CHALK FORMATION ROUND PARIS.

313

The figure on the margin represents the successive strata of the Paris basin, numbered from the surface downwards ; or in the inverse order of the preceding table.

1. Upper freshwater marls and millstones. 2. Marine marls. 3, 4. Second freshwater formation; and gypsum with bones. 5. First marine formation after the chalk. 6. Lowest or oldest freshwater

formation. 7. Chalk with flints.

1. Chalk. The regular beds of black flints which exist in such abundance in the chalk, show that it has been the result of a tranquil deposit under the

ocean.

The chalk masses of the environs of Paris are apparently intersected by nearly vertical fissures, sometimes very narrow, although they extend to very great distances; at others nearly two feet wide. The sides of these rents seem as if embossed, but the convexities and concavities do not regularly correspond with each other. The faces of the fissures look like surfaces which have been worn down, and polished by the waters. In the parts where the rents are so narrow as to allow the two walls to come nearly in contact, there are round vertical holes, irregularly placed, opening into the upper and wider portion of the cleft. Besides being embossed, the walls are pitted, as if exposed to a pelting rain. The flints which project in relief on

the sides of the clefts, have their upper faces covered with crystals of carbonate or sulphate of lime, but their under surfaces are bare.

These effects cannot be ascribed to the action of existing waters, because; 1. The masses of chalk in which they occur, are much above the highest level of the waters of the Seine, and all its affluent streams; 2. The superior strata, and the adjoining hills have too little mass, or elevation to supply currents of water capable of producing these effects; Lastly, water and its springs are so rare in the chalk, that the quarriers at Meudon have been obliged to dig a deep well, in which the water stands 70 feet below the lower level of the quarries. The only metal found in the chalk formation of Paris, is iron in the state of a sulphuret, or globular pyrites, either disseminated, or incrusting the organic debris.

The animal exuviæ give geognostic characters to the chalk, which are clear, essential, and decided. They are very unequally distributed through its mass. Not only are they almost all different from those which occur in the other formations, and particularly in the more recent ones; but they also present notable differences in species and even genera, according as they belong to the upper or lower portions of the chalk. This important consideration leads to the recognition of three members in the chalk formation, distinct in the central portion, but on their confines passing insensibly into each other. These three members which thus differ not only in geognostic position, but also in mineralogical character, are

COMPOSITION OF THE GREEN SAND.

315

1. The white chalk or middle portion; 2. The chalk called tufau in Touraine, the upper part of which is generally grayish and sandy, containing usually hornstones instead of flints; 3. The chloritic chalk, called by Cuvier chalky glauconie (green sand of the English). It is grayish, friable, and all over besprinkled with green particles which closely resemble chlorite. The greenish or reddish nodules from an analysis by M. Berthier seem to consist chiefly of phosphate of lime and iron.

Analysis of the grains of the green sand from Havre, by M. Berthier.

Analysis of the greenish nodules (glauconie crayeuse).

The white chalk alone is found in the vicinity of Paris. This circumstance limits considerably the number of organised exuviæ exhibited there, for it is in the tufau and chloritous chalk that the fossil shells are most abundant. See Plate V.

None of the chalk shells are found in the coarse limestone (calcaire grossier) above it. These two formations are, therefore, perfectly distinct, nor is there any trace of an insensible transition between them, either in the Paris district or probably elsewhere. On the other hand, no such decided dif

ference is observable between the chalk and its subjacent compact limestone. The chalk of other countries, includes species of shells not hitherto observed at Paris, but which are found in the oolitic limestones beneath it. These facts prove that the chalk is not, as many have supposed, of modern origin. We shall find that it has been succeeded by four or five very distinct formations, which indicate a long space of time, and great revolutions, between the period of its deposition, and of those strata from which the surface of our continent has derived the figure which it actually possesses. With the exception of the trochus, there has not been found in the Paris chalk any univalve shell with simple and regular spires. Thus there is no cerite, no fusus, &c. This fact is the more remarkable, as we find a few yards above the chalk, in strata equally calcareous, shells in great abundance of a different structure.

The chalk forms the bottom of the basin or gulf, within which are deposited the several formations of the Paris district. Ere this antique chalk floor was covered by these mineral strata, its surface must have exhibited hollows and prominences in the form of valleys, hills, and terraces. These inequalities are still indicated by the islets and promontories of chalk which rise up through the new formations in certain points. Hence the excavations made in these upper beds reach the chalk at very variable depths. Nor have the inequalities relation with those of the actual surface of the

any

land.

FIRST FRESHWATER FORMATION.

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2. First freshwater formation, plastic clay, lignite, sandstone.-Almost the whole surface of the chalk mass is covered with a bed of clay, possessing very remarkable general characters, amid specific differences at different points. This clay called plastic, because it readily takes and keeps the forms impressed on it, is unctuous, tenacious, and generally composed of about 30 alumina, 60 silica, and 10 water in the hundred parts. It is rarely impregnated with either lime or iron, and makes no effervescence with acids. It is of a fine white aspect at Moret; gray at Montereau; yellow at Abondant; pure slate-gray, or slate-gray mixed with red to the south of Paris from Gentilly to Meudon.

This bed varies in thickness from 50 feet in some places, down to a seam of a few inches in others. Two beds of clay are often met with`; an upper one, which the workmen call fausses glaises (false potter's-earth) is sandy, blackish, and encloses occasionally organic remains; and an under one separated from the former by a bed of sand. The lower stratum which forms the proper plastic clay, is destitute of organic debris; but the upper one is sometimes very rich in fossils, which give it a peculiar character. To this bed and consequently to the plastic clay formation, of which it constitutes a member, belong the sands, and lignites, yellow amber, and numerous fossil shells, some obviously marine, and others natives of fresh

water.

The lignite or fossil bituminous wood (braunkohle of the German mineralogists), sometimes