1815.] Transactions of the Geological SocletT/, Vol. II. 451 



terminate on the south side of Alum Bay, and on the nortli side of 

 tlie chalk there occur a great number of perpendicular beds of clay 

 and sand, often containing abundance of loose pebbles. Mr. 

 Webster conceives it iinpossible that these beds could have been 

 formed in tiie position which they now occupy. He supposes that 

 they were at first placed horizontally over the chalk, and that they 

 were thrown into their present position by some unknown convul- 

 sion. Immediately to the north of these beds is Headen Hill, 

 composed of beds nearly horizontal. These beds Mr: Webster 

 considers as formerly lying over the perpendicular beds of Alum 

 Bay, before they acquired their perpendicularity. 



The perpendicular beds at Alum Bay are analogous to the lower 

 marine formation in the Paris basin ; for all the fossils which they 

 contain, though different from those in the chalk, are of marine 

 origin. The lower fresh-water formation is distinctly seen near the 

 bottom of Headen Hill. It consists of a series of beds of sandy, 

 calcareous, and argillaceous marls, sometimes mixed with brown 

 coal. The thickness of these beds is G3 feet. It contains so many 

 fresh-water shells, and so regularly deposited, that we cannot 

 suppose them to have been carried by rivers into the sea. Besides, 

 in that case we should find a mixture of sea shells, which do not 

 occur in this formation. 'J'his is covered by tiie upper marine for- 

 mation. It consists of clay and marl, is aliout 90 feet thick, and 

 contains a prodigious quantity of sea shells. This is covered by the 

 upper fresh-water formation, about 7^ feet thick, and consisting of 

 alternate beds of sand, lime-stone, and clay. It contains abundance 

 of fresh-water shells, without any admixture of sea-shells. Tliis 

 formation is covered by alluvial soil. 



Mr. Webster conceives the Isle of Wight basin to have been 

 formerly filled with these formations. It consisted of the northern 

 half of the Isle of Wight, extended as far west as Dorchester, and 

 as far cast as Shoreliam, being bounded on the north by the hills 

 which constitute the South Downs. 



The London basin is of much greater extent, but not so well 

 defined. Its southern boundary is matked by Deal, Canterbury, 

 Milton, Chatham, Gravesend, I'lMi'leet, from wliich it nnis south- 

 westerly to Lcatherhead and Guildford, and then bending northerly 

 it proceeds as far west as Hungcrfurd. Maidenhead, Eton, St. 

 Alban's, mark part of its northern ijoundary, and it appears to take 

 in the whole counties of Es«?x, Suffolk, and Norlolk. The bottom 

 of this basin is chalk. Over the chalk lies a bed of sand, or in 

 some places of plastic clay. Over this lies tlie London clay, well 

 known for the great number of marine remains which it contains. 

 This clay is mo'-tly covercrl I)y a bed of gravel of various thickness, 

 obviously alluvial. Mr. V\ ebstcr considers the London clay as 

 analogous to the upper marine formation in the Paris basin. The 

 formations fonnerly placed over tlw; London clay he considers as 

 having been removed by some coiivuls'on. 



Vlil. Remark:; ou the F'UuJied Forts of Scollaiid, By Dr. 



L' K 2 



