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The solid surface beneath the waters would appear to have suffered a long continued and gradual depression, which was as gradually filled, or nearly so, with transported matter; in the end, however, after a depression of several hundred feet, the sea again entered upon the area, not suddenly or violently for the Wealden rocks pass gradually into the superincumbent cretaceous series but so quietly, that the mud containing the remains of terrestrial and fresh-water creatures was tranquilly covered up by sands replete with marine exuviae."
And as the structure of the limbs of several of the gigantic Ornithoscelida, such as Iguanodon, leads to the conclusion that they also may have constantly, or occasionally, assumed the same attitude, a peculiar interest attaches to the fact that, in the Wealden strata of England, there are to be found gigantic footsteps, arranged in order like those of the Brontozoum, and which there can be no reasonable doubt were made by some of the Ornithoscelida, the remains of which are found in the same rocks.
But perhaps it would be safer to allow two or three inches per century, and this would reduce the number of years to one hundred and fifty or one hundred million years. The action of fresh water on the gently inclined Wealden district, when upraised, could hardly have been great, but it would somewhat reduce the above estimate.
Beneath the Atherfield clay or Upper Neocomian of the S.E. of England, a fresh- water formation is found, called the Wealden, which, although it occupies a small horizontal area in Europe, as compared to the White Chalk and the marine Neocomian beds, is nevertheless of great geological interest, since the imbedded remains give us some insight into the nature of the terrestrial fauna and flora of the Lower Cretaceous epoch.
This lower division of the Wealden consists of sand, sandstone, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, predominating somewhat over the arenaceous, as will be seen by reference to the following table, drawn up by Messrs. Tunbridge Wells Sand: Sandstone and loam: 150. Wadhurst Clay: Blue and brown shale and clay, with a little calc-grit: 100.
I have shown, when treating of the Mississippi, that a more ancient delta, including species of shells such as now inhabit Louisiana, has been upraised, and made to occupy a wide geographical area, while a newer delta is forming; and the possibility of such movements and their effects must not be lost sight of when we speculate on the origin of the Wealden.
By referring to what was said of the Iguanodon of the Wealden, the reader will perceive that the Dinosaur was somewhat intermediate between reptiles and birds, and left a series of tridactylous impressions on the sand. To determine the exact age of the red sandstone and shale containing these ancient footprints, in the United States, is not possible at present.
Great rivers and lakes existed in this new continental region, much like those which now exist in Sweden, Northern Russia, and Canada; and the deposits of sand or mud formed at their bottoms or in their estuaries compose the chief part of the Wealden formation in England.
These must have been drifted by a current, probably in water of no great depth. From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden, the whole of it was a deposit in water of a moderate depth, and often extremely shallow.
Wealden : Weald-formation. Portland : Upper oolithic. Oxford : Middle oolithic. Bath : Lower oolithic. Lias : Liassic. Keuper : Upper triassic. Muschelkalk : Middle triassic. Bunter : Lower triassic. Permian : 14. Zechstein : Upper permian. Permian : 13. Neurot sand : Lower permian. Carboniferous sandstone : Upper carboniferous. Carboniferous limestone : Lower carboniferous.
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