are not found in the same state as those of a more recent period-the elephants' tusks and bones of the diluvial gravel, and the osseous caves, which we have hitherto examined; but in all cases, the substances, whether animal or vegetable, have undergone petrifaction-the most delicate plant, the microscopic shell, and the hardest bone, being alike converted into the material of the rock in which they are preserved. How this metamorphosis was produced, by what process of transmutation the original elements were replaced, atom for atom, by matter in some cases so dissimilar-as for instance, vegetable matter converted into silex, or animal substance into sulphuret of iron-without destroying or in the least defacing their external configuration, is an interesting inquiry for the chemist, but it appears to be one of those secret operations conducted in Nature's laboratory, which we shall never perfectly understand. It is from these records, engraven upon stone, as it were by the finger of the Deity, that geologists have been enabled to frame a chronology of the earth, to assign certain periods to certain events, to trace the history of organized beings through a countless succession of ages, up to the period when the Omniscient fiat first said "Let there be life upon the earth," to ascend, in fact, the stream of time almost to its source. But in an inquiry so vast, involving a succession of events so complicated and remote, united by insensible gradations, where shall we commence and how measure our progress?

There being throughout the mineral masses a regular order of superposition, and an apparent gradation in the fossil organic remains which they contain, indicative of their respective ages, we ought in the

highest of the series to find the nearest approach in their organic contents, to the existing animals and vegetables of the countries in which they are situated. Now, were we to examine the bed of the ocean, we should find it filled with shells and other organic remains of animals, corresponding precisely with those which now inhabit it; but extending our observations to the elevated coast which skirts it, our eastern coast for instance, we discover a similar accumulation of sedimentary matter, (the English crag,) containing a large proportion of fossilized shells of the same species as those now existing in the sea below, but including also others, of which there is no existing types; and looking further inland, we see rising from beneath the crag another more extensive deposit, formed at an earlier period beneath the ocean, in which the number of shells corresponding with living species is considerably fewer, and as our observations are continued, the analogy ceases altogether, and the existing species entirely disappear. With the most recent of these formations, therefore, we commence our inquiry, and go back through these various gradations to the oldest and lowest in the series of fossiliferous rocks.

Until a recent period, the newest of the rock formations was supposed to be the chalk, but a most interesting series of beds, which exhibit, in a remarkable manner, the retrogression in their organic contents to which we have just adverted, is known to geologists as the supracretaceous group, a term recently adopted, which expresses their position-supra upon, creta the chalk-they being invariably found resting upon that remarkable rock, which, previous to the deposition of

these superior beds, appears to have undergone abrasion, and to have been hollowed out into deep valleys or depressions in the surface, which these formations now occupy.

The supracretaceous beds constitute a large portion of the surface of Europe, and they have been traced with great diligence from the newest to the oldest, from the Subapennine beds on the shores of the Adriatic, where the line of demarcation between the fossil remains and existing species is scarcely visible, to the chalk with its extinct species; and a sub-classification founded upon their organic contents has been adopted, which is supposed to express their relative ages. To go into a detailed account of this interesting group of stratified masses, even to attempt a bare enumeration of all the localities and remarkable circumstances attending them, would be inconsistent with the design of this "Sketch," in which it is proposed to trace merely an outline of the leading features of geological discovery.

*

* M. Deshayes and Mr. Lyell both suggested, about the same time, this subdivision of the supracretaceous group of rocks: to the former, however, belongs the merit of having most laboriously examined the multitude of shells belonging to these formations, and of having first arranged them in a chronological series. The division, however, adopted, is merely one of convenience, and Mr. Lyell's terms, Eocene, Miocene, and Pliocene are entirely arbitrary, and can never be generally applicable. The circumstances under which distant deposits were effected, must have influenced so considerably the character of the animals and plants entombed in them, that any terms expressive of their relative ages founded entirely upon these, are calculated only to deceive, and to defeat rather than to promote the object of geological research.

By a remarkable coincidence, the two great capitals of Europe, London and Paris, stand in the centre of two of these supracretaceous formations, and it is to the fortunate circumstance of Paris being so situate, that geology owes its present advanced state. The extraordinary discoveries made in the environs of that city by the celebrated Cuvier, and his associate Brongniart, which first exhibited to the astonished world the forms of animals hitherto unknown to the naturalist, and proved that so remarkable a site, had at former periods been the theatre of successive revolutions, both in the animal and mineral kingdoms, formed an era in geology, and gave a new impulse, not only to this, but to the other natural sciences, everywhere an intense desire being excited to become acquainted with the phenomena and causes of these strange mutations. As the formations in the Paris basin as it is termed, have been, from this circumstance, more accurately examined than in any other locality of similar extent, we will take them as a model of the strata under consideration; but it must not be from hence inferred, that the supracretaceous rocks, which form, perhaps, three-fourths of the surface of Europe, are in every instance piled upon each other, in conformity with this model of a district containing only a few hundreds of square miles. There is, however, a remarkable coincidence everywhere observed in the leading features-the alternations of marine and freshwater productions-which characterize these formations, however distant the locality, or dissimilar the circumstances under which they have been produced. *

*It appears from recent descriptions of the great mountain chain of Caucasus, by Klaproth and Kupfer, that supracretaceous

The minute and accurate examination which the country surrounding Paris has undergone, has led to the discovery that an area of ninety miles in diameter, of which that city is the centre, has been at different periods occupied alternately by the sea, and fresh-water lakes or large rivers, whose successive deposits have formed an accumulation of distinct beds of earthy matter several hundred feet in thickness, in each of which indelible traces of the causes by which they were produced may be observed--on the one hand, the sea entombed in immense profusion the remains of the shelly inhabitants with which it was peopled; on the other, the fresh-water enclosed in its sedimentary beds not only its peculiar shells and aquatic tribes, but relics of the strange quadrupeds which frequented its margin, and were accidentally drowned or overwhelmed by inundations, together with specimens of the vegetation of the periodthe trees and plants which grew upon the dry land in its vicinity. Thrice was the sea quietly but effectually displaced by fresh water, during the long interval in which these deposits were accumulating, and at each successive period a decided change in the inhabitants of both sea and land is discernible, indicative, in the main, of that approach to existing species to which we have

rocks (of which the youngest contain shells of species actually existing in Lake Aral and the Caspian sea,) approach to the foot of that lofty ridge of mountains, and being elevated by it, attain a height of two thousand five hundred feet! They alternate with fluviatile beds (deposits from rivers,) even at that elevation, and repose as elsewhere upon a chalk formation, which in its turn covers the oolitic series skirting the older rocks, precisely as we might have inferred from the analogy of the English strata.