The remains of enormous reptiles are numerous in the Wealden formations; crocodiles, lizards, turtles of gigantic size have been discovered, and most curious fossils have been disinterred in the Hastings district. The “Greensands” are separated by what is termed gault, a stiff blue clay found in Norfolk, Essex, and Kent. The Lower Greensand includes the well-known Kentish rag, or limestone, of which so many churches are built. The Upper Greensand is supposed to be a seashore deposit on the sides of an extensive ocean or sea, at the bottom of which the chalk was formed. After the Wealden beds were formed, they were covered by these greensand estuary-beds, or littoral strata. In these series new forms of life appeared, and the waters became the receptacle of myriads of mollusca, etc., which in time formed the great chalk cliffs and downs so often referred to. The chalk is interstratified with sand, which as “gault” and “greensand” was probably the sand of the ocean bed before the chalk was formed upon it, and the seas must have supported many marine reptiles, for stony “nodules,” or coprolites, which are the fossil excreta of the animals, are found, and now used for manure, after being buried for thousands of years. Examination of these remains has resulted in the discovery of the teeth and bones of fish which had been devoured by the gigantic reptiles. An illustration of a shell thus discovered is annexed.
Fig. 677.—Nautilus Inequalis.
Fig. 678.—Ammonite from the chalk.
We have in a former chapter spoken of the chalk and its formation. We know that it is composed of the minute foraminifera. The fossil remains are very numerous in chalk and all of a marine kind, such as the ammonites, belemnites, and such cephalopods, and the echinus, bivalve mollusca, crustacea, etc. We have occasionally flints appearing in the chalk, and this circumstance has given rise to some speculation as to how the flints got there, for they consist of nearly pure silica; and the theory of the petrifaction of sponges, madrepores, etc., has been started to account for their presence. Dr. Carpenter says: “It may be stated, as a fact beyond all question, that nodular flint and other analogous concretions (such as agates) may generally be considered as fossilised sponges or alcyonian zoophytes, since not only are their external forms and their superficial markings often highly characteristic of those organisms, but when sections of them are made sufficiently thin to be transparent, a spongy texture may be most distinctly recognised in their interior.”
It is now generally admitted that the decaying animal matter acts upon the silicious spiculæ of sponges, etc., and the silica is thus deposited.
We may then surmise that at some very distant period the whole extent of the British Isles was submerged, as well as portions of the continent, and after the strata had been deposited the sea and land were disturbed by volcanic action. While the secondary strata were being deposited, very little relative alteration took place, as the deposits are seen to lie “conformably.” But when the great convulsion which upheaved the Apennines occurred, the chalk was raised as we find it in the cliffs and downs, which were the beds of seas. This is the last of the great convulsions which the earth has undergone, for the tertiary strata, which afterwards began to be deposited, rest in the hollows or basins (chiefly in the chalk) then left; the alterations in and since these deposits appear to consist chiefly of the upheaval of certain localities, the depression of others, the evaporation of inland lakes, and the wear and tear of the land from these causes, which are still in continuous action (as from the washing down of cliffs by the sea, and the formation of mud deposits at the mouths of rivers), or the volcanic agencies which in some places (as in Ireland) have cast up basalt over the chalk.
Fig. 679.—Mosasaurus (Maestricht).