Aug. 15, 1872] 



NA TURE 



321 



whence they took their name, and extend tbence continuously to 

 near Horsham, rising into the central ridge of the Wealden ele- 

 vations of St. Leonard's, Tilgate, and AshdowTi Forests. On 

 every side this tract is bounded by the Weald Clay, which ex- 

 tends to the base of the escarpment of the Lower Green Sand, 

 beneath which it passes. 



This surface of fresh-water strata, so defined, extends for 

 seventy miles from E. to W., and has a breadth from N. to S. of 

 thirty-five miles. Over the whole of this area the fresh-water 

 depositions attain a great thickness ; the lower sandy group may 

 be taken at 820 feet, and the Weald Clay at 450 feet at least. 



To realise the conditions under which these accumulations 

 were formed, the now upraised central Sandstone ranges must be 

 put back to their original horizontal position, and the whole 

 series must be regarded as the infilling by fresh-water rivers of 

 ■what was an area of depression, with reference to the terrestrial 

 surface of the time. This Wealden formation can be traced far 

 beyond the limits of the denudation of the S.E. counties. In a 

 southerly direction it occurs in the Isle of Wight, with its two 

 divisions of Weald clay and Lower sands. In this quarter the 

 Weald clay is reduced to a thickness of 68 feet. In a westerly 

 direction (Swanage Bay) the Wealden sands have a great thick- 

 ness, and are surmounted by only a thinnish band of Weald clay 

 or deep-water deposit, and both divisions decrease rapidly, in 

 the e.xtension of the formation across the Isle of Purbeck, and 

 have not been recognised in the Isle of Portland, from which, 

 if they even extended there, they must be denuded off. 



In a northerly direction, several sections about Oxford, as 

 from Shotover Hill to Great Hazeley, from Wheatley to Tets- 

 worth, from Brill through Long Crenden to Thame, from Whit- 

 church to Aylesbury, extending from S.W. toN.E. for a breadth 

 of thirty miles, show Puibeck beds, and fresh-water ferruginous 

 sands passing beneath Cretaceous beds. It is obvious that the 

 Wealden formation has been cut back in this quarter, and that 

 originally it had a much greater extension. In this qiiarter, too, 

 the ferruginous sands overlap the Purbeck beds, showing that 

 the lake had here widened its area beyond the dimensions of the 

 Purbeck lake. 



From Oxford* to the Vale of Wardour is an interval of 

 seventy miles, from over which the Portland Oolite has been re- 

 moved, except at Swindon, at which place there are beds which 

 are unmistakeably referable to the Purbeck group ; and it is a 

 fair inference that it is to this denudation that the absence of the 

 lacustrine depositions is to be attributed, which everywhere on 

 ovir area, and on much of that of Continental Europe which was 

 adjacent, follow next upon the Portland stage. Such being the 

 case, the smallest possible dimensions which can be assigned to 

 the great Wealden lake, are that it extended from beyond Ayles- 

 bury to Portland for 120 miles, and from Portland to the Boulon- 

 nais for 200 mUes. 



From Rye to Portland the Wealden beds pass out of sight 

 beneath the level of the English Channel. The valley of the 

 Channel is the result of the disturbance which produced the E. 

 and W. lines of the South of England, and was produced sub- 

 sequently to the Nummulitic period. 



Dr. Fitton remaiks that the subdivisions of the Wealden for- 

 mation, especially at its upper part, being m some measure 

 arbitrary, it is difficult to determine to which of the three groups 

 any outlying depositions ought to be referred. (Geol. Trans, vi., 



p. 323.) 



Such a difficulty existed when corresponding 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 within 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 most distant 

 argillaceous ar calcareous mud-beds. Considered in this way, 

 the 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 W'ealden series into Purbeck 

 beds, Hastings sands, and Weald clay, must therefore be sub- 

 stituted the more natural divisions of Lower Wealden for the 

 Purbeck series, and Upper Wealden for the series as exhibited in 

 the south-east of England may be of sand and sandstone or 

 Weald clay, according to local conditions of depth. 



There are indications that changes in the area surrounding the 



* F/V*- evidence as to range of Wealden deposits, Phillips' "Geology of 



Wealden formation took place in the progress of that series ; the 

 lower and earlier sandy deposits indicate only inconsiderable 

 depths cf water. Yet the vertical thickness of the series may be 

 estimated at nearly 2,000 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 coarseness of the detritus of the 

 upper, and in the evidence of strong currents settling in definite 

 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 aliemations 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 considerable 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 fresh-water 

 area was subordinate — such, for instance, as would give rise to 

 larger rivers, great 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 

 surface of Portland Oolite, from which they came. This is the 

 Purbeck stage, which commenced with a long period of purely 

 fresh-water conditions. Brackish-water conditions followed with 

 a change of fauna. Mollusca, such as Coibuhi, Cardiuiii, 

 Modiola, Rissoa, appear, presenting — as was observed by the 

 late Edward Forbes — the change of character which the Caspian- 

 sea Molluscs 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 purely fresh-water 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. 



IVfalJcn Formations of the Europtan Surface 



The elliptical form of the Wealden elevation and denudation 

 has its completion on the east in Picardy, across the English 

 Channel. In the Boulonnais there occur ferruginous sands like 

 those of Shotover, fidl of fresh-water shells (Unio) overlying 

 Purbeck limestone, and passing beneath the Cretaceous forma- 

 tion, 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 may safely be placed to the north of the Bouloimais denu- 

 dations ; for the Wealden depositions proper hardly rise to the 

 level of tlie Palaeozoic rocks ot Marquise. The great fissures and 

 pot-holes in the limestones there, which have been produced 

 under suba:rial conditions, and filled with sand, mould, and 

 much vegetable matter, had been produced antecedently to the 

 deposition of the Gault over that area. 



The Wealden beds of the Boulonnais were formed beneath the 

 waters of the same lake as our own. This fresh- water area had 

 an extension southwards ; thus M. D'Archiac refers the mottled 

 clays beneath the iron sands and sandstones at Havre to the 

 Wealden series of this country, so that the limits ot our lake in 

 that direction, or in the south, lay somewhere along the line of 

 the English Channel. 



Sixty miles to the south of the Boulonnais is a district known 

 as the Pays de Bray, which is an elliptical valley of elevation and 

 denudation, like our own Wealden on a small scale, extending 

 from Beauvais to Neufchatel, a distance of forty-five miles. In 

 this denudation the lowest beds exposed belong to the marine 

 Jurassic series (Portland Kimmeridge). Next above the Portland 

 stone is a Wealden formation. " Les depots regardes comme 

 fluviatiles sont les plus voisins de letage Portlandien et forment 

 le groupe infcrieur du terrain Neocomien " (Graves, Oise, p. 55). 

 The remains of the fishes, Cyreiuc, CypriJes, and ferns are such 

 as occur in our Wealden. 



