Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/896

 south-east. These gravels are well seen in the Blackpool section, in the Ribble Cliffs at Red Scar and Balderstone, and in pits near Leyland and Chorley. Shells more or less perfect are invariably found associated with these gravels ; and in those of Leyland I obtained some bones of an herbivorous mammal, which, however, were too fragmentary for Mr. Boyd Dawkins, F.R.S., who kindly examined them, to identify the species.

The pebbles in those gravels are all rounded, and are chiefly from the Lake-district; at Leyland and Chorley, carboniferous rocks and blocks of coal form about 12 per cent. of the larger blocks. With few exceptions, these pebbles are never scratched ; and both the character of the beds, and the species of Mollusca found in them, testify to the great amelioration of the climate which ensued during the Middle-drift period ; this, however, from Mr. Croll's calculations, is only what might be expected.

In the instances where scratched pebbles occur, especially at Samelsbury, the sand is not the clean fine sand usually found in the Middle Drift, but has a loamy, or even clayey, character. This seems to point to special physical conditions ; and it appears not improbable that daring this warm episode in the Glacial epoch stray icebergs from regions still farther north, or from hills cold from their elevation, floated occasionally from the north-west to the south-east in the Middle-drift sea.

Beds of loam varying in thickness from 1 inch to 3 feet occur in the Middle Drift ; and some of them are much used as sand for brass-casting. They generally have a slight dip to the E.S.E. These, from having a certain amount of clay in their composition, support small sheets of water, which, becoming charged with carbonate of lime, often consolidate the surface of the loams into a substance as hard as rock. This also takes place under exceptional circumstances in the gravel-beds, producing masses of conglomerate resembling that portion of the Millstone Grit known as the Kinderscont Grit. These are seen in various places in the banks of the Ribble and in the Blackpool cliffs, where they are used to form rockwork, which has already been described by Mr. Mackintosh in his paper before referred to. In many of the deep-brook valleys in the Preston district the Middle Drift occupies the sides, and the Upper Boulder-clay the top, being separated from one another by a seam of this consolidated loam. These seams often run for miles without a break, though not more than three inches in thickness.

The level of the top of the Middle Drift is an exceedingly variable quantity, varying 60 and 70 feet in half a mile, but it invariably dips from the hills of the Penine Chain towards the sea and the low country. At Blackpool it is 80 feet above the level of the sea, at Chorley 275, at Leyland 150 ; and Mr. Hull gives the elevation of the base of the Upper Boulder-clay in " the valley which runs up from Manchester by Bolton to beyond Sharples," as 275 feet above the sea at Pendlebury, at Clifton 285, at Kearsley 300, at Halshaw Moor 285, opposite Burden Bridge 300, at Bolton 300, and