The Number. 519 



by itself, chiefly in comparatively soft Yoredale shales 

 between the high-terraced hard moorland scarps of 

 Millstone Grit, and the still harder grassy slopes of the 

 Carboniferous Limestone. 



When we come to the other rivers that enter the 

 Humber north and west of the Trent, the case is more 

 puzzling. The Oolites in that region were extensively 

 denuded before the deposition of the Chalk ; so that 

 between Market Weighton and Kirkby-under-dale in 

 Yorkshire, the Chalk is seen to overlap unconformably 

 the Oolitic strata, and to rest directly on the Lower 

 Lias, which there, as far as it is exposed, is very thin. 

 The Chalk, therefore, overspread all these strata to the 

 west, and lay directly on the New Eed beds of the Vale 

 of York, till, overlapping these, it probably intruded on 

 the Carboniferous strata of the Yorkshire hills farther 

 west. At this time the Oolites of the northern moor- 

 lands of Yorkshire seem also to have spread westward 

 till they also encroached on the Carboniferous slopes, 

 the denuded remains of which now rise above the beau- 

 tiful valleys of Yoredale and Swaledale, the whole, both 

 Carboniferous and Secondary, strata having gentle 

 eastern and south-eastern dips. These dips gave the 

 rivers their initial tendency to flow south-east and east ; 

 and thus it was that the Wharfe, the Ouse, and the 

 Swale, cutting their own channels, formed a way to 

 what is now the estuary of the Humber, while the 

 escarpments of the Chalk and Oolite were gradually re- 

 ceding eastward to their present temporary positions. 



That the Oolitic strata spread northward beyond 

 their present scarped edges is quite certain; but 

 whether or not they extended far enough north to cover 

 the whole of the Durham and Northumberland coal- 

 field I am unable to say. Whether they did so or not 



