Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/290



An appearance somewhat analogous is noted by M. M. Cuvier and Brongniart (Essai sur la Geog. Min. des Environs de Paris, p. 17,) in a coarse variety of the French plastic clay which immediately covers the chalk at Meudon; where a breccia composed of fragments of chalk imbedded in a kind of argillaceous paste has filled the fissures and irregularities which existed on the surface of the subjacent chalk before the deposition of the plastic clay.

The same thing may be seen on a small scale in the chalk pit at Woolwich, where there are fissures extending some feet downwards into the body of the chalk, varying in breadth from an inch to more than a foot, and sometimes spreading laterally so as to form considerable cavities, which together with the fissures are filled with sand that has been introduced from the incumbent stratum.

At Reading the chalk is quarried below the green sand containing oysters (No. 2) to the depth of about 25 feet, when the workings are stopped by water at a point nearly on a line with the level of the river Kennet, below which there can be no discharge of water from the chalk, through the medium of the neighbouring springs. In this thickness of 25 feet of chalk, there is but one regular and continuous course of flints, and in this they are disposed in tabular masses, for the most part of about two inches in thickness. (This bed is but a few feet above the water). In the chalk that lies above this siliceous stratum, the flints are disposed irregularly with their usual characters and eccentric forms, derived, in many instances, from the organic remains which they envelope. They are collected for the use of the porcelain manufactories. The chalk itself is extracted largely from under the sands and clays, by means of shafts and levels, to be burnt into lime. There are no septaria or concretions in any of the strata above the chalk, nor the smallest traces of animal or vegetable remains, excepting in the