1871.] JUDD PUXFIELD FORMATION. ' 209 



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 aifairs 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 -mth. Jliwio-marine characters, constituting 

 the weU-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 N^eocomian. 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 aff'ords 

 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-marinc. 



Wealden freshwater. 



Purbeck Formation fluvio-marine. 



Upper Oolite marine. 



II. BiBLIOGRAPnY OF THE SUBJUCT. 



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 kno^vn to local collectors, who 

 have named the more fossiliferous portions the " oyster-beds " and 



* Lettei's in Sir Henry Englefield's ' Isle of Wight ' (Trans. Geo!. Soc. Ser. 2, 

 vol. ii. p. 37). 



