518 S. V. WOOD, JUN., OK THE NEWEE 



which is embraced by Stage II., during which this molluscan fauna 

 changed to that found at Moel Tryfaen. 



The distribution of the Chalky Clay indicates that the ice to which 

 it was due came along the western side of the Wold only ; for, after 

 crossing the "Wash, it overwhelmed so much only of island No. 1 as 

 lay in the direction of this path. Had this ice descended the eastern 

 side of the Wold, it would, we may infer, have overwhelmed all that 

 part of island No. 1 which occupies Sheet 68 ; but this the ice has, so 

 far as it lay outside the track along the west of the Wold, avoided. 

 That such was the case is rendered more probable also by the fact 

 that all the chalk degraded from the Lincolnshire Wold has been swept 

 westwards into the depression of central Lincolnshire, while no trace 

 of the Chalky Clay appears on its eastern side. The escape of the 

 Basement Clay of Holderness from destruction by ice of the Purple 

 Clay, I attribute to this ice having, where the Basement- clay occurs, 

 terminated in the sea, and being thin there, so that the buoyancy 

 of the water prevented that destruction which the ice of the Chalky 

 Clay, where it eventually collected in very thick mass within the 

 broken line of Map No. 1, has caused. 



As by thus dissociating the Basement Clay of Holderness from the 

 Chalky Clay we lose the test of superposition, it becomes necessary 

 to examine how far, or whether at all, this Purple Clay may be a se- 

 parate formation from the Chalky. Among the reasons for regarding 

 them as distinct are the following : — 



(1) The Purple Clay at elevations below 100 feet contains, in its 

 lower part at least, lenticular beds of gravel and sand, the Chalky 

 Clay being destitute of any thing of the kind. This feature, how- 

 ever, I can only assert positively of that arm of the formation which 

 lies east of the Wold and Eastern Moorlands. 



(2) The constitution of the two clays differs, the Purple Clay being 

 crowded with small subangular debris of various hard rocks (prin- 

 cipally from the Eastern Moorlands), while angular debris, other 

 than flint, is very rare in, and, indeed, in most parts quite absent 

 from, the Chalky Clay. 



(3) The presence in the Purple Clay af the shap-blocks, which, 

 from a late communication *, appear to occur at the base as well as 

 in the upper part of it. These blocks seem to occur very nearly as 

 far as this clay extends, but no further, the southermost known 

 being one in the west centre of Sheet 87, which is mentioned in the 

 Geological-Survey memoir for that sheet. They are wholly unknown 

 from the Chalky Clay and from the area occupied by it. 



The position of the Purple Clay is as follows : — It first shows 

 itself above the sea-level in the small and low cliff of Cleethorpes 

 (in the south of Sheet 85) ; and the excavatious for the Grimsby 

 bocks, close at hand, proved it to descend there to a depth of 102 

 feet below high-water mark, and to rest on the chalk direct, except 

 in places wnere it had a few feet of chalky gravel beneath it. 

 Northwards from this it forms the continuous coast- section of South 

 Yorkshire (in Sheets 85 and 94), and there rests on the Basement 

 * Lamplugb, Geol. Mag. for Sept. 1879, p. 396. 



