Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/192

 That this 290 feet is composed of a threefold series of strata, the lowest of which reposes on the Plastic Clay.

That each division of the series commences with a zone of pebbles, and passes upwards from stiff clays to sandy clays and sands *.

Of the lowest series nothing is known with certainty, except its thickness (100 feet) and the mineral composition of its strata, as shown in the Deep-Well boring.

Of the middle series, which includes a thickness of 134 feet, a large portion has been seen in open cuttings on the Works. It includes in its upper part the "Argillaceous sands with Dentalium" and the " Sands with Lingula," described above. The shell-rock in the " Sands with Lingula " at the top of this series probably represents the Bognor Rock of Sussex ; but of this I cannot speak with certainty, as I have no evidence of the height of the Bognor Bock itself above the Plastic Clay.

The upper series, having a thickness of from 50 to 60 feet of clays and sandy clays, including the " Clays with Cyprina" has been sufficiently described above. There can be no doubt whatever that these last beds represent the upper beds of the so-called London Clay of the Alum-Bay and Whitecliff sections. It is evident that the whole of the strata above described are included in group 3 and part of group 4 of Mr. Prestwich's Section of the Whitecliff strata.

The accompanying Sections (fig. 3), which are drawn strictly to measure, exhibit a comparison of the Lower Eocene strata of Portsmouth, with the already well-known Section at Whitecliff Bay, and may, I hope, be ultimately useful for comparison with a much- to-be-desired well-section at Bognor.

I have not thought it necessary to compare the Portsmouth Lower- Eocene strata with those of the London basin, such comparison in effect having been already made by Mr. Prestwich so long since as 1847.

Gravel- and Mud-Deposits resting on the Lower Eocene Strata.

The superficial deposits seen in the excavations at Portsmouth deserve a short description, if only on account of the fine sections exposed.

The first or oldest of these deposits is the gravel-bed shown in the Section (fig. 1) as capping the higher portion of the ground. The base of this gravel-bed stands at from 15 to 20 feet above low-water level, and rests on the unequally eroded surface of the clay. It is a light-coloured subangular flint-gravel, and probably represents the " white gravel " lately described by Mr. Codrington†. The ground on which it rests forms a low escarpment facing to the N. and N.N.E., the direction of the escarpment coinciding with the outcrop and line of strike of the upper beds of the " Clay with Cyprina."

A few patches of contorted gravel containing large, partly rounded flints, have been exposed at a slightly lower level near the edge of

deposits in the Isle of Wight has been noted and commented on by Mr. Fisher (vide Quart. Journ. Geol. Soc. vol. xviii. p. 65).
 * A like sequence in the mineral character of the Lower and Middle Eocene

† Quart. Journ. Geol. Soc. vol. xxvi. p. 535.