost part of the 

 clay remains undenuded, in the highest cliffs, such as those of Dimlington and the 

 " Talbot," it is wholly absent ; showing the gradual release of the chalk country from 

 those degrading agencies which supplied its debris so profusely to the earlier and under- 

 lying East Anglian clay, a. North of Hornsea no trace of a occurs above the beach line, 

 and where the chalk floor comes to the surface about a mile north of the Bridlington shell 

 bed 1 (which is at the beach line immediately on the north side of the harbour), nothing 

 like the chalky clay a appears over it ; but instead, the purple clay c is seen resting 

 direct on the chalk, underlain occasionally by some moraines of rolled chalk, c, that are 

 probably coeval with the lowest part of c further south. The position of the Bridlington 

 bed seems therefore to be about the middle part of the purple clay, c, where there is some 

 chalk intermixture ; and though posterior to the great chalky clay of East Anglia, the bed 

 was evidently succeeded by all that part of the Upper Glacial period in which the rest of 

 the purple clay, both with and without chalk, accumulated. Although the Bridlington 

 fauna does not present nearly such Crag affinities as that of either the Lower or Middle 

 Glacial, and is much more arctic than either, it is nevertheless distinguishable from that 

 of every Scotch, north of England, and Welsh bed, by the presence of those special Upper 

 Crag forms Tellina obliqua, and Nucula Cobboldia, shells not now living, and whose nearest 

 living affinities occur in the Pacific ; while all the species of the Scotch, Welsh, and north 

 of England beds are to be found on one side or other of the Atlantic, or in the Arctic 

 seas connected with it. 2 



The Plateau gravel (No. 10). 



This gravel is everywhere unfossiliferous, and is composed almost entirely of flint. 

 It is difficult in some cases to form an opinion whether it is of Glacial or Post-glacial 

 age. Most of that which is shown in the sections under the number 10 is doubtless 



: The bed was always so covered by the shingle as to be exposed only at rare intervals, and it is now, we 

 believe, buried under the artificial works of an esplanade. Borings in the harbour showed it to be under- 

 lain by twenty-eight feet of the clay c, under which were fifteen feet of gravel, and then the chalk. 



2 At Dimlington Cliff, a little below the beds b, Sir Charles Lyell and Mr. Thomas M C K. Hughes found 

 a thin band of sand intercalated in a which contained Mollusca, some, as Mr. Hughes informs us, having 

 their valves united. The species they found have not been communicated to us. 



