542 TRAlS'gACTlONS OF SECTiOJf C» 



eithel* directly by the encroaching glaciers, or indirectly by the packing of 

 bergs and floes, until the basins themselves possessed a surface upon which tht. snow- 

 fall could accumulate. Thus the basins became great reservoirs of ice, in which the 

 supplies from the surrounding uplands received important augmentation by direct 

 accretion of snowfall ;-^reservoirs, moreover, containing a substance sufficiently 

 rigid not to require retaining walls ; so that, in time, the surface of the ice within 

 the basins rose higher than many parts of the rim. The general movement of 

 the mass within its reservoir then became dependent mainly upon its own 

 configuration, and only secondarily upon the shape of the solid ground. 



These conditions in the North Sea basin had their parallel in the basin of the 

 Irish Sea, in which the * West British ice-lobe ' was developed ; and on the low 

 interior plain of Ireland, where the similar though smaller ' Ivernian ' sheet held 

 possession. 



Now, the crux of the Interglacial problem, so far as the British Islands are 

 concerned, lies in the question whether these huge reservoirs, after their first 

 filling, were completely emptied during the supposed interglacial epoch of warmth 

 named by Professor Geikie the * Helvetian,' and were afterwards refilled for the 

 later ' Polandian ' glaciation, in which, on the evidence of the upper boulder- 

 claySj it is generally agreed that ice-sheets from the basins again closed in 

 upon the land. It is this one intei-glacial or ' middle glacial ' epoch only that 

 most of the British supporters of the hypothesis have demanded, and have 

 attempted to establish in the East Yorkshire sections. 



For my own part, although I have sought long and carefully for evidence of 

 this great interglacial episode in the Yorkshire drifts, and at first with the belief 

 that such evidence must surely be somewhere forthcoming, my search has not only 

 failed to bring to light any adequate proof of its reality, but has yielded many facts 

 which I cannot explain otherwise than by recognising that the ice-lobe continued 

 to occupy the basin of the North Sea during the deposition of the beds claimed as 

 interglacial, though its margin bad for a time shrunk considerably within its 

 earlier limits. 



The ' Purj^le' Bouldei'-Ckiys and Stratified Drifts. — The drifts overlying the 

 Basement Clay in East Yorkshire consist of a complex and very variable series, 

 in which bands of boulder-clay predominate in some places and lenticular sheets 

 of well-stratified material in others. In the cliff"- sections of the ITolderness plain 

 certain bauds of boulder-clay, known as the Upper and Lower Purple Clays, are 

 persistent for many miles ; but when the series approaches the rising ground of the 

 Wolds the individuality of the beds is lost, and they are often replaced entirely 

 by irregular mounds of sand and gravel, 



1 began work on these sections with the then-prevalent idea that every separate 

 band of boulder-clay above the Basement Clay might indicate a separate glacial 

 epoch, and that warm interglacial epochs might be represented by the partings of 

 sand and gravel between these boulder-clays ; and the object of one of my early 

 papers ' was to show that more of these divisions were present than had found 

 place in the scheme of classification then in vogue. But after struggling for a 

 time under an ever-increasing load of epochs I was compelled, in tracing the 

 separate bands northwards, to recognise, as my friend Mr. J. II. Dakyns had pre- 

 viously recognised,- that the whole series underwent protean changes, the boulder- 

 clays sometimes splitting into numerous shreds amid thick sheets of sand and 

 gravel, at other times merging into a single mass to the exclusion of all stratified 

 material, and not rarely presenting a passage from uncompromising ' till ' to 

 stratified gravel, sand, and clay. Hence I was driven to conclude that stratified 

 and unstratified drift must often have been forming simultaneously at places very 

 little distance apart ; and on finding, also, that the whole of the deposits between 

 the Basement Clay and the Upper or ' Hessle ' Clay were not only knit together in 

 this fashion, but were similarly interwoven with the top and bottom of these boulder- 

 clays, I had finally to abandon the Interglacial hypothesis altogether so far as the 



' ' On the Divisions of the Glacial Beds in Filey Bay.' Proc. Forks, Oeol. Soo., 

 vol. vii. (1879), pp. 1C7-177. 



2 ' Glacial Beds at Bridlington.' Ibid., vol. vii. (1879), pp. 123-128. 



