Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/234

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Mr. Webster on the Strata lying over the Chalk.

In the Paris basin the marine marls are covered by a very extensive and thick bed of sand, entirely without animal exuviæ, which has nearly filled up all the cavities which still existed on the surface of the basin, covering the partial formations of marine gypsums, the lower freshwater marls, together with all the other strata, and reducing the surface nearly to a level.

This sand frequently contains beds of sandstone of the same nature with itself, which frequently form the flanks of the hills in the neighbourhood of Paris; and great blocks of these have frequently rolled down into the valleys, the sand having been carried away; such are the sandstones in the forest of Fontainebleau, Palaiseau, &c. The sand of this stratum is often an extremely pure white quartz, and is much used in the arts; but sometimes it is coloured by oxides of iron, or impregnated by carbonat of lime. It forms all the soil of Beauce. The sandstone is very hard, pure, and homogeneous.

This bed is usually covered only by the burr stones without shells, or by the upper freshwater formation. Over it however there is in some parts a sandstone containing marine shells, agreeing nearly with those of the middle beds of the calcaire grossier, or rather with those already observed in this upper marine formation; and in other places a calcareous stratum with shells.

The London clay is in many places covered by an extensive bed of sand, usually called the Bagshot sand. It extends over Bagshot, Frimby, and Purbright heaths, in the county of Surrey, and that on Hampstead heath belongs also to it.

At Purbright, and many parts of the surrounding country, loose blocks of a stone is found similar to what has been called the grey weathers. This stone, composed of siliceous particles cemented together without any intervening substance, may be considered as