1847.] PRESTWICHON THE LONDON CLAY. 361 



ness of two feet, forming a section agreeing perfectly with those of 

 Newbury and Hungerford, where also the band of oysters is far from 

 persistent. (See Plate XIV. Comp. Sec. figs. 5 & 7, points '' e.") 



The structure of the south end of the Kembridge cutting is that 

 which commonly prevails throughout the Hampshire tertiary series 

 south of this locality ; whereas, as modified at the northern end, it 

 represents that of a large portion of the London tertiary district. 

 This section therefore affords, in conjunction with those of Alum and 

 White-Cliff Bays, Southampton, &c., some stratigraphical and zoolo- 

 gical evidence that this lowest division, both in Hampshire and west 

 of London, commenced contemporaneously under very similar condi- 

 tions, and in apparent connexion. 



This point thus probably indicating the early co-relations of these 

 two districts, affords a good base-line from which to measure their 

 progressive development. 



Overlying the sands with Ostrea hellovacina, or rather in further 

 development of the same series, are the mottled clays with interca- 

 lated sands. These are seen in their greatest expansion, and in their 

 most argillaceous condition, at Alum and Wliite-Cliff Bays, and re- 

 appear, I infer, in lesser thickness and more sandy, but in similar 

 superposition, throughout the western portion of the London tertiary 

 district (see Plate XIV. Comp. Sec. fig. 2 to 11). It may be. ob- 

 jected here, that the more variable beds of sands and mottled clays of 

 the London district do not well represent the purer mottled clays of 

 Hampshire ; that the preponderance of sands mark them as a differ- 

 ent group, referable rather to the great central mass of variegated 

 sands and clays at Alum Bay, with which they have been hitherto 

 grouped. Against this it can be shown that the mottled clays and 

 associated sands are subject to great and rapid variations of thickness 

 and structure, both from having been deposited on the inequalities of 

 the chalk, and from their apparently irregular original drift. At Alum 

 Bay they are 86 feet thick ; at White-Cliff Bay, 140 feet ; at South- 

 ampton, 100 feet; at Clarendon Hill, near Salisbury, apparently not 

 more than 50 or 60 feet ; at Pebble Hill, near Hungerford, 45 feet ; 

 whilst at Tilehurst, near Reading, they are said to be 150 feet thick. 



The variation in the mineral character is of little importance. At 

 Pebble Hill we see them subordinate to the beds of sand ; at New- 

 bury the sands and mottled clays are about of equal thickness, and 

 form two beds only ; at Reading we have five beds of mottled clays 

 alternating with seven beds of sand ; at Sonning Hill the sands have 

 almost thinned out, and at Twyford we again have 40 or 50 feet of 

 pure mottled clays, whilst near London they are frequently 80 to 1 00 

 feet thick. In fact, they present a remarkably miiform mineral type 

 of beds of red mottled clay and of siliceous hght- coloured sands, merely 

 subject to variations of proportions. 



They are also characterized throughout the Hampshire and Lon- 

 don districts, whether they are formed of clay beds only, or consist 

 of alternating beds of clays and sands, by the same remarkable ab- 

 sence of animal remains. 



