November 2, 1891.] 



KNOWLEDGE 



217 



sinks beneath the surface at a line joining the towns of 

 Epsom, Sutton, Croydon, Orpington, and Sevenoaks, and 

 to such a depth that, at the spot where a shaft was sunk 

 by the Southwark and Vauxhall Water Company at 

 Streatham a year or two ago, the chalk was only arrived at 

 after traversing ^41 feet of tertiary beds of clay, gravel, 

 and sand. When, however, we pass beyond the northern- 

 most limits of London, we again find the chalk immediately 

 beneath our feet, cropping out from beneath these tertiary 

 beds, at a line joining the towns of Watford, Eickmans- 

 worth, Beaconstield, Marlow, Maidenhead, on to Reading 

 and Hungerford, and forming the long southern slopes of 

 the Chiltern Hills. 



Thus we see that the whole of London, and the beds on 

 which it is built, are held in the hollow of a trough in the 

 chalk, the hills forming a natural boundary both north 

 and south. 



The beds above the chalk may be classified as follows :" — 



1. 



f Made Earth, 

 Alluvium. 

 „ j Brick Earth, 

 (Gravel and Sand. 



3. London Clay. 



4. Oldhaven (Pebble) Beds. 



5. Woolwich & Reading Beds. 



6. Thanet Sands. 



k detailed sketch of each of these is scarcely called for 

 here ; a few characteristics, however, and the places where 

 they can be seen may be of interest. 



The Thanet Sands (6) are one of the most well-marked 

 divisions of the tertiary strata. They attain their greatest 

 thickness on the north-east coast of Kent, being 90 feet 

 thick in the Isle of Thanet, from which they take their 

 name. As we approach London in a \\'esterly direc- 

 tion the bed gradually thins out, so that when we reach 

 the Bank of England it is only 40 feet thick, and at 

 Ealing, to the west of London, it has but a thickness of 

 eight feet. The materials of which the bed is composed 

 have a special characteristic attaching to them, and one 

 which serves to show, to some extent, the source from whence 

 the band was derived before being laid down by the sea which 

 once covered them. Under a microscope the particles of 

 sand are ascertained to be of a regular crystalline form, 

 altogether unlike the rounded grains of sands of which, for 

 instance, the Brighton sands are composed. Now in Belgium 

 there is a wide extent of country, the surface of which is 

 composed of primary crystalline rocks, containing a large 

 proportion of quartz, which is exactly the same as sand 

 and flint in its chemical composition. This fact, together 

 with the fact stated above, that the sands are thickest in 

 the east and thin out to the west, serves to show that the 

 sea denuded the crystalline rocks and washed the quartz 

 westward, laying it down where we now see it, but only 

 transporting the material in diminishing quantities to the 

 west, the beds altogether dying out beyond Richmond. 

 There are some sand pits at Charlton, near Woolwich, 

 where it has a thickness of some 50 feet. Large quan- 

 tities are there quarried and shipped as ballast by vessels 

 going down the Thames. The vertical clifl' caused by these 

 quarrying operations shows us the topmast layer of the 

 chalk at its base, whilst between the two is a thin layer, 

 perhaps six inches thick, of flints. This layer is no doubt 

 owing to the soft chalk having been washed away, the 

 heavier fiints being left behind by the water which denuded 

 the chalk. 



At Charlton the beds known as the Woolwich series (5) 

 arc also well developed. Similarly they are to be seen in 

 full force at Castle Cliff, Newhaven, and at Scaford. The 

 most remarkable feature about these beds is that they 

 include layer above layer of shells, packed tightly together 

 in a matrix of clay. When a mass of this is dried it bears 



' A tier Wliilukoi-. 



a close resemblance to the shell-marble found in the Weald, 

 and known as Sussex or Petworth marble. Much of the 

 shell-sand used largely in London for spreading over 

 garden-paths is obtained from these beds. 



The Oldhaven Beds, next to be mentioned, are recog- 

 nisable at once in the neighbourhood of London. In a 

 matrix of clayey sand are contained large quantities of 

 rounded pebbles not larger than a hen's egg, and generally 

 smaller. These pebbles are noticed in large numbers in 

 Greenwich Park. Blackheath itself sometimes gives its 

 name to the series, whilst those who have visited Croham 

 Hurst, to the south of Croydon, cannot but have noticed 

 the quantities of pebbles which are trodden under foot. 



All these formations, however, sink into insignificance 

 beside the deposit of London Clay (3), which, in some 

 places, is as miach as 90 feet thick. In the London Basin 

 it covers a great part of the surface, and it is also to be 

 found in full force in Hampshire, extending beyond the 

 Solent into the Isle of Wight. Looking at a geological 

 map of the country one cannot help being struck by a 

 conviction that these deposits were once continuous with 

 each other, and that the whole of the area between Hamp- 

 shire and the Thames has since been so completely washed 

 away that now not a trace is to be found in that part of 

 the country. The character of the fossils which the 

 London Clay affords, at once point it out to have been 

 deposited in a sea perhaps quite as deep as that which in 

 a pre\'ious age had laid down the chalk. Most of the low 

 hills around London are formed of this clay, such as 

 Shooter's Hill, the Sydenham Hills, Primrose Hill, &c. 

 The London Clay is a very stiff clay, and is imper\-ious to 

 water, consequently the surface is always more or less 

 damp, although the effects of this are greatly modified in a 

 large part of South London, owing to a subsequent depo- 

 sition of gravel above it. 



After the London Clay had been gradually accumulating 

 for many ages beneath the ocean, the bed of the sea came 

 to be gradually upheaved, until at last it appeared as dry 

 land, the greater part of what are now the British Isles 

 probably partaking in the upheaval. Thus, during the 

 Miocene age which followed, no deposits were laid down 

 in the neighbourhood of Loudon, although during the 

 succeeding Pliocene age the coast-line again approached 

 sufficiently near to allow of the deposition by the sea of 

 those beds known on the Norfolk and Sufi'olk coasts as 

 the Coralline and Red Crags. A gradual declension of 

 climatal temperature had been going on since the tropical 

 times of the London Clay, through the sub- tropical 

 Miocene age, and the temperate Pliocene era, until now at 

 the ushering in of Pleistocene times, the climate was not 

 far short of arctic, and indeed the coimtry, as it then 

 was, soon became covered by an extensive ice-sheet, and 

 glaciers came sliding down from the higher grounds 

 bringing with them the rocks of their place of origin, and 

 depositing their burdens as they melted in places far 

 removed from the land of their birth. 



In the North of England the result has been that thick 

 banks of clay known as " boulder clay " and •' till " have 

 been formed, whilst imbedded in them have been found a 

 few species of shells of a distinctly arctic character, and 

 such as are not now found on the English coasts at all. 

 In the north of London, on the hills of Hampstead and 

 Highgate, a capping of this boulder clay is to be found, 

 the valley of the Thames being a rough boundary corre- 

 sponding to the southernmost edge of the sphere of glacial 

 influence. 



At last the glacial epoch passed away, and with the 

 melting of the ice-sheet the laud became raised above the 

 level of the sea. Tiie Thames, charged with an increased 



