168 PROCEEDIl^GS or THE GEOLOGICAL SOCIETY. 



soon, however, as the higher elevations were submerged, the chalk 

 debris would cease ; and this is the feature presented by the upper 

 portion of the purple clay of Yorkshire, whose purple material 

 seems to us to have been principally derived from the degradation 

 of those Carboniferous and older Secondary rocks that in the north 

 of Yorkshire, and of those Silurian rocks that to the north-west of 

 that county, rise to far greater elevations than the loftiest parts of 

 the chalk area, and which, under the degrading power of an arctic 

 climate, would remain a source of copious sediment after the Wolds 

 had been completely submerged. 



The elevation which the clay with chalk debris attains along 

 the western edge of the Wold, without any trace of the purple clay 

 remaining over it, contrasted with its low position beneath the 

 purple clay on the eastern side, seems to us to require the conces- 

 sion of a very considerable Postglacial elevation of the western Wold- 

 edge at the expense of the eastern. The edge thus elevated into a 

 crest is the feature common to most of the ares or curvilinear sweeps 

 of elevated country that have so intimate a connexion with the 

 direction of the Postglacial denudation, and of one of which, as 

 before mentioned, the Wolds form the principal part. 



The next section, fig. 11, carried from Eand (about the centre of 

 the Praeglacial depression of mid-LincoLnshire) over the Wolds, and 

 thence to the Holdemess coast, will best show the place of the chalky 

 clay on either side of the Wolds, and the position of the pui-ple clay 

 relatively to it, the section beneath it representing what we suppose 

 to have been the relative positions after the deposition of the purple 

 clay, and before the disturbances giving rise to the Postglacial emer- 

 gence and denudation had begun. There is, however, one circum- 

 stance connected with the position of these two clays that requires 

 consideration, which is, the similar absence of all chalk in the pur- 

 ple clay beneath the northern Wold-foot at Speeton to that which 

 obtains where it rests on the Wold-top there and towards Flam- 

 borough. As the W^old in Yorkshire rises in its north-western part 

 to elevations approaching 800 feet, we should expect to find a 

 similar accumulation of chalky clay along that part of the Wold- 

 foot where the presence of the purple clay shows it to have escaped 

 the Postglacial denudation, to what we find both in mid-Lincoln- 

 shire and on the east of the Wold in Holderness ; but nothing of the 

 soli; is there. 



This has, it would seem, been assumed to be the result of the 

 direction of the drift having been from north to south ; but an ex- 

 amination of the case will, we think, show the inadequacy of such 

 an explanation to account for the feature. 



Waiving, for argument's sake, the improbability of the drift of 

 an ice-blocked sea being so absolutely constant in direction as not 

 to permit of any debris from the Wold being carried even the 

 shortest distance northwards, we see that there is a total absence 

 of chalk debris in the clay that rests on the scarp-slope itself. 

 Moreover the general east and west direction of the northern 

 Wold-scarp, for 20 miles west of Speeton, would render a southerly 



