no 



HARDWICKE'S SCIENCE-GOSSIP. 



The causes of such differential movements of parts 

 of the earth's lithosphere were briefly discussed, and 

 referred to contraction of the crust as the prime 

 factor. The Kingsclere-Hindhead axis of elevation, 

 like the Wealden axis of elevation which was 

 connected with it later on, was thus regarded as a 

 slight wrinkle in the crust, or one of a series of slight 

 wrinkles, determined by the slight circumferential 

 approximation of the two great stable archsean and 

 palceozoic masses of Central Europe and France on 

 the one side, and of the north and west of Britain on 

 the other ; the weaker and newer strata of the inter- 

 mediate area getting pinched up and slightly folded. 

 This was shown to be strictly comparable with the 

 effect of the later Tertiary movements of the Alpine 

 chain upon the intervening Secondary and Tertiary 

 strata, as indicated in the section across the Upper 

 Po-Basin, which the author has recently received 

 from Professor Sacco, of Turin.* 



The estuarine character of the London Clay, and 

 its gradual passage upwards from deposits having the 

 character of those of a marine estuary to those of a 

 more constricted river-mouth, were pointed out ; and 

 the evidence adduced from Foraminifera by Professor 

 Rupert Jones, and from the fauna and flora in general 

 by Professor Prestwich, as to the depths of the 

 waters, in which the deposits were laid down, were 

 briefly discussed. The significance of the distribution 

 of septaria and a molluscan fauna chiefly in the lower 

 200 to 300 feet of the formation, as discussed by the 

 lecturer several years ago,t was also pointed out, as 

 well as the indications which the organic remains 

 give of the prevalence of a tropical or sub-tropical 

 climate in Eocene times. Ascertained thicknesses of 

 the London Clay, beneath the overlying Bagshots (i) 

 at Hampstead 300 feet, at Wokingham 270 feet, at 

 Bearwood 250 feet, (ii) at Wellington College 330 

 feet, at Brookwood 370 feet (or more), at Chobham 

 Place 400 feet, at Claremont 450 feet, and at 

 Wimbledon 430 feet, were cited as indications that 

 the line of greatest depression during the London 

 Clay period was some miles further south than the 

 present line of the Lower Thames. The areal 

 extension of the London Clay, and its gradation of 

 thicknesses from east to west, tell us that the area of 

 depression was a synclinal with its axis inclined to 

 the east ; in other words, it took the form of a 

 segment of a cone rather than of a cylinder, forming 

 what Sacco has called a " cone of depression " (" cona 

 di dejezione."). The elevation of the Kingsclere- 

 Hindhead axis subjected the Chalk strata first to the 

 action of sea-waves, which manufactured from its 

 flints the numerous pebbles found in the Reading 

 beds, as well as the few which are scattered through 

 the London Clay ; and we should probably not be far 



* "La Geo-tectonique de la Haute Italie Occidentale," 

 " Bull, de la Soc. Beige de Geologie, S-c," tome iv., 1890. 



t See "Geol. Mag.," 1886, No. 9, on " The Unconformity 

 between the Bagshot Beds and the London Clay." 



wrong if we considered the denudation of the Weald 

 as having in places cut through the Upper Chalk by 

 the close of the London Clay period ; but we have no 

 evidence to show that the Wealden elevation at this 

 early stage was anything more than a part of a 

 plateau-like region similar to that on the Mercian 

 side of the Tamisian area. For all we know it may 

 have been such, and have continued to rise, without 

 much change in its contour-details through the whole 

 of the Eocene, Oligocene, and Miocene periods, until 

 the Wealden anticline proper began to take its 

 shape in the Pliocene period. We shall return to 

 this later on. 



Condition of things in later Eocene time. 



The established attenuation of the sands which are 

 known as Lower Bagshot, which in many of the 

 more central parts of the area seem to form an 

 upward extension of the London Clay formation, and 

 which Professor Prestwich* has recently correlated 

 on such grounds with the London Clay under the 

 name of the " London Sands," tells us of the further 

 accentuation in later Eocene time of the structural 

 features already initiated, as we have seen, for the 

 south of England ; while the fact, that either these 

 sands, or the overlapping Bagshot beds of higher 

 horizons, rest upon the lower portions of the London 

 Clay with molluscan remains and septaria along the 

 northern flank of the area, afibrds indication of the 

 progressive accentuation of the synclinalf during the 

 portion of geologic time indicated by those deposits. 

 Silting up proceeded ; things seem to have become 

 stationary for a period ; deltaic clays and lagoon- 

 deposits were laid down, certainly from as far west 

 as Newbury, to a long distance towards the east, as 

 far, at least, as Essex, as indicated by the section 

 through Brentwood Common, recently published by 

 Professor Prestwich. J 



Wokingham, Bracknell, Warfield,§ and possibly 

 Windsor Park itself, were mentioned, as localities 

 where this transgressive relation of the Bagshot 

 strata to the London Clay seems to be indicated ; 

 while further east, at Hampstead jl and Brentwood^ 

 a similar relation of things can be traced. On the 

 south side of the Eocene formations of the Tamisian 

 area the high dip of the strata has led to more 

 extensive denudation, and so we cannot adduce such 

 good evidence ; nevertheless there are reasons for 

 regarding certain outlying masses of Tertiary sands 



* "Journal of the Geol. Soc," February, 1888. 



f Such a progressive accentuation of a synclinal flexure has 

 been well worked out in the case of the basin of the Po by Dr. 

 Sacco of Turin. See his valuable paper, " Classification des 

 Terrains Tertiaires," "Bull, de la Soc. Beige de Geologie," 

 &c., tome i., 18S7. In that region the accentuation seems to 

 have gone on intermittently all down through Tertiary time, 

 while in the Thames area it appears to have been intercepted 

 by elevation at the end of the Eocene. 



% "Journal of the Geol. Soc," vol. xlvi., PI. vii., Fig. 5. 



§ See the author's papers, ibid., vols, xliii., xliv. 



y "Mem. Geol. Survey," vol. iv., p. 309. 



^ Prestwich, "Journal Geol. Soc," loc. cit. 



