what has generally been given. He proposes to state his opinions on this subject in a subsequent work.

Dr. Buckland has pointed out many situations west of the London clay, where patches of the lower beds occur. These patches indicate that what is called the London Basin, and the basin of the Isle of Wight, were once continuous, and that their continuity was broken by the upheaving of the chalk, which, in several parts, had lifted the portions of tertiary strata that still remain.

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The formation called Norfolk Crag, remains to be noticed as the last of the English tertiary formations.

In the counties of Norfolk and Suffolk, there is a series of irregular beds of ferruginous sand and clay, mixed with marine shells, which has received the name of crag. The beds are much contorted and broken, and are intermixed with London clay and chalk, on which they rest; they are covered in many parts by diluvium. The crag is considered as the most recent of the tertiary beds in England; its true geological position, in relation to the tertiary strata on the Continent, is not precisely ascertained. According to an account of Mr. S. Woodward of Norwich, the crag is of limited extent in situ, in the county of Norfolk. If a line be drawn from Cromer, on the northern coast of Norfolk, to Wayburn, about six miles west, and from thence extending southerly towards Norwich (about eighteen miles,) it will comprise all the regular beds of crag. Mr. Woodward supposes that these beds were deposited in an estuary; eastward of this tract, ligneous and mammalian remains have been found in abundance, indicating that it was once dry land. (Mag. of Nat. Hist. Sept. 1832.) According to Mr. R. C. Taylor, the crag forms the base of the cliffs, from Cromer to Trimmingham. In a valuable paper on the Geology of East Norfolk, by the same gentleman, published in the Philosophical Magazine and Annals of Philosophy, April, 1827, and the following numbers, there is an interesting account of the geological_position of the crag near the coast, with explanatory sections. "The crag rests in part upon the London clay, and a laminated clay without fossils, and partly upon chalk, occupying the lowest sites; rarely rising to eighty feet above the present level of the sea, and in general not more than half that elevation. The average level of its base may be considered to be about that of the present ocean. In certain cases, where the chalk hills attain a higher level than the crag, that deposit could be expected only to envelope or surround their sides, and not to penetrate into the chalk: such eminences would then present the appearance of tongues or promontories of chalk, protruding into the crag; and this circumstance accounts for the occasionally apparent absence of that formation. But the crag has been subjected to abrasion by diluvial currents. Portions of its western edges have been swept away. Their fragments, mingled with those of chalk and preceding formations, piled in enormous heaps, form the cliffs of Cromer and Trimingham,

250 or 300 feet in thickness, upon the original crag, which rests in situ at their base."-No. 4. (new series) p. 228. The fossils in the crag are not mineralized; many of them appear to belong to species living in the present seas. The general characters of the crag are ably given by Mr. Taylor. "A district bordering a hundred miles upon our eastern coast, is occupied by an ancient marine deposit, continually changing its aspect, yet constant in its peculiar characters, and always to be understood by unerring data: now appearing as a ferruginous sandstone, then a compact clay, and again considerably indurated; sometimes blended in a mass of extinct zoophytes, sponges and alcyonites, forming a soft rock; oftener an irregularly accumulated mass of decomposed and broken littoral shells, loosely imbedded in sand like an ordinary sea-beach, yet accompanied with the remains of unknown animals ;-sometimes forming the substratum of a considerable area; or, overwhelmed beneath the debris of older strata, only detected at intervals ;-at one point exhibiting groups of shell fish allied to those of the neighbouring sea; and at another, composed of numerous genera, which are neither to be recognized living in any part of our globe, nor assimilating to the fossil shells of other formations."-Phil. Mag. page 350.

Mr. Taylor, in his account of the Norfolk crag, appears to associate with it the beds which Mr. Woodward describes as diluvium; hence he gives a greater extent to the crag formation than Mr. Woodward. The latter gentleman states, as a well ascertained fact, that the tooth of a mastodon was obtained from the crag stratum at Whitlingham near Norwich; and he has also a fragment of a tooth of a mastodon, which he took out of the crag at Bramerton. These are the only instances at present known, of the remains of this animal being found in any part of Great Britain. Teeth of the fossil elephant or mammoth are very common. A similar formation to crag is said to be discovered on the French coast between Calais and Cape Blanc Nez: also in the neighbourhood of Tangres near Antwerp, and in other parts of the Netherlands.

Mr. Mantell pointed out to me, when at Brighton, that the cliffs there are composed of sand and chalk-flints not worn by attrition, and that they rest on an ancient sea beach, with rolled shingles: in some of their characters, there is a great similarly to the Norfolk crag. The sand is in some parts cemented into hard masses of sandstone, and teeth of the elephant and the horse are found in the cliffs, indicating the high antiquity of this deposition. It has been formed in the valleys or depressions in the chalk, but it is not very easy to explain, how the chalk flints were collected in such masses, and depos ited without having been subjected to attrition. It is probable that future discoveries may make it necessary to place the crag, the Bagshot sands, and the conglomerate in the cliffs of Brighton and other parts of the English coast, among the upper tertiary strata, which will be described in the following chapter.

CHAPTER XVII.

ON THE RECENT TERTIARY STRATA, OR WHAT ARE CALLED BY SOME GEOLOGISTS QUATERNARY.

The Methods for determining the relative Age of Formations explained, and their Value examined.-Evidence from Position.-Evidence from Organic Remains. -System of M. Deshayes founded on Fossil Shells.-Uncertainty attending the Evidence from Organic Remains.-Arbitrary Classifications of Naturalists.Supposed Limits to the Transmutation of Species of Molluscous Animals examined.-System of M. Elie de Beaumont.-Geological Age of Palæotheria-of Mastodons-of Elephants.-Recent Tertiary Strata of the Basin of the Loire.— Of the Subappennine Ranges.─Of the Freshwater Formations in the Appennine Valleys. Remarkable Intermixture of the Skeletons of Whales, Elephants, &c. at Castello Arquata explained by what has taken place in England.--Freshwater Limestone of Eningen, one of the most recent Tertiary Formations.-Human Skeletons erroneously supposed to have been found there.-Observations on the relative Age of the Strata of Eningen.

AFTER the discovery of the true character of the tertiary strata of the Paris basin, and of England, it was, for some time, believed that the former was a complete representation of the whole tertiary formations in every country; and that the strata of the London basin, and of the Isle of Wight, represented a portion of the strata of the Paris basin. It is now, however, ascertained, that in the central and southern parts of France, and in many other countries, there are extensive tertiary formations which contain organic remains, very different from those in the Paris basin.

These strata are, with much probability, believed to have been deposited in detached lakes or estuaries, at a subsequent period to that in which the Paris basin was laid dry.

It appears also probable, that these newer tertiary strata are of different ages; and that some of them approach in their characters to the depositions at present forming on the shores of the ocean, or in the deltas of great rivers, or in freshwater lakes.

The relative antiquity of these recent tertiary formations, is a subject of high geological interest, as it is connected with the history of the latest revolutions of the globe, and the catastrophes that have destroyed the ancient races of its inhabitants.

But how are the relative ages of the strata in different tertiary basins to be ascertained? The relative ages of two groups or formations of strata, or of two strata in distant parts of the same series, may be determined by two methods: one founded upon the evidence of position, the other upon that of organic remains. As the comparative value of these two kinds of evidence, and their relations to each other, has nowhere, that I know of, been briefly and clearly stated, for the benefit of the geological student, I trust I shall be excused for attempting to give a simple and familiar explanation of each method.

The evidence from the superposition of strata, or what the French call gisement, is based upon a self-evident truth. In all stratified rocks that have been formed or deposited by water, the lowest stratum is the most ancient; or, in other words, every stratum is older than the stratum that covers it; unless, by some violent dislocation, the strata have been overturned, or removed from their original position. What is true with respect to two strata, may be applied to two series of strata, that occur under each other: thus, we are certain that the red sandstone and marl under the lias beds, are more ancient than the latter; and as both formations preserve the same character over a great extent, whenever we meet with them in other situations, where the superposition is not apparent, we may safely conclude, that the red sandstone is more ancient than the lias, and occurs under it.

We cannot, however, apply the same evidence to two groups of strata formed in detached lakes or basins, because, being deposited in different localities, they never occur superimposed on each other. Let us suppose that two ancient lakes, situated at a considerable distance, had become dry in remote ages, and that a stratum of calcareous marl were found in the ancient bed of each lake; it would be evidently impossible, from these data, to determine which stratum was the most recent, or whether their ages were coeval. Let us, for the better distinction of the stratum of calcareous marl in each lake, call the one stratum A, the other B. Suppose the geologist, who had seen the marl beds, were to observe, in a neighbouring steep bank or cliff, two marl beds similar to A and B, but separated by a bed of sandstone, he would have no doubt that the lower marl was the most ancient; but he could not apply this to determine the relative ages of the lake-marl strata, A and B. Were he, however, to discover a number of shells of one species in the lower marl bed of the cliff, and another species in the upper marl bed; and were he afterwards to find the same species of shells that were in the lower cliff marl, in the lake-marl bed A, and the species that were in the upper cliff marl, in the lake marl bed B; he would then have strong presumptive evidence, that the lake-marl A was more ancient than the lake marl B. The evidence from organic remains, or what is technically called the zoological characters, becomes more satisfactory in proportion to the number of instances in which it can be supported by the evidence from position.

In the above example of the strata of calcareous marl in the two ancient lake beds, the evidence of their relative ages derives all its value from the original evidence of position observed in the marl beds of the cliff. The evidence from organic remains alone, must ever be attended with uncertainty, unless originally confirmed by the evidence from superposition. Animals whose remains are deposited in distant basins, may be of different species; but this does not prove that they did not live at the same period, as we find in the present

day different species inhabiting different latitudes; and difference of temperature in the waters of different lakes in the same latitude, might occasion a great change in the character of the inhabitants. The consideration that the value of the evidence from organic remains, was originally derived from the evidence of position, and must ever remain, more or less, dependent upon it, appears to have escaped the attention of many geologists, exclusively attached to the study of zoological characters. Among our ingenious neighbours, the French, perhaps too ready to form generalizations from a limited number of facts, the value of the evidence to be derived from the study of fossil conchology is greatly overrated, when they would make it independent of position or gisement. Could the most scientific conchologist or naturalist have discovered from the organic remains in the Wealden beds, whether they were deposited before or after the green sand? Certainly not. He might have ascertained that they were fresh water, and not marine beds; but this would not have assisted him in discovering their relative age. Fortunately, we have here the evidence of superposition; for the green sand lies over the upper Wealden beds, and, therefore, is a later deposition. When the different periods of time in which different species of animals first appeared in different latitudes, shall be known, then, and not till then, can we predicate with certainty respecting the relative. age of strata from their organic remains alone.

I shall now proceed to state the rules attempted to be established for determining the relative ages of the tertiary strata by organic remains.

M. Deshayes considers, that the relative ages of different groups of strata or formations may be determined, by their zoological characters alone; that is, by the species of shells they contain. He forms two grand divisions of stratified formations :

1. Those which contain no species of shells analogous to existing species.*

This division is stated to comprise all the secondary strata.

2. Strata which contain a greater or less number of species analogous to existing species.

The last division comprises all the tertiary formations. Again he subdivides this division into three groups, according to the greater or less proportion of species of shells, that they each contain analogous to living species.

In the more ancient group he places the tertiary formations of the Paris basin, the London basin, the Isle of Wight, and of a part of Belgium, a small part of the Gironde, and the tertiary strata of the Vicentin.

* By espèce analogue, M. Deshayes means identical species.