perhaps is pushing the inference too far, and the conditions of the Purbeck basin seem to me more correctly described as those of a silted-up gulf or bay, portions of which became freshwater lakes or meres, such as formerly existed in the Fenland of Norfolk and Cambridge. It is true that the conditions of sedimentation in the Purbeck bay were evidently different from those in the great bay of the Fenland; the latter has been filled up chiefly by tidal silt, and the areas of freshwater deposits are very small, while in the former the lacustrine areas were large, and silt-bearing currents were absent, so that even the marine deposits were calcareous. Still, there is much analogy between the cases, and I therefore agree with the wording of Mr. Meyer's conclusion that it was "by the co-existence within a comparatively wide area of a fauna suited respectively to freshwater and brackish-water conditions, and by the interchange of such conditions over portions of the same area, supplemented by the occasional intrusion of the ocean, that I would account for the alternation of freshwater, marine, and brackish-water fossils in the Purbeck strata; for neither the conditions of their accumulation nor the life-conditions of their fauna appear to be sufficiently in accordance with an estuarine position" (loc. cit., p. 246). On this view, too, the stratigraphical relations of the socalled Purbeck beds of Oxford and Bucks admit of easy explanation; for if our view of Portlandian geography is correct, it is exactly in this Midland district where we should expect to meet with evidence of the early prevalence of such conditions, and to find freshwater strata of an earlier date than that of the Dorsetshire Purbecks. There we find beds with Portlandian fossils passing up into purely lacustrine strata, which were evidently deposited in the quiet waters of a lake that was never invaded by strong currents either of fresh or salt water. We may M conclude, therefore, that it was a large lake or mere which occupied an area of newly-emerged and low-lying ground, and that its level was maintained by the local rainfall, not by the influx of a large river. My attention has been directed to an essay by Professor Hébert,' in which he discusses the physical history of the Jurassic period in France. He divides it into two portions -a period of depression and a period of upheaval. He considers that the former only lasted till the time of the Great Oolite, and that the Jurassic sea then attained its greatest superficial extension. Instead of admitting a further submergence at the time of the Oxford Clay, he attributes its absence over the exposed Great Oolite area to non-deposition, and thinks the Oxfordian sea was limited to the Parisian gulf, the sea-space becoming smaller and smaller during succeeding epochs, till it was finally upraised in Purbeck times. To most English geologists this must seem a very crude and antiquated view, but I am surprised to find that Professor Gosselet and other French geologists hold similar opinions, and interpret the geologial record of other systems on the same principles. (See Gosselet's "Esquisse Géol. du Nord de la France.") "Les Mers Anciennes et leur Rivages dans le Bassin de Paris. Part I. Terrain Jurassique." 1857. CHAPTER X. THE CRETACEOUS PERIOD. I N the south of England, where the highest Jurassic beds are fully developed, there is a complete sequence through the freshwater Purbeck and Wealden groups to marine deposits of Cretaceous age; but elsewhere throughout Britain and the north of France the marine deposits of the two systems are separated by a marked break and unconformity, representing the interval during which the older rocks were upheaved and remained in the condition of dry land. In the south of France this gap is filled by a complete series of marine deposits, which are known as the Neocomien and Urgonien groups, and these, therefore, are the marine equivalents of our Wealden series. In Yorkshire and Lincolnshire also the freshwater series is partially represented by marine deposits, some of which are probably as old as the upper part of the French and Swiss Neocomian, but there is still an unconformity at their base. The following is a tabular view of the members of the Cretaceous system in the south and north of England respectively, with the names of their French equivalents; the two lower stages of the English succession being grouped as the Lower Cretaceous series, and the four upper constituting the Upper Cretaceous series. |