EocENE.] { { Upper Bagshot Sand ......... ..250 to 300 ft. i o Middle Bagshot beds, includ- . "5 ing Barton Clay (300 feet) Middle‘ 5 ._2 E.nd)Bracklesham beds (100 - ;-_« § eet . IL Lower Bagshot beds ......... ..100 ,, 150 ,, { [London Clay ................... .. 50 ,, 500 ,, IN” -8,_-_ ._-a-,:':’ g E; ()ldhav_en beds . . . . .., ......... .. 20 ,, 30 ,, J E 3 *3 5‘ &"§ Woolwich and Reading beds 15 ,, 163 ,, -1‘: ’= 3-,5; E 55 'l‘hanet Sand .................. .. 20 ,, 60 ,, lrouped in relation to the physical changes which they record, these strata naturally stand in three divisions. At thel base lies a series of berlsdlaiid rloavrfil in ftéesh, e:tig1kt1‘ix1i]:, anc sea-water on an upraise enu e sur ace 0 a . Then comes a, central group bearing witness to the deepen- iurr of these shallow waters and to the advance of the sea fai? up the former estuary. 1 The upper group brings before us proof of the eventual retreat of the sea, and the conver- sion of the area once more into fresh-water lakes and rivers. This arrangement does not quite tally with that which is based on a comparison of the fossils with existing forms, and with those of other Tertiary districts, for it places the London Clay in the middle series, though the fossil evidence distinctly shows that formation to belong to the older Eocene groups. Lower Eocene.—-The Thanet Sand at the base of the London Tertiary basin consists of pale yellow and greenish sand, sometimes clayey, and containing at its bottom a layer of Ureen-coated flints restincr directly on the Chalk. Accord- in; to Mr Whitaker, it isodoubtful if any proof of actual erosion of the chalk can anywhere be seen under the Tertiary deposits in Enrrland and he states that the Thanet Sands everywhere lie Iiponian even surface of chalk with no visible unconformability. Professor Philips, on the other hand, descriges the tchalk lat Reading 211st héivingf been “.l:ll:.CI‘:3.1l1l-)7 groun own 0 a p am or un u a e sur ace as 1 IS IS day on some parts of the Yorkshire coast,” and, having like- wise been abundantly bored by lithodomous shells. The fossils of the Thanet Sand comprise about 70 known species (all marine, except a few fragments of terrestrial vegetation). Among them are several foraminifera numerous lamelli- branchs (Astarte tcnera, 6'3/pr2'7La ]l[o1,'7'isi2', Ostrea Bello- vacina, &c.), a few species of gasteropods (A'atz'ca subcla- pressa, Aporr/e_ru'.s Sowerbii, &c.), a nautilus, and the teeth or palatal bones of fishes (Lamna, I’2'sodus). The Woolwich and Reading beds, or Plastic Clay of the older geologists, consist of lenticular sheets of plastic clay, loam sand and pebble beds. The organic remains show that the sea of the Thanet Sand era gradually shallowed into an estuary. They amount to more than 100 species, and include a few plants of terrestrial growth such as Ficus Forbesi, (w'revillea Hccri, and Laurus H oolccri. The lamelli- branchs are partly estuarine or fresh-water (Cyclas, Cyrena, Dreisscna, Unio), partly marine. Of the latter a character- istic species is Oslrea Bellovacina, which forms a thick oyster bed at the base of the series. Ostrea tenera is like- wise abundant. The gasteropods include a similar mixture of marine with fluviatile species (Ceritlziumfunatmn ]l[elam'a inquinata, Jralica subdeprcssa, Fusus lalus, Palztclizza lenta, &c.) The fish are chiefly sharks (Lanma). Bones of turtles and scutes of crocodiles have been found. The highest organisms are bones of mammalia, one of which, the 002"}/photlon, was allied to the modern tapir. The Oldhaven beds forming the base of the London Clay, though of trifling thickness, l1ave yielded upwards of 150 species of fossils. Traces of Ficus, C-izmamomum, and C0nz_'fcr(e have been obtained from them ,' but the organisms are chiefly marine and partly estuarine shells, the gastero- pods being particularly abundant. The London Clay, as its name implies, is a mass of clay stiff brown or bluish- grey, with septarian nodules. It eiitends through both the GEOLOGY 361 London and Hampshire basins, attaining a maximum thick- ness in the south of Essex. It has yielded a long and varied suite of organic remains, from which we can see that it must have been laid down in the sea beyond the mouth of a large estuary, into which abundant relics of the vege- tation, and even sometimes of the fauna of the adjacent land were swept. Its fossils are mainly marine mollusca, and, taken in connexion with the flora, indicate that the climate was somewhat tropical in character. The plants include the fruits or other remains of palms (Nipa(l2'tes), custard- apple, acacia, gourds, melons, Proteacew, and C'0mfe1'a'. Crustacea abound (Xcmt/mpsis, Iloploparia). Gasteropods are the prevalent mollusks, the common genera being Plcurotoma (45 species), Fusus (15 species), C’;/prcea, Jlfurex, Cassidaria, Pyrula, and Volzzta. The ccphalopods are represented by 6 or more species of Nazztilus, by Belo- sepia sepiozllea, and Beloptera Levesquei. Nearly 100 species of fishes occur in this formation, the rays (]l[ylz'o- batcs, 14 species) and sharks (Lamna, Otodus, being specially numerous. A sword-fish (Tetrapterus p7'isc'us), and a saw-fish (1’rz'stz's bisulcatus) about 10 feet long, have been described by Agassiz from theLondon Clay of Sheppey, whence almost the whole of the fish remains have been obtained. The reptiles were numerous, but markedly unlike, as a whole, to those of Secondary times. Among them are numerous turtles and tortoises, two species of crocodile, and a sea-snake (Palceoplzis), estimated to have been 13 feet long. Remains of birds have also been met with; one of these (L2'tI¢orm's zzultm-inus) appears tohave been allied to the vulture, another (Halcyornis toliapicus) to our modern king-fisher, besides waders and other types. The mammals numbered among their species a hog (I13/rcv cot/cerium), several tapirs (Cor;/plzodon, &c.), an opossum (D-iclelp/:3/s), and a bat. The carcases of these animals must have been home seawards by the great river which trans- ported so much of the vegetation of the neighbouring land. Jl[id(lle.—The Bagshot group consists of sand and bands of clay which, in the Hampshire basin, are tolerably fossili- ferous. As developed in the Isle of Vight and at Bourne- mouth, their lower members have yielded a large number of terrestrial plants, among which the P7'ot€(tc€(e are still numerous, together with species of fig, cinnamon, fan-palm (Sabal), oak, yew, cypress, laurel, lime, senna, and many more. Crocodilian forms still haunted the waters, and have left their bones with those of sea—snakes and turtles and of the tapirs and other denizens of the land, which still, as in the time of the London Clay, continued to be washed out to sea. Among these strata we now find one of the most typical organisms of the Eocene rocks of the Mediterranean basin, a foraminifer termed ]Vm2mz-zclites (IV. lwzvigatct, N. scabra, N. wriolaria). Characteristic fossils are Voluta atlzleta, V. luctatriac, Jllurex asper, Fusus lon_70cz'-us, Cardita planicosta, and Clzama sqmzmosa. Uppe‘r.—The strata of this division of the English Eocene formations are entirely confined to Hampshire and the Isle of Wight. They consist of sands, clays, marls, and lime- stones, in thin—bedded alternations. These strata were accumulated partly in the sea, partly in brackish, and partly in fresh water. They were hence named by Edward Forbes the fluvio-marine series. Among the marine fossils are— Fusus porrectus, Oliva Branclcri, 1l'atz'ca labellata, Ostrea crtllzferrz, and £l'2n2mmlites lcrv2'_r/atcl. The genera Cerit/u'um, Potamomg/a, Paludina, Planorbis, L2'7n71aa_. and Cg/clas are abundant, showing the brackish and fresh-water conditions in which many of the strata were deposited. Remains of turtles, snakes, crocodiles, and alligators continue to occur in these Upper Eocene beds. With these are found the bones of several very characteristic mammals also met with in the Paris basin,———the three-toed I’al<zaotlzerz'um, resem- bling the living tapir ; .-lnaplotlzerizmz, a more graceful
X. — 46Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/375
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