364 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 26, 



At all these places in the London district where the superposition 

 can be traced, these pebble beds are subordinate to sands chiefly yel- 

 low, but sometimes green and dark grey, laminated with brown clays, 

 passing upwards into beds of the same colour and entirely argilla- 

 ceous, with subordinate portions of a dark bluish grey colour, fre- 

 quently very sandy. These strata constitute the great bulk of the 

 London clay. Its upper beds are again generally brown, and near the 

 top usually mixed with light-coloured sands in sufficient quantity to 

 form a good brick clay without any further addition of sand. 



Layers of Septaria occur irregularly throughout the London clay. 

 At the western end of the district the clays frequently contain black 

 flint pebbles, sometimes forming thin layers in the clay, at other times 

 occurring, slightly dispersed, throughout considerable vertical masses. 

 Nearer London these pebbles are of less frequent occurrence. 



If, further, we compare the development and thickness of the Lon- 

 don clay of London and the Bognor beds of Hampshire, we find the 

 analogy continued ; and as this is a point of some importance to the 

 argument, we will confine ourselves to the few sections where we have 

 positive and complete data. 



We have already seen that at Southampton these clays are nearly 

 300 feet thick. The first measurement we can obtain in the London 

 district is at Dogmersfield, near Odiham, where a well has shown these 

 clays to be 320 feet thick, — a very close approximation to the South- 

 ampton section; and at both places they are equally well defined and 

 limited by the sands above and the mottled clays below. Proceeding 

 towards London the clays gradually become thicker ; measuring 400 

 feet at Chobham, and at Highgate and Hampstead about 400 feet* ; 

 the same beds above and below, as at Odiham, serving as hmits of 

 vertical range. (See Plate XIV. Gen. Sec. fig. 1 to 11.) 



We thus have, in mere physical conditions, a fair " a-priori " case 

 for placing the London clay of the neighbourhood of London on a 

 level with the Bognor clay in the Hampshire series. Their struc- 

 ture is analogous, their superposition alike, and their accumulation 

 was apparently nearly coeval in time and equal in mass ; while so far 

 as these conditions are concerned, the presumed synchronism of the 

 Barton clays and Bracklesham sands with the London clay would 

 present great anomalies. 



Organic Remains. 



The foregoing hypothesis of structure, if correct, ought to be cor- 

 roborated and proved by an examination of the organic remains. 



We will commence with an estimate of the total number of species 

 found in the Hampshire and in the London series, and then consider 

 separately the numbers occurring respectively at Barton, Bracklesham, 

 and London ; examine their affinities, and see if any and how many 

 species are peculiar to each of these groups of strata ; and investigate 

 the validity of the palseontological evidence upon which their co- 

 relation stands f. 



* At some places around London they are occasionally thicker than this, 

 t In the determination of the species occurring in the several localities, the 

 Catalogue of British Fossils of Mr. Morris was of the most essential assistance. I 



