90 LONDON DISTRICT. 



owed their position to patches of River Gravel, Hampstead and 

 Highgate to the water-bearing Bagshot Sand. 1 



It was the steam -pump and the iron main that enabled the 

 intervening clay areas to be built over, rather than the discovery 

 that water could be obtained by deep wells sunk through the 

 London Clay. In this way the various hamlets became absorbed 

 in what Cobbett called the ' Great Wen,' till the London of to-day 

 extends in many directions beyond the limits of the Administrative 

 County, constituted in 1888. 



The gravels now have little influence on the growth, though a 

 gravel soil is considered, not always correctly, as an advantage; 

 and where concrete is used, a supply of gravel on the spot is 

 convenient. The London Clay now encourages rather than 

 checks expansion, since it is an ideal stratum in which to 

 construct tube railways. 



BUILDING STONES. 



The economic geology of London might be considered to 

 include that of the stones used for building ; but a complete list 

 of such materials would include those from all parts of Britain and 

 such distant lands as Norway and Greece ; the few that are found 

 within a short journey of London can alone be mentioned here. 



By far the most important building material of to-day is 

 reinforced concrete made from Portland Cement and either river 

 gravel or the sand and pebbles of the Blackheath Beds (p. 87). 



Ferruginous sandstone, ironstone, Kentish Rag, and flint 

 were utilised in Roman times, and these materials with tiles 

 were employed in the construction of London Wall; a good 

 example of flint -work may be seen at the Tower. Of building 

 stones, that of the Upper Greensand of the neighbourhood of 

 Reigate, Gatton, Merstham and Godstone was much used in 

 olden times, as, for instance in Old London Bridge. The harder 

 rock, known as firestone, was employed not only for buildings, 

 but for floors of furnaces ; it was utilized in Hampton Court 

 Palace; hearthstone is obtained from the softer calcareous 

 sandstone. Chalk has been used at the Tower and in many old 

 churches. 



Blocks of sarsen or greywether sandstone from the Chiltern 

 Hills have frequently been used for pitching in London ; the 

 Round Tower of Windsor Castle is built of this material. 



The ancient monastic barn at Harmondsworth is built of 

 puddingstone. 



The iron railings round St. Paul's were made from Wealden 

 iron ore at Lamberhurst at a time when charcoal was used in 

 the furnaces. 



1 See Prestwich, Presidential Address to Geological Society, 1872; Bonney, 

 T. G., in Besant's ' Early London.' 



