4. on the Section Between BLACK-GANG-CHINE and Arher- FIELD POINT

190 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. l~Iaidstone .function of the South-Eastern Railway), seem to be identical with the marine clays found at Hythe and at Atherfield in the Isle of Wight. He adds, " There is also a bed of stone, not a continuous bed, but in concretionary masses, just above the junction, from which I obtained fossils, and which, I consider, re- presents the Atherfield rocks. This bed is also similar to the blocks taken from the cutting in the vicinity of Red Hill, near Reigate." He adds, that the same junction can be traced from the Teston Cutting in the direction of Maidstone, to near the Farley Cutting through the Kentish Rag. The junction of the Wealden and greensand clays is at the bottom of the valley, near the banks of the river Medway. 4. On the Section between BLACK-GANG-CHINE and ArHER- FIELD POINT. By CAPT. L. L. B. IBBETSON, and PROF. EDW. FORBES, F.R.S. THE accompanying Table exhibits the succession of strata presented in ascending order from the Wealden to the top of the Upper Green Sand in the Isle of Wight between Atherfield Point and St. Catherine's Down. The measurements of the upper portion were ascertained by trigonometrical survey, by Capt. Ibbetson, during the years 1833---38, those of the lower portion during the winter of 1842-3. The following observations refer to that portion of the section which includes the Lower Green Sand strata, visited by Capt. Ibbetson and Prof. Forbes in March~ 1844. Between the Gault, as seen near Black-Gang-Chine, and the Wealden at Atherfield Point, there are sixty-three distinct strata, the total thickness of which is 843 feet. § 1. Description of the Strata. The lowest of these is a brown clay 3 feet thick, the base of which, at the junction with the Wealden, abounds in remains of fish. Through this clay are scattered many fossils, none of which are peculiar to this lowest bed, but mostly such as run on through the fossiliferous clays of the Lower Green Sand. This is suc- ceeded by a harder bed or rock of a sandy texture, 2 feet thick, characterised by the presence of numerous fossils, among which the most remarkable is the Perna Mulleti, peculiar to this bed. The clays which succeed are fossiliferous at the lower part, but very slightly so in the middle, where they contain numerous crys- tals of sulphate of lime. The uppermost of these clay strata, called the Lower Lobster-bed, is an impure fullers' earth, abounding with fossils, the most characteristic of which are numerous remains of .4stacus scattered here and there, and found in so perfect a state that no time could have elapsed between the death of the animal and its entombment in the strata, sufficient to permit decomposi- tion to take place. These clays present a thickness of 99 feet. The hard noduliferous bed which succeeds, termed the Lower IBBETSON AND FORBES ON THE LOWER GREEN SAND. 191 Craehers, is full of Gervillia avieuloides and other fossils, and a similar stratum immediately above (the Upper Craehers) abounds in fossils peculiar to itself; indeed it is in this bed that most of such of the species as are limited in their distribution, occur. The Crackers occupy a thickness of 18 feet. A clay bed, 20 feet thick, having the properties of fullers' earth, and similar in appearance to that preceding the Crackers, succeeds : it is very fossiliferous, and, like the other, abounds in Crustacea, mostly of species identical with those in the Lower Lobster Bed. This is termed in the section the Upper Lobster Bed. Am- monites and several bivalves accompany these crustacea. A dark sandy clay succeeds, and is very fossiliferous ; the cha- racters of the fossils do not for the most part differ from those in the lowest clays. It is 20 feet thick. This is capped by a band of Terebratulze (mostly T. Gibbsii) imbedded in the stratum of dark sand, 22 feet thick. The Tere- bratulte are in immense abundance and accompanied by Serpul~e. A series of beds containing zones of Gryph~ea sinuata imbedded in dark sand succeed. The Gryph~ea zones mostly alternate with rows of large nodules containing Crioeeras and Seaphites. This assemblage of Gryph~ea zones is interrupted in the centre by a bed of sandy clay, 34 feet thick, very fossiliferous, and in which a great many of the fossils of the lower clays reappear. These Grypheea and Crioceras beds, with the included clays, have a thickness of 155 feet. Thirty feet of dark sand, containing prolific zones of Terebratula 4 chiefly T. biplicata, succeed, and form the base of a newsuccession of Gryph~ea bands imbedded in dark sand; but the Crioceras nodules are absent. Twenty-four feet is the extent of this uppermost series of Grypheea zones. Above this the beds become ferruginous, and are occasionally, though rarely, mixed with dark blue clay. Fossils in some parts are abundant, but mostly in the state of casts, and no new forms appear. A lignite bed occurs in the lower part of these ferruginous beds, the lignites being arranged in zones. There are also here and there rows of calcareous concretions, usually of an oblong shape, and mostly having a direction towards the S.E., like the lines of oblique bedding occasionally presented in this part of the series. At the top of Black-Gang-Chine waterfall, a series of indurated ferruginous sand rocks alternating with dark sandy clays appear. The sand rocks are composed of quartz grains, and exhibit lines of oblique bedding. They contain no fossils. At the uppermost part of the Lower Green Sand is a series of thin beds, alternately ferruginous and sand, lying immediately below the gault. Casts of a Solarium (species unknown), and of an Ammonite, were found in these bands. § 2. Grouping of the Strata. The 63 strata enumerated may be grouped under three divisions, from their general mineral character. ][92 PROCEEDINGS OF TtIE GEOLOGICAL SOCIETY. A. The lower asscmblage of clays, mostly fullers' earth, abounding in fossils, and in which the Perna sand-rock and the Cracker nodules are exceptional strata indicating temporary conditions. B. The region of Gryphcea sinuata sands in which the Tere- l~ratula bands and upper clays are exceptional strata. This region may be subdivided into three portions, the two lower containing Crioceras nodules separated by the clay, and the upper containing no nodules. The noduliferous part of the series, and that which is free from the nodules, have each a zone of Terebratulee for a base. C. The region of ferruginous sands, which may itself be divided into two or three sections, the lowest of which is fossiliferous. § 3. Chemical Peculiarities of the Beds. A chemical analysis of the composition of thc several strata was next given ; the principal results of which, affecting the distribution of the organic remains, are the following : The beds which are most fossiliferous are those containing most carbonate of lime. In the ferruginous beds, whether upper or lower, there are no traces of lime ; but large quantities of peroxide of iron. This is true as well of the fossiliferous as the non-fossiliferous parts. The gault which caps the iron bands at Black- Gang-Chine contains but few fossils, and those occur rarely. On analysis it was found to exhibit no trace of carbonate of lime, but a little gypsum ; whereas the fossiliferous gault of Folkstone and other places abounds in carbonate of lime. § 4. IndicationsofConditions underwhich these Beds were deposited. At the close of the deposition of the Wealden, there appears to have been a sudden depression of the bed of the great freshwater estuary, and an influx of the sea. The first effect of such an influx would be the destruction of the animals in the estuary not adapted for living in salt water; hence we find a total destruction of the Wealden animals, the remains of which accumulate towards the point of the junction of that formation with the Lower Green Sand ; a fact which indicates the nature of the change. Even the Cerithium, although belonging to a genus many species of which are capable of living in the depths of the sea, was destroyed- not- withstanding that its appearance, only in the uppermost beds of the Wealden, indicates that its presence there was due to the com- mencement of the very state of things which eventually destroyed it. That the depression was of some extent, though not, perhaps, of very many fathoms, is indicated by the nature of the animals which lived in the first-formed sea-bed, and which, when they died, were often imbedded in the fine and, probably, fast depositing mud, in the vertical position which it is the habit of the animals of such genera as Pinna and Panop~ea to assume when alive. After this, a temporary change followed, when an influx of sand, mingling with the calcareous mud, caused a state of sea-bottom peculiarly fa- vourable to the presence of animal life. In this way were called IBBETSON AND FORBES ON THE LOWER GRV-EN SA~D. 193 into existence a multitude of species which were added to those which had appeared before them. This was, in fact, such a state of sea-bottom as is now presented by great shell banks ; but it does not seem to have lasted long, and new depositions of mud appear to have extinguished some forms, whilst others suffered by the change only in the diminution of their numbers.

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