TRANSACTIONS OF THE SECTIONS. 95 



been reduced by denudation ou tlio north and pass beneath the Cretaceous series 

 on the south. The original Like must have had a westerly extension seawards ; and 

 its area must have equalled that oi' 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 allthe streams de- 

 scending from the western slopes of a terrestrial suiiaco of very ancient date, namely 

 the oTanitic district of Central France. 



In North Germany there is a well-exhibited Wealden formation, extending from 

 Bentheim by Rheine, with a breadth from N. to S. of twelve miles. From Ibben- 

 hiiseu it reaches on the S. side of the Tri.issic and Pala3ozoic axis of Osnaburg for 

 many miles. It is everywhere in an intermediate position betwixt the Upper 

 Jurassic and Lower Cretaceous formations. Ou the N. of the axis it spreads for 

 seventy miles to Miuden, certainly as for N. as the Steinhuder Meer near Hanover, 

 and as far S. as the Ilils district. From W. to E. the ascertained extent of this 

 lake is upwards of 120 miles. 



At Bentheim the dark Wealden clays, with bands of limestone and spathic iron- 

 ore, with Cyrenep, Melanice, &c. like those of Sussex here, are 400 metres thick ; 

 so that the real dimensions of this northern lake were very much greater than 

 those here given. 



These largo lacustrine areas imply that there was at that time a corresponding 

 extent of terrestrial surface. And 'it may fairly bo asked, what is the geological 

 evidence of such a condition r* There occur over parts of Belgiimi the remains of 

 such a terrestrial condition of surface beneath the lower Cretaceous beds there 

 (Tourtia), consisting of variegated sands and clays, with much diffused vegetable 

 matter, and occasionally with bods of ligiiite ; sucb surfaces can be traced along 

 the line of the Belgian coal-field (Mons), and overlying parts of the Pa-lasozoic 

 series. These beds are not of suthcient dimensions to be termed lacustrine, but 

 have all the characters of the deposits of ponds and marshes ; and M. Bumont has 

 properly referred them to the Wealden period. Suchlike 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 freshwatej," 

 sands with PterophijUiim, Fecoptvris, Cycadites, &c. of our Wealden arefound. 



The break betwixt the marine Jurassic and Cretaceous formations is very dis- 

 tinct, physically and zoologically ; and it may be fairly asked, in what way do the 

 forms entombed in the products of the intercalated period of terrestrial-surface con- 

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



time? . , . , . 



That the earliest Purbeck-Wcalden fauna should have Jurassic relations^hat is 

 to say, that it must have synchronized with such wherever that formation was 

 beinn' continued, is only whai might be expected ; for the whole of tlio bed of the 

 Jurassic seas in the northern hemisphere was not converted into subaerial surface 

 at once. Blidway in the course of the Purbeck- Wealden series there is evidence 

 of the recurrence of marine conditions, with Portlandian forms, such as Ostrea 

 tlistorta and Hcmicidaris juirbcclu'iisis. It was on this gTOund that Prof. E. Forbes 

 suggested the propriety of placing the Purbeck series with the Jurassic in syste- 

 matic o-rouping ; 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 hemisphere 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 (Stonesheld), or between the lower and middle (Brora, StaiKn), or 

 at the uppermost stage (Portland), would be merely the addition of so much more 

 to the existing land. 



The forms of life which would colonize such new surfaces would be such as 

 migrated from the older adjacent lands; if any change took place in the fauna 

 or liora of such old land-surface in the coiu'se of the production of the marino 

 Jurassic series, it would be recorded in the forms entoniled in tl;c lacustrine 

 formations of the several stages here alluded to. 



The fossil plants and freshwater shells from Brora, Loch Stailln, and the 

 Wealden seemed at hrst to certain well-known and competent naturalists to show. 



8* 



