hills, and the surfaces also of some of the plains between them. Such is a brief transcript of the admirable memoir of MM. Cuvier and Brogniart on the most interesting formation hitherto explored by the joint resources of geology and physiology. It exhibits an unfading picture of the convulsions which the primeval globe suffered, a few centuries prior to its close. The chalk beds of both the Paris and London basins evidently stood at first, near the level of that ancient ocean, as may be deduced from the billowy undulations on their surface. Had they lain deep under water, the rolling of the waves could not have chequered their face with those remarkable ridges and excavations, which still exist to indicate the cause of their origin. 1. It was about this geological epoch, that some of the great fields of basaltic-lava were poured forth from the interior of the globe, spreading over the secondary strata of chalk and sandstone, as we see exemplified in the north of Ireland, in Scotland, and many other countries. The epoch of the covering over of the great chalk formation of Antrim with basalt, coincides with the covering of the chalk basins of Paris and the Isle of Wight, with their tertiary strata. The acts of eruptive violence which immediately clothed the submarine chalk surface of one region with an immense sheet of lava, may have upheaved the chalk slightly above the level of the sea in another district not far distant, thus converting a marine basin into an estuary of great extent, which probably comprehended at once the CAUSES OF THE PHENOMENA. 339 London, Isle of Wight, and Paris basins. For the depression between them that forms the bed of the English Channel, is very trifling in proportion to its extent of surface; and is very much exceeded in depth by the large continental lakes. The greatest observed depth of the water at the western extremity of the Channel, between the Land's End and Ushant, is only 70 fathoms, the distance between the two places being about 30 leagues of 30 to a degree; whereas the lake of Geneva is nearly double that depth between Lausanne and Evian, while the distance between the two latter places is only about 8 leagues. The greatest depth of water in the narrowest part of the Channel, between Dover and Cape Blanc Nez is thirty fathoms, and the distance 6 leagues; and between Cape La Hogue and the Isle of Portland, a distance of about 16 leagues, the depth of water is only 45 fathoms.* 2. The estuary cut off from the sea by alluvium at its mouth, would become progressively fresh by the influx of river streams, and change into a lake, on whose bottom, clay, vegetable remains, and freshwater shells would be deposited. 3. But as the phenomenon of trap veins transpiercing one another, proves that a series of submarine convulsions took place at successive periods, a second eruption of basalt might inundate the lake for a season with a shallow body of sea-water, introducing with it a colony of new inhabitants. By successive deposits of their exuviæ, along with calcareous matter and sand, the bottom would eventually rise above * Mr. de la Beche; Geol. Trans. 2d Series, Vol. I. p. 89. the tide level; and the sea would thus be slowly shut out, as the floodgate became more complete; the production of sea-shells would proportionally diminish, and cease entirely, as the sea withdrew, and the ground became once more, first a brackish marsh, and then a freshwater lake. (Peu a peu cette production de coquilles diminue, et cesse tout-afait; la mer se retire, et le sol se couvre de laes d'eau douce.) 4. Now were formed the solid deposits of marly limestone, as in the travertino of Italy: now the gypseous waters let fall their precipitate of soft plaster, which gradually consolidated round the bones of the anoplotheria, palæotheria, birds, crocodiles, trionyx tortoises, palms, and other productions of the tropical climate which then obviously must have prevailed in the latitudes of London and Paris. The period of the deluge was now drawing nigh, and partial dislocations of the strata, began to betoken that awful catastrophe. The ocean bed once more upheaved by a submarine expansive force, caused its waters to surmount the embouchure of the Parisian basin, and make it again an estuary of the sea. 5. Now the third marine formation of Brogniart or second above the chalk commenced, with its yellow clay-marls, its sandstone, and marine sands, its marl limestone, and upper sea marls. These deposits on reaching a certain height, necessarily became a barrier to the sea, reconverting the basin into a freshwater lake. 6. In this state it continued till by successive depositions, and the drainage of the waters, after RANGE OF FRESHWATER STRATA, EXTENSIVE. 341 the deluge, it was finally brought into the existing condition. Thus the vicissitudes of the land and ocean, portrayed in the tertiary formations, harmonise perfectly with other terraqueous phenomena of the same geological period. The whole may be regarded as characteristic preludes of that storm, which ere long destroyed the old world and its inhabitants. § VIII. CONCLUDING OBSERVATIONS ON TERTIARY STRATA. These tertiary strata which a few years ago had been noticed only in the basins of Paris and London, are found to be most extensively distributed over the surface of the globe. Their existence is now familiar to us, in almost every state in Europe, particularly in the Sub-Appenine formations, where they have been so well described by Brocchi, and are now receiving further illustration from the able hand of Professor Guidotti of Parma. Again we trace them round the shores and in the islands of the Mediterranean; at Montpellier and Nice; at Savona, Volterra and Rome:-in the fish-beds of Mount Lebanon,—and the nummulite limestone that forms the foundation of the pyramids of Egypt. We recognise them also along the northern shores of Africa, and in Malta, Sicily and Sardinia. Mr. Strangways has traced them largely in the Steppes of Southern Russia-and on the shores of the Black Sea and the Caspian. The Russians in their expedition to Bokaria, have found them on the borders of Lake Aral; and finally on the authority of Mr. Crawfurd's discoveries, we establish them in a considerable district of the Burmese empire beyond the Ganges.* Mr. Colebrooke has pointed out analogies between the tertiary deposits of England and those of Caribari, which form a band at the base of the Thibetian mountains, and extending to Silhet, contain nummulites similar to those imbedded in the tertiary limestone of the Egyptian pyramids, and of frequent occurrence in the calcareous rocks of the same epoch in northern Italy. The beds of Caribari present the very association of organized remains, that characterise the tertiary strata of Europe, in which extinct genera of the Pachydermata have been discovered.t The distribution of the tertiary strata in discontinuous patches and basins, is another interesting phenomenon in geology which has lately received illustration from the indefatigable science of Dr. Buckland. Thus the tertiary beds of the London formation become gradually contracted in their progress westward through Berkshire, until they terminate in a point at Savernake Forest, between Hungerford and Marlborough. The strata of chalk on which these formations repose, dip inwardly from the circumference towards the axis of the basin, and sink nearly on all sides, beneath overlying beds of the plastic and London clays. To this arrangement, a remarkable exception occurs near the south-western extremity of the basin, a few miles to the south of Rev. Dr. Buckland. Geol. Trans. 2d Series, Vol. II. + Mr. Pentland, ibid. p. 394. P. 387. |