362 W. WhUaker — Chalk of the London Basin. 



of the Middle Chalk, again, is defined by beds of hard chalk, known 

 as the Melbouru Eock, of a more distinctly bedded character than 

 the chalk above, and having usually a nodular structure and some- 

 times a brecciated one. As far as our work in dividing the Chalk 

 has yet gone, this bed is proved to be of constant occurrence, though 

 varying in its amount of distinctness. [Since this paper was written, 

 it has been found that the Melbourn Kook is absent in the far west, 

 in Devonshire.] 



The Lower Chalk is distinguishable from the Middle Chalk by 

 the much thinner character of its beds, the planes of bedding being 

 sometimes emphasized by marly partings [especially in the Belemnite 

 Marl, which next underlies the Melbourn Rock] It is often slightly 

 clayey, and the Chalk Marl is sometimes particularly so. I'his 

 lowest member is in places separated fairly sharply from the rest by 

 a third hard bed, the Totternhoe Stone, a gritty, brownish-gre}', firm 

 bed. There are no flints in the Lower Chalk of the London Basin. 



Any attempt to divide the Chalk into White Chalk and Grey 

 Chalk is, I think, inconvenient. Colour is generally a very bad 

 guide in making stratigraphic divisions, and the greyness of chalk 

 sometimes depends on its dampness rather than on any inherent 

 colouring. Moreover, the term Gi^ey Chalk has been used in so 

 varying a way as to have become confusing. Sometimes it has 

 been limited to the Chalk Marl ; sometimes it has been used for the 

 whole of the Lower Chalk; now Prof. Dawkins seems inclined to 

 include the Middle Chalk also under this head. In the first two 

 cases the term is simply a synonym, and therefore useless ; in the 

 last it joins together two diviisions which are distinctly separable, 

 and is therefore worse than useless. 



As the divisions, Upper, Middle, and Lower Chalk, have been 

 mapped, more or less completely, by the Geological Survey, in the 

 London Basin, from Hants and Wilts through Berks, Oxon, Bucks, 

 Beds, Herts, Essex, Cambridgeshire, and Norfolk (partly), and also 

 outside the London Basin, in parts of [Devon], Somerset, Dorset, 

 Wilts, and Hants, and all through Sussex, I think that they may be 

 accei^ted as practical. 



3. Flow of Water in the Chalk. 



The passage of water through the Chalk occurs in three ways. 



(1) Through the pores of the rock. As these are very small, 

 sometimes indeed practically closed, this is of course a slow process, 

 and results, in underground work, merely in the weeping or oozing 

 out of water from the cut suifaces. Nevertheless, there may he 

 exceptional cases where, from greater openness in the rock, a greater 

 quantity of water may pass through. 



(2) Along planes of bedding, or the originally horizontal layers 

 of which the Chalk is built up. Sometimes the passage of water 

 along these planes is insignificant, but sometimes it is very marked, 

 and this naturally occurs where the bedding is more definite or 

 somewhat open, as along layers of flints. Where, too, marly layers 

 occur, their more impermeable character, as compared with the rest 



