TRANSACTIONS OF THE SECTIONS. 93 



whicli of the three gi'oups any outlying depositions ought to be referred. (Geol. 

 Trans, vi. p. 323.) 



Such a difficulty existed when con-esponding portions of a formation were 

 supposed to require an agreement in mineral character and composition ; but it 

 happened at all times, as now, with respect to the depositions withiji areas of water, 

 whether of lakes or seas, that the beds which were strictly equivalent in respect 

 of time, varied, from place to place, from marginal shingle to submarginal sand- 

 zones, and deeper and more distant argillaceous or calcareous mud-beds. Con- 

 sidered in this way, tlie distant Oxford and Buckingham portions of the Wealden 

 formation are referable to the submarginal accumulations of the great lake, and 

 may be synchronous with " Wealden clays." For the threefold division of the 

 Wealden series into Purbeck beds, Hastings sands, and Weald claj', must there- 

 fore be substituted the niore natural divisions of Lower Wealden for the Purbeck 

 series, and Upper Wealden for the series as exhibited in the S.E. of England, which 

 maybe of sand and sandstone or Weald clay according to local conditions of depth. 



There are indications that changes in the area surrounding the Wealden forma- 

 tion took place in the progress of that series. The lower and earlier sandy deposits 

 indicate only inconsiderable depths of water. Yet the vertical thickness of the series 

 may be estimated at nearly 2000 feet: for that area, at least, progressive depression 

 must have been going on, but not uninterruptedly. As regards the upper and 

 lower divisions of the formation, the difference consists in the greater coarsenessof 

 the detritus of the upper, and in the evidence of strong currents setting in detinite 

 directions, in an extension of the area and of an increased depth — so that at the 

 later stage a central area of deep-water depositions may be defined, as well as the 

 directions in which such conditions thinned away. Great changes took place in 

 the depth of the water of the lake, as indicated by the alternations of the drift- 

 sand beds with deeper- water mud deposits, and in places by the conversion of lake- 

 bed into land-surface, upon which plant-growths established themselves for con- 

 siderable periods of time, and which were again submerged. 



Such changes as these seem to imply change in the physical geography of the 

 land region to which this great freshwater area was subordinate — such, for in- 

 stance, "as would give rise to larger rivers, gi'eater influx of fresh waters, and 

 stronger currents. 



The successive conditions indicated by the great Wealden group as a whole are, 

 for the first stage, that of an extensive shallow lake, or sound, at the sea-level of the 

 time, the inflowing waters to which were largely charged with lime derived from 

 the sm-face of Portland Oolite, from which they came. This is the Purbeck stage, 

 which commenced with a long period of purely freshwater conditions. Brackish- 

 water conditions followed, with a change of fauna. Mollusca such as Corhula, 

 Cardium, Modiola, Rissoa appear, presenting, as was observed by the late Edward 

 Forbes, the change of character which the Caspian-sea mollusks have at present 

 in adapting themselves to brackish water. 



During the Middle Purbeck series the alternations from fresh- to brackish- water 

 conditions were frequent and apparently of short duration, till finally it was closed 

 as it commenced by a thick set of pm-ely freshwater depositions. 



The changes in the Purbeck series are readily accounted for by reference to areas 

 of water such as occur on the American coast at present, and which may be salt or 

 brackish, according to the extent to which the sea-waters are excluded by sand- 

 bars from mixing with the fresh waters flowing from the land. 



The S. and E. coast-line of our Wealden lake must be looked for beyond the 

 area of our island. 



WeaWen Formations of the European Surface. 

 The elliptical form of the Wealden elevation and denudation has its completion 

 on the east in Picardy, across the English Channel. In the Boulounais there occur 

 ferruginous sands like those of Siiotover, full of freshwater shells {Unio), over- 

 lying" Pm-beck limestone, and passing beneath the Cretaceous formation, just as 

 happens in this country. These Wealden beds are not now of any considerable 

 thickness, having been reduced by the denudation of the district. They are so 

 mixed up with pebble-beds in places as clearly to indicate a marginal line, which 

 1872. 8 



