ALLUVIAL DEPOSITS. 
17 
nea palustris^ Limnea limo.sa, V'alvata piscinali.s, 
Paludina impura ; in the lower part, Lutraria com- 
pj’essfiy T'ellina aoUdula^ Cardium edule, Turbo ulva^ 
and the Indmiay or cases of the larvae of Phrpga- 
necPy in ahimdance, with minute shells of the genera 
Planorhis and Limnea adhering to them, the inter- 
mediate layers containing an intermixture of both. 
Hence it appears, that after the catastrophe which 
broke through the chalk hills, and thus formed the 
transverse valleys of the South Downs, the basins of 
the chalk were filled with salt water ; the currents 
of fresh water flowing from the interior brought 
down clay, silt, and decaying vegetables, and soon' 
occasioned an intermixture of lacustral testacem, 
and at length so far changed the nature of the ele- 
ment, as to render it fit for the habitation of fresh- 
water shell-fish only.* The transition of the 
ancient lake into a narrow river has probably 
been occasioned partly by natural, and partly by 
artificial causes ; within the last fifty years the 
levels were covered with water, during several 
months in the winter season ; and even now it 
requires all the resources of art, to confine the 
river within the limits of its own bed.t 
* This conclusion naturally results from the occurrence of marine 
shells in the lower beds only, and of freshwater in the upper, the two 
being intermixed in the intermediate layers, since the experiments of 
M. Beaudant have shown, that if freshwater mollusca be suddenly in- 
troduced into sea water, they die in a very short time ; but if the fresh 
water be gradually impregnated with salt, they will live in it when 
of the strength of sea water withqut any injury : the same experiments 
repeated on freshwater mollusca gave similar results. — Vide Anna!, 
de Chim. et Physique, ii. 32. 
f The alluvial deposits above described are clearly of very remote 
antiquity ; as is evident from the superficial situation in which ancient 
C 
