584 MESSES. GAEDNEai, KEEPIlirG, AJSB MOlS^CKTOIir 0^ THE 



British representatives, isolated from tlie more northern basin in 

 ivhich the London Clay was formed. The mingling of the seas 

 apparently lowered the temperature of the water to an extent suffi- 

 cient to drive away such essentially tropical forms as the larger 

 Cones, Cowries, Bullas, Harps, &c.*, without, however, greatly affect- 

 ing the character of the contemporary land vegetation. 



We cannot say that the whole period was one of sustained and 

 continuous elevation, but the drifts of broken shells at the close of 

 the Middle Bartons indicate an upheaval and the presence of strong 

 currents. These shell-drifts thicken and become more numerous to 

 the east and north. The protracted elevation soon afterwards forced 

 back the sea, and converted the former estuary into a brackish- 

 water reach of our great Eocene river. The Middle and Lower 

 Bartons were evidently deposited in almost pure sea-water, though 

 the considerable number of rare freshwater shells occurring in 

 Edwards's list, even from Highcliff, would imply a river-current 

 strong enough to have carried them along. Prestwich, moreover, 

 mentions the occurrence of Cyrena ohovata and Potamides cinctus in 

 them at both Alum and Whitecliff Bays. It seems almost super- 

 fluous to point out that if the Barton Beds were estuarine, quite 

 different deposits would be forming synchronously in the higher 

 reaches of the river as well as further out to sea. Something not 

 very different in quality from the freshwater Eocenes, which occur 

 below as well as above them, must, in fact, have been forming in 

 their vicinity or at no great distance ; while the oceanic deposits of 

 this period are probably preserved in the bed of the Atlantic, and 

 have not been exposed to view. In dealing with our Eocenes it 

 neither follows that distant beds are synchronous because they are 

 similar, nor that they were separated by any intervals of time 

 because they are dissimilar. 



DESCEIPTIOlSr OE THE BedS. 



The Barton Section (fig. 2, facing p. 594). 



Of all the sections exposed, by far the most important and the 

 classic one is that occupying the fine open bay of Christchurch, facing 

 the Keedles, and terminated westward by Hengistbury Head, and 

 eastward hj Hurst Castle. Towards the centre of the bay, where 

 the Bartons are developed, the cliffs average little short of, and in 

 places exceed, one hundred feet in height. Their summits are 

 nearly level, and they terminate rather abruptly near Muddiford to 



* The following are the principal types driven away or extinguished, and all 

 of them have a highly tropical aspect : — Cyprcea Bowerbankii, C. tuberculosa, 

 C. glohularis ; Voluta cithara and V. muricina ; Conus deperditics and C. diadema ; 

 Pleurotoma attenuata ; Harpa, all sp. ; Cassis gigantica ; Natica cepacea, N. 

 hyhrida, N. ponderosa, N. pachycheila ; Turritella sulcifera ; Bentalium 

 grande ; Cerithium giganteum ; Bulla Edwardsii ; Hipponyx cornucopice ; 

 Sanguinolaria HoUoiuaysii, Cardita planicosta. Area Branderi ; also the corals 

 Ocidina, Siderastrcea, &c. Some of the more temperate of the London- 

 clay genera, such as Trivia, reappear, but not Astarte, Cyprina, Verticordia, 

 Vermetus. The only species of large size to appear for the first time in the 

 Bartons is Voluta luctatrix. 



