not suppose that the rate of subsidence was hastened at the period when the displacement of a great body of fresh water by the Cretaceous sea took place. Suppose the mean height of land drained by the river of the Wealden estuary to have been no more than 800 or 1000 feet; in that case, all except the tops of the mountains would be covered as soon as the fundamental Oolite and the dirt-bed were sunk down about 1000 feet below the level which they occupied when the forest of Portland was growing. Towards the close of the period of this subsidence, both the sea would encroach and the river diminish in volume more rapidly; yet in such a manner, that we may easily conceive the sediment at first washed into the advancing sea to have resembled that previously deposited by the river in the estuary. In fact, the upper beds of the Wealden, and the inferior strata of the Lower Greensand, are not only conformable, but of similar mineral composition. "It is also a remarkable fact, that the same Iguanodon Mantelli which is so conspicuous a fossil in the Wealden has been discovered near Maidstone, in the overlying Kentish-rag, or marine limestone of the Lower Greensand. Hence we may infer, that some of the Saurians which inhabited the country of the great river continued to live when part of the country had become submerged beneath the sea. Thus in our own times, we may suppose the bones of alligators to be frequently entombed in recent fresh-water strata in the delta of the Ganges; but, if part of that delta should sink down so as to be covered by the sea, marine formations might begin to accumulate in the same space where freshwater beds had previously been formed; and yet the Ganges might still pour down its turbid waters in the same direction, and transport the carcasses * of the same species of alligator to the sea; in which case their bones might be included in marine as well as in subjacent fresh-water strata." 4. LITHOLOGICAL STRUCTURE OF THE IGUANODON COUNTRY.-The nature of the rocks and strata of which the country of the Iguanodon † was composed is also a subject of considerable interest; and from the first moment that the fluviatile origin of the Wealden suggested itself to my mind, to the time when I was compelled by ill health to quit the field of my early researches, I lost no opportunity of obtaining data by which the problem might be solved, and carefully examined the pebbles and boulders, and the fine *The Rev. C. Fisher points out the exceptional unconformability of these deposits in Dorsetshire. Trans. Camb. Phil. Soc. vol. ix. p. 556, See also Petrifactions and their Teachings, chap. 3. detritus as well as the coarser materials of which the Wealden beds are composed. My lamented friend, the late Mr. Bakewell, kindly afforded me his valuable aid in determining the nature of the rocks whence the debris was derived; but the materials were too scanty to throw any satisfactory light upon the inquiry. In the "Fossils of Tilgate Forest, a bed of conglomerate, near Cuckfield, is described as containing pebbles of white, yellow, pink, and mottled quartz, jasper, flinty-slate, and indurated sandstone: and from this deposit I expected to obtain more satisfactory information than from the fine detritus of which the sands, sandstones, and clays consist. But with the exception of small pebbles of rock-crystal, the substances above mentioned comprise all the transported minerals that have come under my observation. The abundance of micaceous particles in many of the sands and sandstones, and the prevalence of argillaceous and arenaceous deposits, seem to indicate a region in which were primary rocks, with sandstones and clay; for the quartzpebbles and the micaceous sands and clays may have been derived from decomposed granitic and felspathic rocks. Extraneous fossils are rare both in the Wealden and the Chalk. Debris of oolitic fossils, however, occur in some beds of the Lower Greensand in Surrey. The observations of Dr. Buckland on this problem entirely accord with the result of my own investigations. "The general absence of pebbles shows that the lands were distant * Small rock-crystals are often found in the sandstone near Tunbridge Wells, and are cut and set in rings and brooches by the lapidaries of that town. A rolled fragment of coniferous wood, which, from its state of mineralization, there is reason to conclude is from the Oolite, was found by Henry Carr, Esq., in a block of white chalk from a railway-cutting near Epsom. See Geology of the Isle of Wight, p. 140. Dr. Fitton mentions having found a rolled ammonite in the Wealden. from whence the fine particles of sand and clay were transported. We should be inclined to look for these lands either in Devonshire or Cornwall, on the west; or in the nearest primary and transition mountains of the Continent, viz. in Normandy and Brittany, on the south-west; or in the forest of Ardennes, on the south-east. It is not probable that the materials of the Wealden formation have been derived in any great degree from the detritus of the Oolitic series, because in such case we should have found among them an admixture of pebbles of Oolite, none of which have yet been noticed."* Of the seaward extension of the delta of the Iguanodon river, no certain indications have yet been obtained: † but it is evident that there must have been intercalations of the detritus and organic remains of the land and fresh-water with those of the sea into which the mighty stream discharged its waters; and there can be no doubt that sooner or later such fluvio-marine Wealden strata as those at Punfield and at Beauvais, when studied more extensively, will be found to throw much light on the history of the Wealden. 5. LOWER SECONDARY FORMATIONS. In accordance with the chronological arrangement (p. 202), I proceed to the consideration of the antecedent, or lower, group of the secondary formations, namely the Oolite, Lias, and Trias.. As a whole, the series consists of alternations of clays, marls, limestones, sands, and sandstones, abounding in marine exuviæ, and which have evidently been deposited in the basin of a sea or seas. With these strata are intercalated in some localities beds of fluvio-marine detritus, in *Geol. Trans. 2nd series, vol. iii. p. 430. + Mr. Godwin Austen's Map, Pl. I., vol. xii. Geol. Soc. Journ., and his observations, op. cit. p. 66, should be consulted on this subject. The Oolitic and Liassic rocks may be conveniently considered under the single term "Oolitic," or, still better " Jurassics," As the Jura mountain-range comprises the whole series of these rocks, the continental geologists, dividing them into groups, know them as the "White (or Upper) Jura," "Brown (or Middle) Jura," and "Black (or Lower) Jura," and collectively as the "Jurassic formation." which vestiges of terrestrial animals and plants are imbedded. (I shall also comprise in this Lecture a notice of the Permian formation, although belonging to the palæozoic system, because the mineral and some of the organic characters are conveniently described when treating of the Trias, and that the Carboniferous system may be considered in a separate discourse.) The fossil remains of the inhabitants of the Jurassic sea comprise a prodigious number of foraminifera, zoophytes, echinodermata, mollusca, crustacea, fishes, and reptiles, nearly all of extinct forms. The fossil marine plants of this series belong to several species of algæ; and those of the land consist of such as were transported into the ocean by rivers and currents, namely, trunks and branches of coniferous trees, cycadeous plants, ferns, &c.; sometimes occurring in the state of lignite and coal. The relics of the land animals are principally of insects, with bones and teeth of numerous reptiles; and of three genera of small terrestrial mammalia. Evidence is thus afforded of the existence of countries clothed with a luxuriant vegetation, and inhabited by numerous reptiles and a few warm-blooded quadrupeds. 6. GENERAL VIEW OF THE OOLITE AND LIAS, OR THE JURASSIC FORMATION.-This formation may be described as consisting of three principal argillaceous and of an equal number of calcareous deposits; namely, Upper or Portland Oolite, and the Kimmeridge Clay; Middle or Coralline Oolite, and Oxford Clay; and Lower Oolites, and the Liassic Clays. The leading subdivisions of these strata, as they occur in England, and the names by which they are generally distinguished, are expressed in the following table. The Lias is included; for, though, in conformity with the usual geological classifications, this group of strata is placed as a separate formation in the synopsis (p. 202), we shall find it convenient, and, I believe, more in accordance with the origin and nature of the deposits, to comprise it in a general survey of the Oolitic or Jurassic series.* THE OOLITIC OR JURASSIC GROUP. UPPER OOLITE of the Isle of Portland, Wilts, Bucks, Berks, Oxfordshire, &c. MIDDLE OOlite. Wilts, Oxfordshire, Yorkshire. LOWER OOLITE. Wilts, Somerset, Gloucestershire, Oxfordshire, and Northamptonshire. OOLITE. 1. Portland Stone, and Portland Sand;-limestone, usually white, with layers of cherty flint; green and ferruginous sands: Ammonites, Terebræ, Trigonia, with other marine exuviæ; some bones of reptiles, and drifted wood. 2. Kimmeridge Clay;-blue clay, with septaria, bands of sandy concretion, and bituminous shale; marine reptiles, fishes, shells, corals, and other organic remains. 1. Calcareous Grit and Coral-rag ;-sandy limestone, shell-limestone, and limestone composed of corals, with shells and echini. 2. Oxford Clay ;-blue clay, with septaria; abounding in fossils: a ferruginous sandy limestone, called Kelloways-rock, full of shells, is subordinate to this clay. 1. Cornbrash ;-coarse shelly limestone, with clay. 2. Forest Marble, and Bradford Clay; -fissile arenaceous limestone, coarse shelly oolite, sand, grit, and clays. 3. Great Oolite;-Oolitic limestone and calcareous 4. Fuller's Earth ;-marls and clays, with fuller's limestone. *In the Map, Pl. I. vol. i., the Oolite and Lias are denoted by the same colour and number (4). |