W. Whitaher — Chalk of the London Basin. 363 



of the Cbalk, is likely to cause a flow of water; so also may an 

 alternation of more open with more compact rock. 



(3) The chief ineans of water-communication through the Chalk 

 is by the more or less vertical fissures that occur, either along planes 

 of jointing (structural planes produced after solidification, and not 

 connected with bedding), or more rarely, along faults or cracks. 

 Joint-planes are universal in the Chalk and mostly very plentiful, 

 though often they are so closed as to allow the passage of but little 

 water. They can readily be seen in pits, and it is generally found 

 that the chief ones have a more or less definite direction over large 

 ti'acts, a good example of this being given by the highest Chalk of 

 East Kent, in which the chief joint-planes run in a north-westerly 

 and south-easterly direction, as is the case also near Grays, in Essex. 

 The planes of master-jointing are connected by other planes running ' 

 across them. ^ 



In underground work it is sometimes found that the chalk is 

 affected by a great number of these joint-planes, none being open, 

 and each giving passage to only a small quantity of water, though 

 the result of the whole n\f\y be very satisfactory. In other cases 

 a long extent of chalk may be passed through with but very slight 

 result as regards water-yielding fissures, and then a large fissurS 

 may be suddenly struck and a great quantity of water found. 



It is very likely that in many of the swallow-holes (which will 

 be described in a separate paper) the water enters the Chalk at 

 a fissure or crack, in fact along a plane of weakness. 



The above remarks refer to the ways in which water gets down^ 

 wai'd into the body of the Chalk ; it may be well to consider its more 

 horizontal course toward the parts where it finds an outflow in springs. 

 The fissures alluded to naturally form a means of communication, 

 not only more or less vertically downward, but also lengthwise 

 along their course, from the higher to the lower grounds. 



The same, too, would be the case with planes of bedding, where 

 they are fairly marked ; but it has occurred to me that sometimes 

 too much may have been credited to bedding-planes in this matter. 

 From the fact that the Chalk dips inward, from its generally lofty 

 escarpment to the lower ground formed by the margin of the 

 Tertiary Basin, and that therefore the dip-slopes of the Chalk form 

 by far the broadest parts of the outcrop, it follows that the water in 

 the Chalk would have a general flow in that direction, or inward to 

 the central part of the Basin. This flow in the direction of the dip 

 is, of course, subject to lateral change, where valleys are cut through 

 the Chalk to some depth, and an outlet for water is made. 



When, however, we examine other smaller tracts, where the 

 slope of the surface does not go with the dip of the beds, we find 

 a wholly different state of things. The one constant and recognized 

 exception is the face of the great escarpment, that is the outward 

 and generally abrupt slope of the Chalk ; the crest of the escarpment 

 forms a watei'shed, and the underground water flows outward, in 

 the direction of the slope, which is the reverse of the dip. The 

 many springs that occur at or near the foot of the escarpment are 

 proof of this flow. 



