WOOLWICH AND BEADING SEKIES. 
269 
but sometimes brackish. At Woolwich a body of river-wa¬ 
ter must have flowed jDermanently into the sea where the 
CyrencB lived, and they may have been killed suddenly by 
an influx of pure salt-water, which invaded the spot when 
the river was low, or when a subsidence of land took place. 
Traced in one direction, or eastward towards Herne Bay, 
the Woolwich beds assume more and more of a marine char¬ 
acter ; while in an opposite, or south-western direction, they 
become, as near Chelsea and other places, more fresh-water, 
and contain Unio^ Paludina^ and layers of lignite, so that 
the land drained by the ancient river seems clearly to have 
been to the south-west of the present site of the metropolis. 
Fluvictiile Beds underlying Deep-sea Strata, —Before the 
minds of geologists had become familiar with the theory of 
the gradual sinking of land, and its conversion into sea at 
difierent periods, and the consequent change from shallow 
to deep water, the fluviatile and littoral character of this 
inferior group appeared strange and anomalous. After pass¬ 
ing through hundreds of feet of London clay, proved by its 
fossils to have been deposited in deep salt-water, we arrive 
at beds of fluviatile origin, and associated with them masses 
of shingle, attaining at Blackheath, near London, a thickness 
of 50 feet. These shingle banks are probably of marine ori¬ 
gin, but they indicate the proximity of land, and the exist¬ 
ence of a shore where the flints of the chalk were rolled into 
sand and pebbles, and spread over a wide space. We have, 
therefore, tirst, as before stated (p. 268), evidence of oscilla¬ 
tions of level durino; the accumulation of the Woolwich se- 
ries, then of a great submergence, which allowed a marine 
deposit 500 feet thick to be laid over the antecedent beds 
of fresh and brackish water origin. 
Thanet Sands (C. 3, p. 252).—The Woolwich or plastic clay 
above described may often be seen in the Hampshire basin 
in actual contact with the chalk, constituting in such places 
the lowest member of the British Eocene series. But at 
other points another formation of marine origin, character¬ 
ized by a somewhat diflerent assemblage of organic remains, 
has been shown by Mr. Prestwich to intervene between the 
chalk and the Woolwich series. For these beds he has pro¬ 
posed the name of “ Thanet Sands,” because they are well 
seen in the Isle of Thanet, in the northern part of Kent, 
and on the sea-coast between Herne Bay and the Reculvers, 
where they consist of sands with a few^ concretionary masses 
of sandstone, and contain, among other fossils, Pholadomya cu- 
neata,^ Cyprina Morrisii,^ Corhula longirostris^ Scalctria Boicer- 
hanhii,^ etc. The greatest thickness of these beds is 90 feet. 
