322 



NATURE 



[Aug. 15, 1872 



The thickness of this fresh-water formation is inconsiderable 

 compared with our Wealien. The separation of the fresh-water 

 formation from the marine Portland is well defined ; not so that 

 betwixt the Wealden and Xeocomian. Here, as in the Punfield 

 section, the fresh-water and marine conditions seem to have 

 alternated, and the manner ia which this takes place suggests the 

 supposition that the influx of a considerable body of fresl) water 

 from the land of the time took place not far from this place. 



Neufcbatel is seventy miles south of Boulogne ; the Wealden 

 beds, as we have seen, indicate that the series extended south- 

 wards from Marquise ; and it is no unreasonable supposition that 

 the deposits of this Pays de Bray were formed under the waters 

 of the same lake as were those of our own Wealden. 



Such, then, were the dimensions ^of the Wealden Lake or 

 Sound ; it extended from parts of Buckingham, on the north, 

 half across the English Channel on the south, a breadth of 160 

 miles. In the contrary direction it reached from Wiltshire into 

 France, beyond Beauvais for 250 miles. 



In another part of France, Depart, de I'Aube, Mr. Cornuel 

 has described a fluvio-Iacustrine formation between the Jurassic 

 and Cretaceous formations at Vassy, containing Igiianodon, 

 several species of Unio and Planorhcs. The lacustrine formation 

 at Cimey is in a corresponding geological position. 



In the Jura, Villers, Forcine-le-has, the Portland beds are fol- 

 lowed by hard bluish marls, calcareous marls, and gypsiun. the 

 whole very like our Purbeck series. Tnese lacustrine formations 

 are interesting, as they seem to show the existence of a chain of 

 lake? stretching across France into Switzerland for 260 miles, 

 with a general direction parallel to the axis of Artois, and thus 

 connected as part of one great lake system with our Wealden. 



Ill France, Depart, des Deux Charentes, some 350 miles due 

 south of our Sussex coast, there occurs a great fresh-water for- 

 mation in intermediate position between the Portland Oolite and 

 what were then the lowest beds of the Cretaceous series. Like 

 our own Wealden, this also is exhibited over a surface from which 

 the Cretaceous strata have been denuded. This formation has 

 engaged the attention of many French geologists, more particu- 

 larly of M. Coquand, who has determined its ai;e and purely 

 lacustrine character, and who puts it as the equivalent of the 

 Purbeck beds of England ; in this he seems to be guided by the 

 general likeness as to composition and the presence of Physa 

 Bristoioi, a well-known Purbeck species. 



The sequence of events at this place was as follows : — Subse- 

 quently to the formation of the Portland Oolite the sea-bed 

 became terrestrial surface, and subsequently again to that, a 

 depression extending from Chateauneuf, near Angouleme, to 

 beyond the Island of Oleron, became the site of a great fresh- water 

 lake. From St. Jean d'Angely to Chateauneuf is a distance of 

 thirty-five miles, and from Chateauneuf to Oleron, S.E. to N. W. , 

 is upwards of lOO miles ; but then figures do not give the full 

 dimensions of this fresh-water area, as its deposits have been 

 reduced by denudation on the north, and passes beneath the 

 Cretaceous series on the south. The original lake must have 

 had a westerly extension seawards, and its area must have 

 equalled that of Lake Ladoga. 



The feeders of this lake are more easily accounted for than in 

 the case of our own Wealden. Such a lake would necessarily 

 have received all the streams descending from the western slopes 

 of a terrestrial surface of very ancient date, namely, the granitic 

 district of Central France. 



In North Germany there is a well-exhibited Wealden forma- 

 tion, extending from Benlheim by Rheine, with a breadth from 

 north to south of twelve miles. From Ibbenhiisen it reaches on 

 the south side of the Triassic and Palicozoic axis of Osnaburg 

 for many miles. It is everywhere in an intermediate position 

 betwixt the Upper Jurassic and Lower Cretaceous formations. 

 On the north of the axis it spreads (or seventy miles to Minden, 

 certainly as far as north as the Steinhuder Meer to near Hanover, 

 and as far south as the Hils district. From west to east the as- 

 certained extent of this lake is upwards of 120 miles. 



At Bentheim the dark Wealden clays, with bands of limestone 

 and spathic iron ore, with Cyretur, MdanicCj &c., like those of 

 Sussex here, are 400 metres thick, so that the real dimensions of 

 this northern lake were very much greater than has been here 

 given. 



These large lacustrine areas imply that there was at that time 

 a corresponding extent of terrestrial surface. And it may fairly 

 be asked, what is the geological evidence of such a condition ? 

 There occur over parts of Belgium the remains of such a terres- 

 trial condition of surface beneath the lower Cretaceous beds 



'here ( Pourtia) consisting of variegated sands and clays, with 

 much diffused vegetable matter, and occasionally with beds of 

 lignite ; such surfaces can be traced along the line of the Belgian 

 coal-field (Mons), and overlying parts of the Palaeozoic series. 

 These beds are not of sufficient dimensions to be termed lacus- 

 trine, but have all the characters of the deposits of ponds and 

 marshes ; and M. Dumont has properly referred them to the 

 Wealden period. Such like evidence of terrestrial conditions 

 recur over a wide European area ; such are the subcretaceous 

 beds of pisiform iron ore, of subaerial origin, and the wide area 

 over which fresh-water sands with Ptcrophylliim, Pecopteris, 

 Cycadises^ &c., of our Wealden are met. 



The break betwixt the marine Jurassic and Cretaceous for- 

 mations is very distinct, physically and zoologically ; and it may 

 be fairly asked, in what way do the forms entombed in the pro- 

 ducts of the intercalated period of terrestrial surface conditions 

 serve to throw any light on what took place during that long 

 interval of time ? 



That the earliest Purbeck-Wealden fauna should have Jurassic 

 relations, that is to say, that it must have synchronised with such, 

 wherever that formation was being continued, is only what might 

 be expected ; for the whole of the bed of the Jurassic seas in the 

 northern hemisphere was not converted into sub^rial surface at 

 once. Midway in the course of the Purbeck-Wealden series 

 there is evidence of the recurrence of marine conditions with 

 Portlandian forms, such as Ostra distorta and Ileinkidaris pur- 

 liakcnsis. It was on this ground that Prof E. Forbes suggested 

 the propriety of placing the Purbeck series with the Jurassic in 

 .systematic grouping ; for it showed that up to the time of the 

 Middle Purbeck beds the marine fauna of the nearest seas was 

 still Jurassic. 



The considerable extent of land surface in the northern hemi- 

 sphere during the whole of the marine Jurassic period, and the 

 local conversion of any portions of such sea-bed into land, 

 whether in the course of the deposition of the Lower Jurassic 

 series (Stonesfield), or between the Lower and Middle (Brora, 

 Staffin), or at the uppermost stage (Portland), would be merely 

 the addition of so much more to the existing land. 



The fonns of life which would colonise such new surfaces 

 would be such as migrated from the older adjacent lands ; if any 

 change should have taken place in the fauna or flora of such old 

 land-surface, in the course of the production of the marine 

 Jurassic series, it would be recorded in the forms entombed in 

 the lacustrine formations of the several stages here alluded to. 



The fossil plants and fresh-water shells from Brora, Lock 

 Staffin, and the Wealden seemed at first to certain well-known 

 and competent naturalists to show that an identical set of forms 

 ranged throughout. A minutely critical examination has since 

 indicated shades of difference ; yet it may be questioned whether 

 such are greater than different localities in the same zoological 

 province now present, allowance being made for differences of 

 these old estuarine and lacustrine areas. 



The relations of the land-surface forms of the Wealden forma- 

 tions of the European surface has been recognised by all natura- 

 lists as being Jurassic rather than Cretaceous. In this the 

 Purbeck-Wealden group offers an exact counterpart zoologically 

 and geologically, of the Permian-Trias group ; just as the marine 

 zoological relations of the Permian are PalLCOzoic, so those of the 

 Purbeck are Jurassic ; and when next after each of these, and 

 after the wide spread of purely marine conditions over the 

 northern hemisphere at each period, the marine fauna is seen to 

 have undergone a complete change, in the one case Palnjozoic 

 forms go out, and for ever, to be succeeded by Mesozoic or 

 Jurassic ; in the other Jurassic forms go out and the Lower Cre- 

 taceous come in, and are those which interchange with the upper- 

 most Wealden fauna at Punfield and the Pays de Bray. 



Did time allow, I might call attention to the results of the 

 labours of the distinguished pakvontologists who have described 

 the forms of life of the Wealden period, both of animals and 

 plants. From them we know that crocodiles and chelonians, 

 referable to many genera, abounded in the Wealden waters. 

 These, with the cycade^ of the land, sufliciently mark the tem- 

 perature of that time as being much higher than it is here at 

 present. With respect to the numerous large terrestrial Dino- 

 saurs, it is observable that as yet they are ne.arly all peculiar to 

 our Wealden lake. The relative level of this lake seems through- 

 out to have been such as to have admitted of easy communica- 

 tion anfl interciiange w^ith the waters of the sea ; and this cojuii- 

 tion may serve to account for some of the peculiarities which its 

 fauna presents. 



