166 J. L. LOBLEY — THE EOCENES OF ENGLAND 



Clay epoch. In Sheppey the remains of more species of turtles 

 than are now known to exist have been found. Nautili and volutes 

 and allied genera are abundant in the London area ; while the fossil 

 fruits of the palm {Nipadites) are extremely common in the 

 Sheppey district. 



Well-sinkings in the London district give a maximum thickness 

 of 400 feet to the London Clay. This is much diminished in the 

 lower parts of the Thames Valley, which has been cut out of this 

 formation by the river. Thus, near the river we find the clay 

 about 200 feet thick, while at Hampstead it is about 400 feet. The 

 upper beds are sandy in character, and contain a small Pcdunculm 

 {P. decussatiis) in abundance, with nautili of several species. 



It is an interesting question, "Whence came this large accumulation 

 of argillaceous matter ? Since each of the sedimentary rocks is made 

 up of the waste of those deposits which have formed the land of 

 the period of its deposition, we must look to older rocks than the 

 London Clay to find the source of the material of which it is com- 

 posed. The decomposition of one of the hardest rocks — granite — 

 affords a pure clay, and doubtless a large portion of the London 

 Clay was derived from the waste of the granitic rocks to the west, 

 though much may have come from Gault and Oxford Clay areas. 



The London Clay is not made up entirely of argillaceous matter, 

 for wo find in it concretionaiy masses of carbonate of lime, with an 

 abundance of crystals of selenite and nodules of iron pyrites. The 

 first and last of these mineral concretions are so abundant that in 

 Sheppey the disintegration of the sea-cliff's yields them freely enough 

 for their collection to form a considerable industry. The concretions 

 of carbonate of lime (known as septaria) are ground for cement, 

 and the iron pyrites is used for the manufacture of sulphuric acid. 

 Of the age of the London Clay also, at Bognor in Sussex, siliceo- 

 calcareous masses form what is called the Bognor Kock, which is 

 so hard as to well withstand the waves of that exposed coast. 



Though the fossils are of many species and very numerous, they 

 are not scattered uniformly through the formation, biit are in 

 colonies as it were. Thus we have Chelonia and plant-remains 

 abundant in Sheppey, nautili and volutes common at Highgate 

 in the upper beds, Ditrupa in masses in a band in the Isle of 

 "Wight, fish and other vertebrate remains in the lower basement 

 beds. The following are a few of the characteristic fossils of 

 the London Clay : — Vcrtebrata : Hyracothcrium leporinum, Chelone 

 hreviceps, Palmoplm Toliapicus, Lamna elegans. Invcrtebrata : 

 Nautilus imperidh's, Aturiaziczac, Vohcta nodosa, Aporrhais So/rcrhi/i, 

 Roiiti'Il(iri(( r/u/osfi, P/io/ai/owi/a uiarf/ar/faofa, rccfiuiculas decioisatus, 

 VcDittcuhiria bognoriensis, Ditrupa plana, Ophiura IVetherelli. 

 Plants : Nipadites ellipticus, Ptcrophiloides Richardsoni. 



The Middle Eocenes. 



The Middle Eocenes of England have a total thickness equal to 

 that of the Lower, and consist of tlie Lower Bagshot Sands, the 

 Bracklesham and Barton Beds, and the Upper Bagshot Sands. 



