398 PROCEEDINGS AT MEETINGS OF

"Tliat floods, probalily ansing fn.)in the causes I have mentioned, did sweep down our valleys seems j^roved by the fact, that particularly in the naiTow valleys of the Avon, all the hill-sides against which the stream would have imi>inged present bold esoiu-jjments, whilst the hills at the sides of the valley not exposed to this wearing action have the usual Kwelling outline so characteristic of a chalk district. " But we have not only to take into account this far greater mechanical excavating action of the Quaternary rivei*s. The surface of the chalk beneath the gravel is extremely uneven, and deep 'put-holes' are of frequent occurrence; some of these extend downward into the very sul>- Ktance of the chalk to the depth of from 20 to 30 ft. This wiusting of the chalk is due, nut to any mechanical force, but to a powerful chemical erudiiig action arising from the presence of carbonic acid in the water, and this chemical action was doubtless a very active accent in deepening our valleys. The rain that falls upon our downs by contact with decay- ing vegetable matter bccomt's charged with carbcjuic aci'i, and, in conse- quence, acquires the property of iicting as a solvent of chalk or any other calcareous rock with wiiich it may come in contact Nor is the quantity thus removed year after year inconsiderable, for every gallon of spring water, in a chalk district, contains about seventeen grains of carbonate of lime, and by calculation it has been found that in each square mile of such a district upwards of one hundred and forty tons of chalk are thus dissolved, and carried away l)y our rivers, annually.' " But when due allowance has been made for the great excavating power of the Quaternary rivei"s, there still remains the fact, that, since ti»e i'ahcolitliic implements fuund in the valley-gravels were fiishioned, a period has elajjsed uf sufficient duration to deepen our Wiltshire valleys some 70 or 80 ft.

• ' There are also other indications of the anticpiity of the Fisherton bods,

and conseiiuently of the Hint im[)lements found in them. ' Ilivoi-s in a state of flood, or passing even at a moderate s[)eed over soft or inco- herent soil, are always turbi<l, owing to the presence in their waters of earthy matter which they are transporting towards the sea.' ' " This solid matter, l>eing only held in suspension by the velocity of the current, smks to the bottom, and forms shallows or l)anks, wiien the waters from any cause become still. Hero and there, at the sides of our Wiltshire valley.s, are patciies of bri;k-eu'th which were deposited, in the manner rlescrilml, i)y tiie Qiiattjruiry rivers. " Tlic streams of the Avon and Wiley unite at Fisiiorton, and the very extensive deposit of brick-o irth that occurs thoro indie itos the position of the still water which in iUo (Quaternary pi-rio I existe I behind their actual point of union. Tins brick-earth attains a miximnin thickness of no less than 30 ft. in Harding's an<l in Baker's j»its. . d yet tiiis de- posit is evidently U'lt the result of citadysmic action, for delicate and minute shells perfectly uninjured occur throughout tho doposil. .Iore- t)er there is distiui-t liimiii itiou in the strata, showing that if was a • Kvanii, "Ancient Stone Iwi|il<*tnrMtN," if 1,200 yanlM, fino j^ravd; nml if ft Iitlli» |>. 4'<!1'. iiv<T Iwii niilun |M>r lioiir it in I'lipiiblu uf ' Ihid. , p. Hf^'.i. If tin* velocity f>f ft rivfT trnnH|Mirtinf( »hiviTy ftngiiiiir ntunoA oi b« 30u jftribt [H)r hour, it io Miiflicinnt to tlio niiu uf An «gg. tvar ii|> Hnu cUy; if 'iOO yanU, lin<< A.kn<l;