W. Whitaker — ChaJh of the London Basin. 365 



to the downward sinking of water in the rock. In the matter of 

 fissures there may be little difference between the two divisions, and 

 the Middle Chalk suffers from having mostly a far greater mass of 

 other beds over it, which would tend to close its fissures. 



The Chalk Rock, which forms the top surface of the Middle Chalk, 

 where well developed, is favourable to the passage of water along 

 it, being usually markedly jointed, and so, lohen below the level of 

 saturation, is often a water-yielding bed ; but when above that level, 

 there is nothing to stop its being dry. 



Where the escarpment forms a long gentle slope, as in Cambridge- 

 shire and Norfolk, the outcrop of the Middle Chalk is fairly broad, 

 and much water must sink through the rock and find its way out 

 in the springs on the westward slope. 



Where, too, the outcrop of the Middle Chalk, though usually 

 much less in width, is cut back by valleys that breach the upper 

 part of the great escarpment, as is the case with the valleys of the 

 Lea (in Beds), of the Gade and of the Bulbourn (in Herts), of 

 the Misbourn and of the Loudwater (in Bucks), of the Thames 

 itself and of the Lambourn and of the Kennet (in Bei'ks and Wilts), 

 there, again, much water is collected by this division, and many 

 springs issue from it, and feed the Thames by its various tributaries. 



The Melbourn Eock, at the base of the Middle Chalk, is some- 

 times water-bearing, by reason of the bed next below (known as 

 the Belemnite Marl) being mostly of a rather clayey character ; 

 so that, under favourable conditions, it may delay the downward 

 passage of water. 



The Lower Chalk, being generally more clayey than the upper 

 two divisions, is less free to the passage of water. Moreover, its 

 outcrop is mostly narrow, and it is not laid open along valleys as 

 the Middle Chalk often is. Circumstances, therefore, are clearly 

 against this division in the matter of water. 



There are, however, places where, from favourable conditions 

 arising, much water occurs in the Lower Chalk, and three special 

 cases may be noticed, the more so as they have largely to do with 

 the lowest and usually the most clayey part of the division, the 

 Chalk Marl. 



(1) In Bucks, Beds, Herts, and Cambridgeshire, there is, at the 

 top of the Chalk Marl, a marked hard bed, of a somewhat gritty 

 character, known as the Totternhoe Stone, which is permeable. As 

 the water sinking into this, either directly or through the overlying 

 chalk, is stopped in its downward course by the more clayey beds 

 below, springs are of frequent occurrence along the line of outcrop 

 of the stone. 



(2) In Bucks there is sometimes a hard permeable bed in the 

 Chalk Marl, which also gives rise to springs. 



(3) In Western Norfolk the Chalk Marl loses its clayey character, 

 and passes northward into hard jointed chalk. The Lower Chalk 

 as a whole, too, is harder than usual, and more permeable. The 

 result is that powerful springs occur either at the very base of 

 the Chalk or some little way above it, and are probably the best 

 source of local water supply. 



