Dr. Aubrey Strahan — Geology at the Seat of War. 71 



the close correspondence of the strata in detail gives a peculiar interest 

 to a study of these parts of England at the present time. 



In the Loudon Basin we recognize three groups in the lower 

 Tertiaries, and in the Hampshire Basin the same three groups with 

 other higher Tertiary strata which have been removed from the 

 London area by denudation. The lowest group, resting directly upon 

 the Chalk, is known as the Reading Beds and Thanet Sands with pebble 

 beds. The thickness of the group amounts to 100 feet in places. At 

 its base lies a layer of unworn chalk-flints coated with a green silicate 

 of iron, and interbedded in the sands are clays, often of a vivid red 

 colour. 



These characters are reproduced in the Landenien of Belgium. The 

 interbedded clay is known as the Argile de Louvil. 



The Beading Beds pass below the London Clay, and under suitable 

 circumstances the water in them is held down under pressure by that 

 impervious covering. In such cases, when a hole is bored through 

 the clay, the water rises from the sand and overflows at the surface. 

 This used to be the case years ago in parts of London, and well- 

 borers sometimes found themselves in unexpected possession of an 

 uncontrollable fountain which flooded their own and their neighbours' 

 premises. On the Continent an artesian supply is still available in 

 suitable conditions within the margin of the Tertiary basin. The 

 water, however, is potable only near the margin ; in the inner 

 parts of the basin, far away from the outer edge, it is too heavily 

 loaded with mineral matter to be usable. 



The London Clay, which comes next above the Thanet Sand, has 

 a thickness of 400-450 feet. It corresponds in character, thickness, 

 and fossils to the Argile de Flandres, or the Ypresien of Belgian 

 geologists, except that in Belgium it consists in the upper part of 

 alternating bands of sand and clay. London Clay has its uses. 

 Almost the whole of the system of tube-railways under London has 

 been constructed in this watertight material. The earlier under- 

 ground railways, the sewers, and other works were situated nearer 

 the surface, and encountered large quantities of water in the 

 superficial gravels ; the tubes were protected by clay above and 

 below, except in a few exceptional localities. 



The Bagshot Sands come next above the London Clay. These 

 attain a thickness of approximately 1,000 feet in Hampshire, but in 

 the London Basin have been for the most part denuded away. Parts 

 of them, however, still remain near Aldershot, Bagshot, and Ascot 

 and on the tops of Highgate and Hampstead Hills. Wherever they 

 exist they make their presence apparent by a characteristic scenery 

 of heath or pine forest. In Belgium they are represented by the 

 Paniselien and Bruxellien Sands, and there also they produce a type 

 of country which is in strong contrast with that produced by the 

 Ypresien Clay. The conspicuous hill on which Cassel stands is 

 composed in the main of Paniselien Sand, though it includes some 

 later formations on its summit. Eastwards, as the Belgian Tertiary 

 basin is appi'oached, the Paniselien Sand comes on in greater force. 

 It forms the bold range of hills which surround Ypres on its southern 

 and eastern sides, and which includes the site of the famous Hill 60. 



