648 PROCEEDINGS OF THE GEOLOGICAL SocieTY. [June 22, 
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 exten- 
sively 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 be- 
tween 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 thick- 
ness of more than 100 feet. In the Preston district, as described 
above, the various brooks haye 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 reexcayated 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 
mollusea found with it. Both clays (see Appendix) contain more 
Silurian erratics in the N.W., and more Carboniferous erratics in 
the 8.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 Heematitic 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 le, being 
nearly the same when it occurs on the Silurian, Carboniferous, 
Permian, or Trias. 
Here and there, in both the Boulder-celays, faint indications of stra- 
tification may be 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. 
