Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/849

Rh AROUND THE ESTUARY OE THE DEE. 737 of such around the estuaries of the Dee and Mersey. Even on Hope Mountain the apparently mamillated form of the surface is chiefly the structure of the beds of Millstone -grit developed by denudation. Horizontal and Vertical Hang e of the two Boulder-clays of the Basin of the Irish Sea. — These two Boulder -clays are often separated over large areas by middle sand and gravels, as proved by the geological surveyors in Lancashire, and as may be seen in Cumberland between the estuary of the Duddon and llavenglass (Geol. Mag. for June 1871). To the south of the Mersey the upper clay is generally found lying on the surface of the middle sand ; and, excepting in hollows, there is very little lower clay further south than Chester, though, under the middle sand, it is often represented by a loam with erratic stones, but without shells, which would appear to be the equivalent of the base of the formation further north. In many places in Shropshire this loam is the only drift-deposit. The small quantity of typical lower clay south and south-east of Chester may be accounted for partly by supposing a southerly diminution in the supply of subglacial clay, and partly by the evident using up of the stony contents of the lower clay during the accumulation of the middle drift which, for a great distance south and south-east, con- tains lower-clay erratics. 'The lower clay is much the same along the shores of the Irish sea from the Solway Frith to the neighbour- hood of Chester, and from that neighbourhood to Anglesey, as I have had many opportunities of ascertaining. The upper clay is likewise the same from the banks of the Eden, near Carlisle, to Crewe in Cheshire, and from Crewe to Anglesey. The greyish- faced fractures are not always present ; but they frequently recur. At the Dalton brick-pits, Barrow-in-Eurness, and at Crewe, they impart a general bluish-grey colour to the clay ; and all the way between these two places they may often be seen. They likewise frequently recur in Shropshire and Denbighshire. They resemble the ash-coloured partings of the Hessle clay described by Mr. Searles Y. Wood, jun. As regards vertical range, the lower clay nowhere, so far as I have seen, maintains its low-level or shelly character at a greater height than from 100 to 150 feet, as Mr. De Bance long ago pointed out ; but it is certainly continuous with a deposit of mixed loam, clay, and gravel which runs up the hill-sides to a great altitude. I have never seen typical upper clay at greater heights than from 400 to 600 feet ; and it generally loses its shelly character before it reaches these heights. Mr. S. Y. Wood, jun., has not found the Hessle clay (with which he is disposed to correlate the north- west-of- England upper clay) at a greater height than about 300 feet in Yorkshire, while in South Lincolnshire it does not reach higher than about 50 feet above the sea-level. The upper clay of the north-west does not penetrate into the valleys of the Lake-district, or into the valleys of Wales, with the exception of the Yale of Clwyd and the valley traversed by the Mold-and-Denbigh railway, in which it does not rise to the level of the water-parting. On the east slopes of Halkin Mountain it thins out upwards at about 400 feet, and east of Glossop at about 600 feet.