The bottom of the shaft was 30 feet under the bed of the river Thames. The unfortunate failure of the project, from the river bursting in before they had completed a drift, is well known.
At Reading there are several pits dug for the purpose of procuring brick clay, some of which reach to the chalk; an account of the strata at this place is given us by Dr. Brewer in the Phil. Trans. 1800.
Immediately incumbent on the chalk is a stratum 2 feet thick of green sand containing numerous oyster shells. Many of these are entire, having both their valves united; but the animal matter being entirely gone, and the shell not having undergone the process of petrifaction, they are white and extremely brittle, and separate into laminæ. Fishes teeth are also found with them. Over this is a bed 8 feet thick of a bluish rough clay, then fuller's earth 21 feet, and fine white sand 7 feet. Next is a stiff red clay, probably the plastic clay, of which tiles are made. This is much thicker than any of the other beds; and over is the alluvial soil. These strata are known to extend for several miles with little variation.
At Brentford, on the borders of the Thames, about 6 miles from London, many important discoveries have been made in the grounds of Mr. Trimmer; an account of which has lately been read before the Royal Society. Here the chalk lies at a great depth, as they have dug 200 feet through the blue clay without coming to it. Immediately upon this clay are a few feet of sand and gravel with water; over that from 1 to 9 feet of calcareous loam; then sandy gravel 7 feet, and lastly calcareous loam or common brick earth 9 feet. In the blue clay were found the usual fossils of this stratum, being entirely marine. The three beds just over it contained a vast collection of the bones of elephants, both African and Indian, of the hippopotamus, the horns and jaws of oxen, the horns of deer, and both land and freshwater shells.