450 EOCENE PERIOD. CHAP. 



sandy, the lower part strong clay, with septaria and bands of shells. 

 This clay, cut through many years since in the ' Archway' at 

 Highgate, furnished many shells which were figured in the early 

 numbers of Sowerby's valuable Mineral Conchology. This ' London 

 clay' is about 400 feet thick at Highgate, and nearly 500 feet 

 in Sheppey Island, where it yields abundance of fruits and some 

 animal remains indicative of warm climate h . 



It is covered at Hampstead considerably, and at Highgate 

 partially, by a sandy deposit which is more extensively spread about 

 Bagshot Heath, and therefore called by that local name. In all 

 this tract it is but little fossiliferous ; but on the south coast, as 

 in Bracklesham Bay, where the full type seems to be exhibited, 

 fossils are very numerous, and on the whole differ considerably from 

 those of the London clay 1 . The highest land which it reaches 

 in this area is the summit of Hampstead Heath, 430 feet above 

 the sea. 



This is the latest eocene deposit in the London basin. There 

 are no meiocene or pleiocene beds, as those terms are commonly 

 understood, nor any sure indication that such ever existed here. 



In considering these remarkable strata, which were accumulated 

 in a period so near, geologically speaking, to our own, we are 

 presented with problems of great interest, which, if they can be 

 solved, will have more than local application. 



Whence came the materials, the clay, the sand, the pebbles; 

 in what direction ; by what forces urged ? what were the tracts 

 of sea and land ; how deep the water, how high the land ? what 

 is the explanation of the appearance of fluviatile shells among 

 oceanic exuviae? 



Of the materials a great part can be found in the country 

 surrounding the drainage of the Thames on the north, west, and 

 south; some of them can be had only within the drainage, as 

 the flint pebbles which are certainly the gift of the neighbouring 

 chalk, and specially the upper chalk. The clays and sands can 

 be sufficiently matched by deposits of the earlier cretaceous, oolitic, 

 and liassic ages, and conditions of land and sea may be imagined 

 such as to allow of these materials arriving by ordinary means 



h At High Beech, and Langdon in Essex, greater heights are assigned by the 

 Ordnance Survey, but the measurements are not free from doubt. 

 1 Prestwich, Journal of Geol. Soc. vol. x. p. 434. 



