1847.] PRESTWICH ON THE LONDON CLAY. 363 



and a few subordinate sandy beds, the whole forming a total thickness 

 of 300 feet. Sometimes the clays are of a dark grey, but brown is the 

 predominating colour. As a mass the mineral character is almost 

 entirely argillaceous, carbonate of lime though present being in com- 

 paratively very limited quantity, and the admixture of green and 

 yellow sands occurring chiefly at the top and bottom of the deposit. 



Another lithological character is the presence of two or three layers 

 of round black flint pebbles, as well as their occasional dissemination, 

 denoting an increased transporting power of water, but no change of 

 condition. They are more numerous at Alum Bay than at White- 

 Cliff Bay, and still more so at Southampton. 



We will now pass over to the London district, and examine the 

 strata there superimposed to the mottled clays. Commencing at 

 Basingstoke and Odiham, we meet with in this position a thick and 

 well-marked deposit of brown and bluish grey clays. To their litho- 

 logical characters we will first confine ourselves, and ascertain whether 

 these present any discrepancies with the Bognor beds of Hampshire. 

 So far from this being the case, their agreement is remarkable, — far 

 greater than could have been expected. 



This great argillaceous deposit commences here, as in the Isle of 

 Wight, with a mineral character different from that of the beds below 

 it. At its base, a bed of sand with rolled flmt pebbles almost invariably 

 occurs, indicating therefore, along with other phsenomena, that the 

 London tertiary district probably underwent at this period a change of 

 condition similar to that we have shown to have occurred in Hamp- 

 shire. 



Thus at Chinham near Basingstoke, we find the mottled clays and 

 sands overlaid by a yellow clayey sand, with a few pebbles and patches 

 of green sand, passing upwards into the mass of the London clay ; at 

 Itchingwell, near Kingsclere, by a thin band 9 inches thick of pebbles, 

 over which is a brown sandy clay passing upwards into a tough brown 

 clay, forming the lower part of the London clay ; at Pebble Hill the 

 same layer of pebbles, overlaid by dark sandy clays with London clay 

 fossils. At Reading also the mottled clays are capped by brown clays 

 and sand, with pebbles and patches of green sand, and again at Son- 

 ninghill : so also at various places along the southern edge of the 

 London district, as at Guildford. A similar mechanical structure pre- 

 vails at the base of the London clay at Hampstead, where however 

 the sand and pebbles are cemented by carbonate of lime, a character 

 of not unfrequent occurrence, forming m such instances large tabular 

 masses, as at Hedgerly, Sonning, and various other places around 

 London, and in Hampshire at Alderbury near Salisbury, and South- 

 ampton. In all these places this conglomerate band is usually very 

 fossiliferous. (See PI. XIV. Comp. Sec. 6, 7, 11. point J.) 



Further, the peculiar water-worn and irregular surface of the 

 mottled clays and sand exhibited at their junction with the brown 

 clay deposit in the Isle of Wight is also distinctly visible in the London 

 district. This was very apparent in the long railway cutting at Sonning 

 near Reading. The whole line of junction was there uneven and wavy, 

 forming depressions and curves as mach as 1 or 2 feet deep. This 

 structure is also observable in a lesser degree at Chmham near Basing- 

 stoke, Pebble Hill, and Guildford. 



VOL. III. — ^PART I. 2 C 



