LECTURE IV.

1. Introductory remarks. 2. Secondary formations. 3. The chalk for mation. 4. Chalk and flint. 5. Flint nodules. 6. Organic remains in flint. 7. Sulphuret of iron. 8. Mæstricht beds. 9. St. Peter's mountain. 10. Mosasaurus of Mæstricht. 11. Lower group of the chalk. 12. Organic remains of the chalk. 13. Fossil vegetables. 14. Fossil zoophytes. 15. Radiaria and crinoidea. 16. Echinites.

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17. Shells of the chalk. 18. Cephalopoda. 19. The belemnite. 20. The nautilus. 21. The ammonite, or cornu ammonis. 22. Turrilite, hamite, &c. 23. Spirolinites. 24. Infusoria in flint. 25. Crustacea of the chalk. 26. Fishes of the chalk-sharks. 27. Fossil salmon, or smelt. 28. Macropoma, and other fishes of the chalk. 29. Reptiles of the chalk. 30. Review of the chalk formation. 31. Geology of the south-east of England. 32. Geological phenomena between London and Brighton. 33. The wealden. 34. Wealden of the Sussex coast. 35. Pounceford. 36. Sub-division, and extent of the wealden. 37. Quarries of Tilgate forest. 38. Rippled sandstone. 39. Wealden of the Isle of Wight. 40. Isle of Purbeck. 41. Petrified forest of the Isle of Portland. 42. Modern submerged forest. 43. Fossils of the wealden. 44. Fossil vegetables-ferns. 45. Clathraria and endogenites. 46. Seed-vessels. 47. Fossil shells. 48. Sussex marble. 49. Fossil cypris. 50. Fishes. 51. Reptiles of Tilgate forest. 52. Fossil turtles. 53. Fossil crocodiles. 54. The Swanage crocodile. 55. The plesiosaurus. 56. The megalosaurus. 57. The iguanodon. 58. The Maidstone iguanodon. 59. Size of the iguanodon. 60. The hylæosaurus. 61. Flying reptiles. 62. Fossil birds. 63. The country of the iguanodon. 64. Sequence of geological changes. 65. Retrospect of geological mutations.

1. INTRODUCTORY REMARKS.—The knowledge we have acquired from our investigation of the phenomena described in the previous lectures, will materially facilitate our geological progress, by enabling

us to comprehend the former effects of those agencies, by which the surface of the earth has been renovated and maintained.

The elevation of the beds of seas and rivers, and their conversion into fertile countries-the submergence of islands and continents beneath the waters of the ocean-the rapid formation of conglomerates from shells and corals on the sea shore-the accumulation of beach and gravel, and the inhumation of animals and vegetables-the slow deposition of sediment by lakes and rivers-the imbedding of innumerable generations of insects, and the formation of limestone from their almost invisible skeletonsthe construction of solid stone out of fragments of bones, and rocks, shivered by earthquakes-the engulfing, in estuaries and inland seas, of land animals, birds, and reptiles-the consolidation of both organic and inorganic substances into rock, by the infiltration of flint and lime by thermal waters the transmutation of submerged forests into coal and lignite-the destructive and conservative effects of volcanic eruptions-the conversion of sand, gravel, and clay, into homogeneous masses by heat, and even the production of metalliferous veins of gold and silver-all these phenomena have passed in review before us, although our inquiries have extended through periods which, however vast and remote in relation to the records of our race, are but brief and modern in the physical history of the earth.

The geological events previously described, although forming a connected series, may be divided into periods, each of which is marked by certain zoological characters; namely, 1st. The modern, or human epoch; 2d. the elephantine, characterised by the preponderance of large pachydermata; 3d. the palæotherian, in which animals allied to the tapir prevailed, and Europe presented a system of gulfs and lakes.

2. SECONDARY FORMATIONS.-I hasten to the consideration of another and antecedent geological epoch, that which comprehends the Secondary Formations. Hitherto our attention has been principally directed to deposits confined within comparatively limited areas, as the basins of lakes, gulfs, estuaries, and inland seas; and accumulations of drifted materials produced by the action of torrents, rivers, and inundations. We have now arrived on the shores of that ocean, of whose spoils the existing islands and continents are principally composed; the fathomless depths of the ancient seas are spread before us, and the myriads of beings which sported in their waters, and lived and died in those profound abysses, remain, like the mummies of ancient Egypt, the silent yet eloquent teachers of their own eventful history.

A reference to the Tabular Arrangement of the Strata (Plate VII. and page 194) will show that the secondary formations constitute nine principal subdivisions, forming four natural groups, viz.

U

the cretaceous, oolitic, saliferous, and carboniferous systems, each containing littoral, marine, and oceanic deposits; sandstones having been formed amidst the agitated waters of the sea shores, clays in tranquil bays and gulfs, and limestones in deep water. I purpose, in the present discourse, to explain the geological characters of the first two of the series, namely, the CHALK and the WEALDEN.* The former is composed of rocks that have been accumulated in the depths of a sea of great extent; the latter, of the sediments of a vast delta; the one affording a striking illustration of the nature of oceanic, and the other of fluviatile deposits.

In the diagram (Plate VII.) the wealden (3*) is represented as an intercalation between the chalk and the oolite (3, 4), because it is of limited extent, and where absent, as in the midland counties of England and on the continent, the chalk lies upon the oolite, as will be shown in the next lecture. As both the chalk and the wealden are fully developed in the south-east of England, the phenomena about to be described may be examined with but little inconvenience; and an extensive collection of the peculiar fossils of these formations may be seen in my museum.+

*The term Weald is derived from the German Wald, a wood or forest. The Weald of Sussex was formerly an impenetrable forest, called Anderida by the Romans, and Andredswald by the Saxons.

Now in the British Museum. See Descriptive Catalogue of the Mantellian Museum, 8vo. fifth edition.

3. THE CHALK FORMATION.-The pure white limestone, called chalk, is known to every one; but in the nomenclature of geology, the name is applied to a group of deposits very dissimilar in their lithological composition, but agreeing in the nature of the organic remains which they contain, and evidently referable to the same geological epoch. The series essentially consists of green and ferruginous sands, clays, marls, and grey and white limestones, abounding in marine remains. With this explanation it will be convenient to employ the term in its extended sense. The chalk formation comprises the following subdivisions:

Glauconie crayeuse.

Glauconie sableuse.

6. Shanklin, or lower green sand The chalk is generally white, but in some countries is of a deep red, and in others of a yellow colour; nodules and veins of flint occur in the upper, but seldom in the lower chalk. The marl is an argillaceous limestone, which generally prevails beneath the white chalk; it sometimes contains a large intermixture of green sand, and then is called firestone, or glauconite. The galt is a stiff, blue or black clay, abounding in shells, which frequently possess a pearly lustre. The Shanklin, or lower green sand, is a triple alternation of sands and sand