according to its true characters, connects itself in many points with the general causes before explained, and it remains only to ascertain its peculiar features. I have already stated, why, in the revolutions that happened to the ancient sea, islands came to be formed it was because fresh quantities of the liquid having penetrated through the fractures that were made in the crust, and its surface thus sinking at the exterior, those portions of its bottom, which had not sunk so low as the rest, were then left dry. Now it was upon some of these islands that the peat was formed; in the same manner as it is formed on numerous islands of the North Sea. I have likewise shown, from very striking phenomena, that the supports on which those portions of the crust, which, in the catastrophes that happened to the bottom of the ancient sea, rested, were themselves exposed to subsidence, when, in the course of a period more or less long, the action of the liquid on the interior pulvicules extended beneath them. Then the portions, which had been thus supported for some time, themselves sank down, and if they happened again to pass under the surface of the liquid, there they received a fresh accumulation of strata. Here then, I observe, is a general cause, which, in its different modifications, is impressed in a thousand several ways on the surface of our continents, in such sort that the phenomenon of coals is only a branch of them, modified by the peat. When the islands, of which I have been speaking, came to sink, the peat was covered with other strata, owing to the precipitations that continued to take place in the liquid. When afterwards a new quantity of the liquid was absorbed in any part, and

its surface sank sufficiently for those islands again to appear, fresh peat was formed on them, which again sank beneath the liquid, by a new depression of their base. Lastly, in some of the subsequent great revolutions at the bottom of the sea, these masses, composed of vegetable and mineral strata, underwent a very great catastrophe; they broke and sank down afresh, and the state to which these strata were then reduced, differs nothing from what we observe in our mountains, except that they belong to a lower stage of ruins, by the very repetition of their subsidences; this is likewise the case in regard to all our plains, in which the same disorder is observable, whenever there is sufficient inducement to be at the expense of making deep excavations in them.

OF THE STRATA OF CHALK.

26. The production of chalk is an operation which I also place in this same period, but in coming to it immediately after having treated of the formation of coal, I do not pretend to follow the order of time. This order, indeed, is marked with precision in each place, as much by the superposition of strata of different species, as by the traces of successive accidents the same successions are also repeated in a number of places, but they are not general, and we find in the intervals other kinds of successions, also more or less frequently repeated, as well as numerous monuments of more particular effects. It is only, therefore, by persevering in our observations, that we shall be enabled to unravel this chaos; in respect,

however, to the general course of the operations, no doubt can arise.

27. Certain naturalists, who from want of sufficient knowledge of mineral substances, or from inattention to those which have preceded the existence of marine animals-who even have never thought of enquiring whence those animals could have derived the substance of their shells and other habitations, have imagined that all fossil calcareous strata have been produced from their spoils; and they consider chalk as a first state through which these substances have passed before they became lime-stone; but the following are direct proofs of the contrary. Owing to the catastrophes which the entire mass of our strata has formerly so often undergone, we observe in many places the superposition of chalk upon the lime-stone, in large sections of the two kinds of strata, forming abrupt faces in some hills or cliffs on the sea-shore; and here we find that the first stratum of chalk, namely, that which rests on the lime-stone in contact with it, is likewise similar to all those of the same mass, so that the two classes of strata are completely distinct from one another. How then is the supposed effect of time here perceptible? Each of these classes of strata contain marine bodies, but they differ considerably from each other; for, to confine myself to one instance only, in the lime-stone, we find quantities of the cornua ammonis, of which we have no traces in the incumbent chalk, which serves directly to prove, that the liquid of the sea had undergone some essential change at the time when these last strata were formed, as it affected, not only the precipitations, but the sea animals. Lastly, one of the

characters of the chalk is, that it contains a great abundance of flints, some of them forming, as it were, a pavement between some of those strata, others disseminated through their mass, and likewise forming some veins. These hard bodies would again recur in the lime-stone below, if it were only more ancient chalk; but we do not find them'.

1 At the time when I wrote this first demonstration of the error of those who have imagined that chalk was a first state of calcareous stone, which differs only by a greater hardness produced by time, although I had observed, in various countries, and with great attention, calcareous strata, and their extraneous bodies, I had no where seen any that contained cornua ammonis, or that were without flints; and it was those chalks which I had seen resting on strata of calcareous stone, containing no such flints, and where cornua ammonis were abundant; a circumstance which sufficed to show that calcareous stone and chalk were very distinct precipitations in the ancient sea. But I have here a new fact to adduce, which leads me to make a general remark: viz. that when in certain spots we find abrupt changes of certain strata to other strata of different species, we may likewise elsewhere meet with gradual transitions in the same beds; and this is in particular the case in respect of calcareous stone and chalk. Of this, I am about to bring an instance, which will confirm what I have said in the text, of the influence exercised over organized beings, living at the bottom of the ancient sea, by the changes of precipitations which took place in its liquid; and at the same time there will hence result a new degree of probability in favour of the opinion given in the next section, on the origin of silex.

One of my nephews, another of whose observations is mentioned in that section, having passed some time in the neighbourhood of East-Bourne, on the coast of Sussex, availed himself of the opportunity thus offered for studying with care the strata of which the cliffs of that coast are composed. From Beachy-Head to EastBourne, the coast runs from S. W. to N.E.; its cliffs, as is always the case, are a section towards the sea of the hills of the country; and along one of those cliffs, the greatest elevation of which, at Beachy-Head, is about five hundred feet, on following the beach,

28. I have stated, in the eighteenth and twentyfourth of my Letters in the "Journal de Physique," my reasons for believing that the flints are local trans

the sections of different species of strata are found, which all rise towards the N.E. in an angle of from ten to fifteen degrees, and terminate upwards; for their irregular sections form the upper part of the cliff, as well as of the hills to which it belongs, which hills themselves terminate abruptly towards East-Bourne. That peculiar situation of the strata allows of their being observed separately without quitting the beach, where their abrupt section terminates like a wall, against which the waves, during the high tides, have formed a strand. Thus then, following the foot of the cliff, all its strata are successively seen throughout a considerable extent, by reason of their slight inclination, from those which were formed the first, and which terminate towards its extremity on the side next East-Bourne, to the last, which constitute the greatest elevation at Beachy-Head.

The former of those strata, which are truncated to the N.E. towards the part where the cliff ceases, are of calcareous stone, without flint; and contain, among other shells, many cornua ammonis. Upon this first kind of strata, there rests, with the same inclination, another very distinct species, which, like all the others, constitute successively from N.E. to S.W. for a certain space the foot of the cliff. Here we already find chalk, but it is grey, and its strata have different degrees of hardness; there is as yet no flint; and with regard to the shells, which are there of different species, the softest strata, those which lie nearest to the calcareous stone, contain like it cornua ammonis; but they are no longer met with in the following strata, which are harder, and where echinites begin to appear, which are not found in the calcareous stone. My nephew estimated that this mass of chalk might be a hundred feet in thickness. On these strata there rests a distinct mass of about two hundred feet in thickness, consisting of strata of whitish chalk, which in their turn, form for a considerable space the foot of the cliff, where they all appear in succession, in consequence of their inclination: there is as yet no vestige whatever of flint, and the cornua ammonis disappear in them to such a degree, that, with the greatest attention, my nephew discovered only one,