Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/339

 and, in some cases, are subordinate to marine strata of Neocomian age.

3. The Wealden of northern Germany is the deposit of a different river from that which formed the Wealden of England and France ; and the period of its formation, while it commenced at the same epoch, was of much shorter duration, coming to a conclusion before the end of the Lower Neocomian.

In this memoir I propose to describe in detail some of the observations on which these conclusions are founded. Other portions of the evidence I am compelled to defer to a future period, the present state of affairs in France having prevented me from making certain investigations essential to the completion of the subject. At the base of the great freshwater system of the Wealden there is found a series of beds with fluvio-marine characters, constituting the well-known " Purbeck Formation." At the top of the Wealden there occurs another series of strata of similar character, less known, it is true, but not less interesting and important, for which I propose the name of the " Punfield Formation." These two series of beds, consisting of finely laminated clays, sands, and limestones, while they present many points of resemblance to one another, are very distinct both in mineral character and the nature of their fossils from the purely freshwater and generally brightly coloured beds of the Wealden proper, as well as from the truly marine beds of the Oolite and Neocomian. The Purbeck formation is shown by its stratigraphical relations, and the fossil contents of its marine beds, to have very close relations with the Oolitic System. The Pun- field formation is, on similar evidence, clearly referable to a portion of the Neocomian System. In both the Purbeck and Punfield formations, we find evidence of the alternation of freshwater, brackish-water, and marine conditions : while the former affords proof of the gradual transition of the marine beds of the Upper Oolites into the freshwater strata of the Wealden, the latter as clearly indicates the equally gradual return of marine conditions, which, at the termination of the Wealden period, ushered in the Upper Neocomian.

These several formations occur in unbroken sequence as follows : —

Upper Neocomian ("Lower Greensand") marine. Punfield Formation fluvio-marine. Wealden freshwater. Purbeck Formation fluvio-marine. Upper Oolite marine.

II. Bibliography of the subject.

Although no special mention of the strata which I am about to describe is found in the earliest memoir (that of Webster*) on the Wealden and Purbeck of the Isle of Wight and Dorsetshire, some of their peculiarities have long been known to local collectors, who have named the more fossiliferous portions the " oyster-beds " and

vol. ii. p. 37).
 * Letters in Sir Henry Englefield's 'Isle of Wight' (Trans. Geo). Soc. Ser. 2,