Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/847

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