and Macclesfield have a difference of elevation of more than 1000 feet, or nearly 200 fathoms, we find the same assemblage of shells of Mollusca.
On the slopes of the knolls, and in the hollows between them, the surface of the Middle-drift sand is found to have been extensively denuded, or eroded : whether this was caused by the action of currents between the banks, or whether, between the Middle - drift and Upper Boulder- clay periods the surface of the former was heaved up into land and subjected to subaerial disintegration followed by subsequent subsidence and the deposition of the latter, is at present doubtful ; but the phenomena of erosion are observable over a very large area, including the Manchester district described by Mr. Hull.
Upper Boulder-clay. — This deposit covers the whole country between Ormskirk and Wigan, Wigan and Preston, Preston and Lancaster, as well as the districts of Ulverstone and Blackpool, with a vast sheet of clay, in some instances reaching a thickness of more than 100 feet. In the Preston district, as described above, the various brooks have often cut through this deposit to a great depth, exposing the sands and gravels of the Middle Drift beneath. The hollows in the Middle Drift are always filled up with Upper Boulder - clay, but have been occasionally reexcavated by brooks acting along the lines of the old hollows, producing " drift- valleys" within drift- valleys, double valleys formed entirely in Glacial and Postglacial times. Thus we often find the base of the upper drift, in the brook-cliff, 50 or 60 feet below its base in the adjoining upland plain, less perhaps than 500 yards distant. At Leyland, the top of the Middle Drift descends more than 70 feet in less than half a mile.
The Upper Boulder-clay resembles, in the whole of Lancashire, from Ulverstone to Manchester, ' the Lower Boulder-clay of the southern part of the low country in its physical character, chemical composition, included erratic fragments, and the species of shells of mollusca found with it. Both clays (see Appendix) contain more Silurian erratics in the N.W., and more Carboniferous erratics in the S.E. of Lancashire ; both are of a dull Indian-red-coloured tint, caused by the presence of iron derived partly from the Triassic rocks and partly from the Haematitic deposits of Furness ; in which district the Upper Boulder-clay has a deep, almost lurid, colour. The colour of the Boulder-clay, as has been observed by Mr. Hull*, is perfectly irrespective of the rocks upon which it may lie, being nearly the same when it occurs on the Silurian, Carboniferous, Permian, or Trias.
Here and there, in both the Boulder-clays, faint indications of stratification maybe occasionally observed; but the pebbles and boulders are imbedded pell-mell and at all angles in the mass. Large boulders are comparatively rare in both clays, and are generally composed of granite or porphyry, and, more rarely, of lake-district Silurian -grits. Shap-granite pebbles occur in both clays, as far south as Liverpool :
- Geology of the country around Oldham (Mem. Geol. Survey), p. 48.
