PLIOCENE PEEIOD IN ENGLAND. 525 



accumulated. This accumulation having been put an end to only by 

 the passing away of the Glacial period (for with these clays do I 

 define the close of the Glacial period, referring the ice-extension of 

 the Hessle Clay to a brief return of extreme cold during the Post- 

 glacial), the sea deposited gravel over the clay which the ice had 

 deserted up to that level to which the emergence at this time had 

 reduced it. The cemetery sand and gravel is therefore synchronous 

 with the gravel /, which I have already described as synchronous 

 with the latest part of the Purple Clay rather than with e ; and 

 where at lower elevations it is overlain by the Upper Clay of the 

 north-west, so as to become the Middle Sand, it is synchronous with 

 beds still later. Prom Cheshire, where the Lower Clay of the north- 

 west is thus overlain, to the edge of the Chalky Clay in Sheet 62 

 there is a large interval where sand and gravel only occur ; and 

 here, from there having been no intrusion of moraine, the sand and 

 gravel series is uninterrupted, just as it is in the southern part of 

 fig. YL, in the eastern part of fig. YII., in the Surrey and Hamp- 

 shire areas illustrated by figs. III. & Y., and in the Chiltern, 

 Marlborough, and Cotteswold districts. 



As I shall describe in the second part of this memoir, marine con- 

 ditions prevailed uninterruptedly on the western side of the Pennine 

 until the Upper Clay of that region emerged ; for in this clay, at low 

 levels, marine shells occur, and no denudation of the Lower Clay 

 into the form of hill and valley therefore took place in the interim, 

 such as occurred between the Purple Clay and the Hessle beds in 

 Holderness ; in consequence of which the Middle Sand and Upper 

 Boulder-clay rest upon the Lower in such a way in the north-west 

 as make the two clays difiicult of distinction there. This feature 

 has induced some geologists to insist that there is no distinction 

 between the Upper and Lower Clay of Lancashire, and that they and 

 the Middle Sands of that region are all one formation, as in the sense 

 I have mentioned of uninterrupted marine conditions being main- 

 tained they are. On the east side of the Pennine, in Holderness at 

 least, this is diff'erent, for the Hessle sand and gravel, which is the 

 equivalent of the middle sand of Lancashire, is, where fossiliferous 

 at all, fluvio-marine, and rests at Hessle on a land-surface ; and it 

 occupies along the coast-section valleys eroded in the Purple Clay, 

 which sometimes, as at Dimlingtoii, are cut quite through this and 

 down into the Basement Clay beneath it, while the Hessle Clay, which 

 overlies this and is coeval with the Upper Clay of the north-west, has 

 yielded no trace of shells. 



Thus, so far as I have been able to trace them, the whole train of 

 facts, perplexing as they have long been to me, now appear to me 

 harmonious. As regards the phenomena presented by Wales, I 

 know too little of them to say more than this, viz. that the gravels 

 at high elevations accumulated there during the culmination of 

 Stage II., of which the patch at 1350 feet on Moel Tryfaen is a 

 remnant, appear to me to have been mostly destroyed by the ice 

 which during the rise increased over Snowdon, as it did on the 

 Pennine and Westmoreland mountains; but Snowdon being an isolated 

 mountain mass and not flanked, as are the Westmoreland mountains, 



