GRAVEL. 87 



The Blackheath gravel, as noted by Mr. T. V. Holmes, had 

 a high reputation in the 18th century and was sent to great 

 distances. 1 Being composed almost wholly of flint-pebbles, it 

 may have been used for pavements, grottoes, etc. 



On the higher Chalk tracts flints picked off the fields are 

 utilized for road-metal, for which purpose they are the more 

 serviceable as they are well-seasoned; flints do not, however, 

 bind sufficiently well for modern traffic, unless mixed with other 

 stone; crushed flint is used in great quantity for drives and 

 roads designed for light traffic only. 



The term ' ballast ' as applied to gravel probably arose from 

 the fact that such material has been extensively used for 

 ballasting vessels. 



The term ' Thames ballast ' has been applied to material 

 taken from the bed of the river, comprising about two parts of 

 pebbles to one of sharp sand, and adapted for making concrete. 2 

 This material, as we are informed by Mr. C. W. Osman, is not 

 now obtained of such good quality as formerly. 



The Thames sand after being washed has been used with 

 Portland Cement for making bricks and concrete. It makes 

 good mortar. 



In early days querns or hand-mills for grinding corn were 

 made from the Hertfordshire puddings tone. Specimens have 

 also been cut, polished, and made into lids of snuff-boxes, as 

 mentioned by Kalm in 1748. 



HISTORICAL. 



The influence of the geology and topography on the selection 

 of the site of London in the first place, and on the subsequent 

 growth to the '. Greater London ' of to-day, forms an interesting 

 study, of which the essential points may be briefly touched on 

 here. 



The reasons for the choice of the site have been discussed by 

 many writers, some of whom have, unfortunately, based their 

 remarks on a wrong idea of the geology. The best account is 

 given by Mr. Hilaire Belloc, who makes use of the geological 

 maps described in this Memoir. 3 



As may be seen from the map or gathered from the preceding 

 chapter, the Thames below London Bridge flows through a wide 

 spread of alluvium, which must originally have been marsh : 

 this ground is now protected from the tide by banks which are 

 usually thought to date from the Roman era. At various points 

 down stream, e.g., Purfleet, Woolwich and Greenwich, the river 

 touches solid ground, but only on one side. Near London 

 Bridge, however, gravel spreads on either side approach each 



1 Geol. Mag., 1882, p. 137. 



2 See ' lire's Dictionary,' Ed. 7, by R. Hunt and F. W. Rudler, Art. Concrete, 

 vol. i, p. 906. 



3 ' The River of London,' 1912, Chap. iii. 



