ABOUND THE ESTTJAKY OF THE DEE. 735 



which produced the sandy and gritty part of the formation ; and I 

 believe the same remark applies to the clay of which the formation 

 mainly consists. Icebergs from an ice-sheet, or from glaciers, in 

 the Lake-district and south of Scotland, could not have brought the 

 whole of the deposit with its local grit, erratics, and clay, while ice- 

 bergs, carrying both clay and erratic stones, and dropping them at 

 intervals into the slowly accumulating local materials, would have 

 left a deposit much less uniform in its character and structure*. 

 Wide-spread and thickly congregated masses of floating coast-ice from 

 the Lake-district &c. may have brought the erratic stones (with more 

 or less sand and clay). The structure therefore of these deposits 

 can perhaps be best explained by supposing a threefold origin : — 

 (1) the local grit and sand furnished by ordinary sea-action ; (2) the 

 clay washed out from beneath the ice-sheet or glaciers of the Lake- 

 district, and generally distributed by currents ; (3) the stones, prin- 

 cipally erratic, but to some extent local, supplied by floating coast- 

 ice. There is generally more grit, and a greater number of stones 

 in the lower than in the upper clay ; but still they are too much 

 alike to require a different explanation. 



Persistent Line of Demarcation between the Middle Sand and the 

 Upper Clay. — When the upper clay rests not only on surfaces of 

 rock or rock-sand, but on the middle sand-and-gravel (in which 

 position it is chiefly found), there is almost universally a clean and 

 straight or undulating line of junction, without the slightest com- 

 mingling of materials ; so that, even within the vertical space of an 

 inch (as in the recent long railway-cutting between Chester and 

 Delamere), the typical clay, with parts of intensely glaciated stones, 

 is separated from the equally typical sand with entirely unglaciated 

 stones, excepting in the case of a few instances of very limited ex- 

 tent, where the sand has been cleanly interlaminated with loam or 

 clay. The sharpness of this line of junction and its persistence over 

 large areas would seem to indicate the shaving-off or denudation of 

 the sand before the deposition of the clay. It would likewise seem 

 to show that the clay could not have been brought by land-ice, 

 because land-ice could not have pushed its moraine profondc for scores 

 of miles over an extensive deposit of yielding sand and gravel with- 

 out confusedly mixing up the two formations. But the most difficult 

 fact to explain is one to which Professor Hull, Mr. I)e Ranee, and 

 others some time ago called attention, and which is strikingly ex- 

 emplified in the above new railway-sections, namely the evidence of 

 a great leap from an interglacial sand-and-gravel period, when no 

 glaciated stones were floated by ice over the submerged plains, and 

 when scarcely any clay was deposited, to a period when clay with 

 intensely glaciated stones began to accumulate f. 



* It may likewise be remarked that the depth of the sea, indicated by the 

 vertical range of either of the Boulder-clays, could not have floated icebergs. 



t Mr. Shone, F.Gr.S., of Chester, can corroborate what I have here stated. 

 He has now collected a great number of shells from the upper clay of the 

 Chester and Delamere railway-cutting, and has found additional species indi- 

 cating a very cold climate. 



