200 J. S. GARDNER ON THE LOWER LONDON TERTIAR1ES. 



its bulk to have been hardly inferior to that of the largest existing 

 rivers. 



If we admit the preponderating action of this stupendous river in 

 the formation of all Eocenes in England and France, their strati- 

 graphy becomes comparatively simple ; but if we try to explain them 

 by any other means, we are forced to suppose conditions which have 

 no parallel at the present day. 



With the probable physical features of our area during the Eocene 

 period before us, we are better able to appreciate the relative values 

 of the divisions of the Lower London Tertiaries. 



The Lower London Tertiaries are divided by Prestwich and the 

 Survey into the purely marine Thanet Beds, the nuviatile, estuarine, 

 and marine Woolwich-and-Eeading Beds, and the marine Oldhaven 

 Beds. The object of the present communication is more especially 

 to question whether more than one Eocene sea encroached upon 

 our area prior to the sea w T hich deposited the London Clay. 



The Thanet Beds. 



The limits of the area over which the Thanet Beds were deposited 

 were pretty accurately traced by Prof. Prestwich ; and it is quite 

 unlikely that they ever had any very considerable extension beyond 

 their present limits as mapped by the Survey. They are almost 

 wholly unfossiliferous west of Rainham* in Kent, and, indeed, present 

 no features of interest outside the Canterbury and. Isle-of-Thanet 

 districts. The following diagram from the Survey Memoir of 1876, 

 p. 56, explains their distribution. 



Pig. 1. — Diagram illustrating the- Distribution of the Thanet Beds. 

 w. E. 



Epsom. Rochester. Sittingbourne. Canterbury. 



I ill 



a 



e. Fine sand with occasional layers of sandstone, with fossils, sometimes silici- 



fied, thins westward, from 40 feet in thickness, and passes down into 

 d. Bluish-grey sandy marl, with green grains and fossils, thins westward, and is 



almost confined to East Kent. 

 c. Pine sand, without fossils ("at least but a very few remains have been 



found ") ; it is 60 feet thick in West Kent, where it forms nearly the whole 



of the Thanet Beds, thinning out westward, in Surrey and eastward in East 



Kent. J 



b. Alternations of brown clay and ioam, without fossils, thin and local (in part 



of East Kent). * 



a. The "base-bed," clayey greensand with unworn green-coated flints resting on 



the Chalk, thin (rarely over 5 feet), but constant. 



The division c is typically exposed in railway-cuttings, especially 

 about Swanley and St. Mary Cray on the London, Chatham, and 



* Otterham Quay, E. of Kainham, is the spot furthest west, except Upnor, 

 from which fossils are recorded by the Survey and by Prestwich. 



