genera of testaceous animals, that have left their remains in chalk and the lower strata, appear to have been extinct before the deposition of the London clay. Nautilites are, however, found in it, similar to the species inhabiting the Indian Ocean, and bivalve and univalve shells are so numerous, that it would be difficult to select any particular species, as peculiarly characteristic of this formation. The shells mostly belong to genera inhabiting our present seas; yet slight variations of form may be perceived, which have induced naturalists to regard them as distinct from living species.

The springs that rise in the London clay are generally impregnated with sulphate of iron and sulphate of lime, and some of the springs contain sulphate of magnesia; the quality of the water, however, varies much in different situations, and at different depths. To obtain soft water, it is necessary to bore or sink through the London clay to the sand above the chalk, and sometimes into the chalk itself. * The London clay and the under beds have been perforated to the depth of three or four hundred feet in some situations, before good water could be obtained; when the stratum is pierced which holds the best water, it rises almost immediately, and sometimes overflows the surface. This admits of an easy explanation, by referring to the section of the Vale of Thames. (Plate IV.) The

* At the village of Wilsden, three miles north-west of London, the boring for water was made two hundred and eighty feet into the clay, and seventy-five feet below it into the chalk, when the water immediately rose to within thirty-five feet of the surface. Chalk rocks, and other calcareous rocks in which the strata are divided by fissures that are not filled with clay, always contain water in the fissures when the strata dip under the surface of the ground, or when they are covered by argillaceous beds. This is also the case with coal strata; and the presence of water is necessary to keep the coal in good condition. If the water be entirely drained from a bed of coal a considerable time before it is worked, the quality of the coal is much deteriorated. This may be occasioned by air penetrating the fissures, and promoting the decomposition of pyrites in the coal.

water which enters the edges of the porous strata, say at xx, descends to the lowest part of the trough or basin, and when perforated would rise to near the level of xx, were the strata deposited in a circular basin, the edges of which rose on each side from the bottom of the Vale of Thames; but the strata are deposited in a longitudinal basin or trough between the chalk hills of Hertfordshire and Surrey, and the river Thames cuts through the porous edges of the strata below Greenwich; so that the water being there let out, can seldom rise in wells much above the highwater mark. Were it not for this, we might have natural jets d'eau of considerable height and magnitude in all the squares of London, to cool and refresh the air during the summer months, and supply the inhabitants in the vicinity with salubrious water. In order to preserve the water pure, that is obtained from chalk or the sand over chalk, it is necessary to line the inside of wells, or to put down tubes, to prevent the water from the London clay, intermixing with the pure water from below.

As the plastic clay and London clay contain wood coal, or lignite, which is supposed to be characteristic of these beds, probably the strata with wood-coal at Alpnach (see Chap. VIII.), may be regarded as belonging to a similar epoch. Some French geologists would place these strata still higher in the tertiary series. The strata at Alpnach are peculiarly remarkable for containing the remains of the narrowtoothed mastodon, and of other mammalia, at the depth of nearly three hundred feet from the surface. The annexed cut is taken from a drawing of one of these teeth in the possession of the late Professor Meisner, of Berne, who also gave me specimens of the strata below which the tooth was found.

It is deserving notice, that teeth almost exactly similar were found on the volcano of Imbaburra in the Andes, which is ten thousand feet above the level of the sea. I have one tooth in my possession from thence, purchased at the sale of the late M. Fau

jas de St. Fond, of which the annexed cut may also serve as a correct representation.

The strata at Alpnach consist of the following beds,

in a descending series:

-7

小

1. Light grey sandstone

2. Light grey limestone like Jura limestone

-13. Different beds of Molasse or

6. Bituminous shale in layers

7. Stinkstone, a bituminous lime

227 0

60

1 6

7 0

stone with bones and river 1 to 2 feet.
shells, the roof of the coal

8. Coal

9. Bituminous schist

0 6

In.

06 to 8

The bituminous strata, and shaly limestone, possessed all the characters of beds in the regular coal formations in England: probably the fetid quality of the limestone No. 7, was derived from the abundance of animal matter which it might contain. No. 2. is subcrystalline, and bears a near resemblance to mountain limestone in its mineral characters.

Above the London clay there is no calcareous formation, except in the Isle of Wight, but in the Paris basin there are two; of which the lowest is called Calcaire grossier.

Le calcaire grossier, or coarse limestone of Paris, is deposited upon the plastic clay, as the latter is upon the subjacent chalk: between the plastic clay, however, and the calcaire grossier, there is a bed of sand; but geologists are not determined, to which of the two formations it belongs. The calcaire grossier differs in its quality in the different beds, but it may be described generally as a yellowish earthy limestone, which bears some resemblance to Portland stone in its fracture, texture, and colour; but it is not oolitic. The strata of limestone alternate with argillaceous marl and shale, and with calcareous marl.

The lowest bed of calcaire grossier is soft, and much intermixed with green particles and sand; it contains a great number of the fossils called nummulites, on account of their being flat and round, and resembling in shape a small coin. The shells in this bed are in high preservation. In the beds immediately above, called the middle beds, there are a prodigious number of marine shells, and also the stems and impressions of leaves of plants that are not marine. In the lowest and middle beds of the calcaire grossier, no less than six hundred different species of shells are found.

In the upper part of the calcaire grossier, the

strata are several feet thick, and yield a hard coarsegrained and durable limestone: it is from these strata that the best building-stone is procured. It is often nearly filled with shells of the genus cerithium, and has hence been sometimes called calcaire à cerites.

Between the strata of building-stone, there often occur thin strata of flint or chert; in some parts these siliceous strata enlarge into thick beds of chert (silex corné), or into beds of sandstone containing marine shells; in the beds of this sandstone, at Pierrelaie, freshwater shells have been discovered, mixed with numerous marine shells. The total thickness of the beds of calcaire grossier, near Paris, is about ninety feet.

No beds of limestone resembling the calcaire grossier of Paris, are found in the tertiary strata of England. The calcaire grossier in the departments of La Dordogne and La Gironde, and other parts of France, presents a considerable difference from that in the Paris basin. In Hungary, extensive strata of the calcaire grossier have been described by M. Beudant; they are in every respect analogous to the strata in the Paris basin, both in their mineral and zoological characters. The lower beds also are intermixed with shelly sand, and green particles, which bear a close resemblance to the shelly depositions in the plain of Lombardy. M. Humboldt thinks he discovered a formation similar to the calcaire grossier, in some parts of South America.

Calcaire siliceur is composed of limestone, sometimes grey and compact, and sometimes tender and white: it is penetrated by silex in every direction, and in all its parts. According to the early opinion of M. Brongniart, the calcaire siliceur occupies the place of the calcaire grossier where the latter is wanting; others regard it as an upper formation above the middle gypsum. Some of the beds of the calcaire siliceux furnish mill-stones, and contain river shells. In this bed, the siliciate of magnesia was discovered by