The point of chief importance in this boring is the occurrence, at 93 feet below the lowest beds exposed in the open cutting, of a band of rounded flint pebbles, similar to that at the base of the " Clays with Cyprina planata." The discovery of this second band of pebbles has given a clue to the position, in relation to the underlying Plastic Clay and Chalk, of all the strata seen on the works, as I shall show by comparison with the records of two previous borings in the Dockyard.
For permission to avail myself of the records of these two borings, which probably have not been hitherto published, I am indebted to the kindness of Mr. Wood, the Superintending Civil Engineer at H. M. Dockyard at Portsmouth.
The first of these, known as the Deep-Well boring (fig. 3, c), is situated at the distance of 3000 feet from the boring on the Extension Works, or only about 170 feet S.S.W. of the line of strike of the highest strata above described. It was carried down to the depth of 1037 feet. In this boring the chalk is reached at the depth of 408 feet. The thickness of the Red or Plastic Clay amounts to 118 feet. A layer of rounded flint-pebbles is shown at the height of 100 feet above the Plastic Clay, or about 190 feet from the surface. There can, I think, be no doubt whatever that this pebble-bed represents the pebble-bed cut through in boring on the Extension Works. It is curious, however, that the upper pebble-bed is not shown in the Deep-Well section.
The second boring (fig. 3, b) in the Dockyard (the first, perhaps, in point of date, as it was made in 1825) is at the distance of nearly 1000 feet S.S.W. of the line of strike of the highest beds on the Extension Works. It has been carried down to a depth of 290 feet. In this boring both the pebble-beds are shown ; and the interval between them corresponds exactly to that between the two pebble- beds on the Extension Works. The same beds have been also shown to occur, in the same relative position, in other borings at no great distance from the above, as well as in well-borings through corresponding strata at Southampton*. Two pebble-beds are shown by Mr. Prestwich as occurring in about the same position at Alum Bay† .
It is shown, then, by the evidence of these borings as explained by the open cuttings on the Extension Works, that the strata there exposed range from the height of 163 feet to that of 290 feet above the Red Clay ; and by the same evidence the position of the remarkable shell-beds underlying the zone of pebbles and " Clay with Cyprina " may be placed at the height of 233 feet above the Red Clay, or within from 60 to 70 feet of the base of the Lower Bagshot.
[Note. — The term Red Clay here made use of answers to the Plastic Clay of most authors.]
Taking now a more general view of the London and Bognor strata of the Portsmouth district, we find : —
That their thickness certainly exceeds 290 feet.
- For details of the Southampton well-borings I am indebted to Mr. Bristow,
of the Geological Survey.
† Quart. Journ. Geol. Soc. vol. ii. pl. ix.
VOL. XXVII. PART I. G
