Bonchurch and Ventnor. A list of the fossils obtained by Dr. Fitton from these strata, chiefly from the cliff at Western Lines, is given in "Geol. Trans." vol. iv. p. 202.

THE GALT. This is a deposit of very dark blue marl, having an average thickness of 100 feet; the clay in many places has a large intermixture of greensand, and is traversed by thin layers of red marl. It presents no distinctive mineral aspect, but is characterised by the abundance, peculiarity, and state of preservation of its fossils. Numerous species of ammonites, hamites,. and other cephalopoda, with their nacreous shells entire, and bivalves and gasteropodous univalves, seldom if ever found in the strata above or below, may be collected from most localities where the galt is exposed in natural or artificial sections of any considerable extent. This deposit is seen on each side of the anticlinals on the southern coast, and in some parts of the Undercliff, near Ventnor and Bonchurch; I believe only a few fossils have been observed.

THE GREENSAND.-This division of the cretaceous system comprises numerous alternations of argillaceous and arenaceous strata of great thickness and extent, constituting an important group, which is well defined by the fossil remains.

It consists of cherty and calcareous sandstones, and sands of various colours, white, green, grey, red, brown, fawn, yellow, and ferruginous: the predominance of the green sands in some districts having suggested the term now employed to designate the group. It was called the ironsand where the ferruginous character predominated, as in the Isle of Wight.* Thick beds of sandy and ferru

* The whole series is described by Mr. Webster as the ironsand, in his various memoirs on the geology of the Isle of Wight; and the unfortunate application of this term to the sands of the Wealden districts of Surrey, Kent, and Sussex, retarded, for a long period, the true exposition of the nature and relation of those deposits.

ginous clay, are intercalated with the arenaceous strata. The greensand is separable into three divisions, as was first shown by Dr. Fitton, whose indefatigable and successful labours have so largely contributed to the elucidation of the characters and relations of the deposits between the chalk and oolite in England.

In the upper or Shanklin sand series, the strata are chiefly ferruginous sands with ironstone and concretions of chert, with dark clays in the lower part. The middle group is marked by the prevalence of green and grey sand with beds of calcareous sandstones, and cherty limestone, called Bargate-stone in Surrey, and Rag in Kent. In the lowermost or Atherfield series the argillaceous character predominates. The beds of sand abound in peculiar fossils, and the concretionary limestones and sandstones contain characteristic organic remains. This general notice will suffice, as the details of the sections exposed along the southern coast of the Island have been worked out by Mr. Webster, Dr. Fitton, Captain Ibbetson, Professor Forbes, and other eminent observers, and are published in the Transactions and Journal of the Geological Society.

The following species of shells from the Greensand of the Isle of Wight, are figured in Plates IV. and V :— Corbis corrugata, Pl. IV. fig. 1. This and the following figure represent but one side of the bivalves, and therefore do not convey an idea of the form of the entire shell; but the markings on the surface are distinctly given, as affording the easiest comparison for the inexperienced collector. Trigonia caudata, Pl. IV. fig. 2. This shell is so remarkable both in its form and sculpture, as to be easily identified. Gervillia anceps, Pl. IV. fig. 3. Entire beds of this shell occur in some of the sands at Atherfield. The species is generally more elongated than in the individual figured.

Venus striato-costata, Pl. IV. fig. 4. A small and easily recognised shell.

Venus parva, Pl. IV. fig. 7. In ironstone nodules in Shanklin

cliff.

Arca raulini, Pl. IV. fig. 5.

Perna mulleti, Pl. IV. fig. 6. This large and remarkable bivalve occurs in great numbers in a thick layer immediately above the junction of the greensand with the Wealden, at Atherfield and Sandown-bay. The prolonged hinge line is commonly broken off in extracting the shells from their matrix, unless due care is taken.

Thetis minor, Pl. V. figs. 1, 2. These are only casts in ironstone, the shell having perished. They are abundant in the concretions from the ferruginous sand on the top of the cliff at Dunnose Point, that have fallen on the shore.

Gryphea sinuata, Pl. V. fig. 3. specimens are numerous

This shell attains a large size; along the shores at Shanklin,

This shell in a very perfect

Ventnor, and in Compton-bay; being washed out of the

cliffs of greensand.

Terebratula sella, Pl. V. fig. 5.

state is found by hundreds in the greensand at Atherfield, &c. It occurs in great numbers in the greensand-pits at Faringdon, in Berkshire.

Nucula scapha, Pl. V. fig. 6. This and the following small shells abound in the Atherfield Crackers-rock.

Tornatella albensis, Pl. V. fig. 4.

Natica rotundata, Pl. V. fig. 7.

Pterocera retusa, Pl. V. fig. 8.

Rostellaria robaldina, Pl. V. fig. 9.

Cerithium turriculatum, Pl. V. fig. 10.

Scaphites gigas, Pl. V. fig. 11. This shell attains a large size; specimens two feet long are sometimes found. The collections of Capt. Ibbetson and Mr. Bowerbank contain splendid examples. Imperfect specimens of the spiral part have been described under different generic names.

SUMMARY.-The character of the cretaceous system, as shown by these investigations, is that of an ocean-bed formed in a vast basin by successive accumulations of sedimentary detritus, transported by currents, and thrown down in the tranquil depths of the sea; arenaceous and argillaceous deposits prevailing in the lower, and cretaceous in the upper division of the series; periodical intrusions of heated fluids charged with silex having taken place at uncertain intervals. The fossils prove that the ocean

swarmed with innumerable beings of the usual orders of vertebrate and invertebrate marine organisms, belonging for the most part to species and genera now unknown; and in the chalk are seen for the last time that numerous tribe of cephalopoda, the ammonites, of which not a single species is known either in the tertiary strata or in any more recent deposits: so far as our knowledge at present extends, with the chalk the whole race of ammonites disappeared.

With regard to the vegetable kingdom of the cretaceous period, the presence of numerous fuci attests the nature of the marine flora; and the fragments of drifted coniferous wood, fir-cones, stems and leaves, which are found in the flint and chalk in some localities, prove that the dry land was clothed with pine-forests, and cycadeous plants. The occasional discovery of bones and teeth of reptiles, shows that the islands and continents were tenanted by oviparous quadrupeds.* Of birds and mammalia not a vestige has been discovered.

rence.

*It is worthy of remark, that fossils and fragments of other rocks are very rarely found in the white chalk. Pebbles of quartz and sandstone are the only extraneous minerals of frequent occurA solitary instance of fragments of green chlorite schist, in chalk marl, near Lewes, was discovered by myself some twentyfive years ago. My friend, Henry Carr, Esq., C.E., recently found some water-worn pieces of fossil-wood imbedded in white chalk, from the cutting of the railway between Epsom and Croydon. This wood, in its mineralogical character and organic structure, is unquestionably identical with the well-known fossil-wood of the Portland oolite, and I have no doubt is a portion of a mass that was drifted into the bed of the cretaceous ocean.