126 HEX. J. C. CLrTTEEBUCK GEOLOGY AND WATEE SUPPLY 



It may be remarked that on the immediate surface of the Chalk 

 there is usually a thin band of dark red clay, called by Dr. Buck- 

 land a "pargetting," said often to contain traces of manganese. 



Passing over the valley of the Colne by the lofty "saaduct, the 

 Chalk and Tertiary beds may be seen in a chalk pit to the east of 

 the line, and on the entrance of the cutting on the London side of 

 the Bushey Station. The first partly exposed bed consists of sand 

 sometimes intermixed with rolled pebbles, sometimes coloured, 

 sometimes of a singular purity and brightness, known as silver 

 sand, and quai'ried for household, trade, or horticultural purposes. 

 Above the sand beds are the clays formerly bearing the term 

 "plastic," as indicating their nature. They are of various and 

 beautiful colours and consistency. A good exposed section may be 

 seen in Mr. Blackwell's brick field. Immediately resting on the 

 clay in the cutting is a bed of black silt in which sharks' teeth are 

 found. This is capped by an outcropping bed of the London Clay, 

 which here completes the series, the whole dipping towards and 

 passing under London and finding its outcrop in the bed of the 

 Thames above "Woolwich, which gives a name to some of these 

 Tertiary beds. The silt bed seems to crop out in the river about 

 the spot where the tunnel was made, and it is believed that much 

 of the difficulty encountered in its construction was due to the 

 fact that this bed being of the nature of quick or running sand, 

 the water of the river thereby found a too ready access to the 

 works, hence their consequent serious hindrance. This is con- 

 firmed by the fact that a recent subway has been constructed near 

 the Tower of London with little difficulty, where the stiti' London 

 Clay was pierced, and the ingress of water thereby prevented. 



It is now to be considered how the geological condition of the 

 district will account for the presence or absence of water. To 

 begin with the upper beds — those found on the higher levels. It 

 may be sufficient to speak of these as beds of sand and gravel 

 resting on clay. It may be remarked that such spots were fixed 

 on of old for habitation ; a rule which will extend to cities and 

 towns as well as country villages. The site of ancient London is 

 nearly if not entirely co-extensive with beds of gravel resting on 

 the London clay. Hampstead, Highgate, Harrow, Hendon, and 

 other towns were placed where there is a certain amount of water, 

 though the increase of population has required additional (quanti- 

 ties to be obtained often at groat trouble and ex])ense. The water 

 in such places is easily reached ])y sinking shallow wells into the 

 gravel or sand, the water being upheld by the clay beneath. AMierc 

 there is only a thin bed of gravel or sand, ponds are sunk, Avliich, 

 if they receive the drainage of a considerable area, will sometinu^s 

 furnish a supply through the year. These sources of water when 

 they show themselves above ground are usually called land s])rings ; 

 they will run till the bed of water in the sand or gravel is 

 exhausted or reduced to so low a level that the pressure of the 

 water is not sufficient to force it to the stiri'ace. The decj^er or 

 more abundant sources of water are usually called main si)rings. 



