a 
On the Weald. 75) 
have been formed to them, by the action of the rain and 
Bourne streams, and one or the other of these causes seems. 
sufficient to account for all the valleys as they now exist. 
It must be borne in mind that all these valleys are formed out of 
the Chalk, which is acted upon by rain in a very peculiar way. 
Rain is, of course, nothing more nor less than distilled water. 
During its passage through the air it absorbs from it a con- 
siderable quantity of carbonic acid. This, falling on the 
chalk, dissolves and carries away with it a large quantity of 
chalk, as Dr. Carpenter showed us at one of our recent 
meetings, and so its action is to keep constantly increasing the 
size and depth of every valley and depression which exists in 
the chalk. It cannot, however, dissolve the flints with which 
the chalk abounds. These are set free, and, if left to them- 
selves, either remain exposed on the surface, or drop down 
into the valleys, where they accumulate. In the neighbourhood 
of Kingsdown, which is near Dartford, in Kent, they may be 
seen, in thousands of tons, in large heaps, in the bottom of the 
valleys, though here they have, no doubt, been greatly assisted 
in their descent by the constant ploughing of the land. If 
these flints had come within the reach of the streams they 
would have been carried along by them, and ultimately have 
come to form part of some of the numerous beds of angular 
gravel which abound in all chalk districts. The flints out of 
some of the Wandle valleys which we have considered this 
evening must now form part of the beds of angular gravel 
which are to be found under Croydon and in its immediate 
neighbourhood, but the question by what precise means they 
were transported to and deposited in their present resting 
places is not at all an easy one to answer. 
I have not by any means exhausted the subject of the 
Wandle basin, but I have only time this evening to call. 
attention to two more points of interest connected with it. 
I have mentioned above that the underground flow of water 
does not always follow the same course as the surface 
drainage. The Wandle basin seems to afford a remarkably 
good illustration of this. This most interesting subject, 
that is, the underground flow of water in the Wandle 
basin, is now, and has been for some years, the special 
study of one of our members, Mr. Baldwin Latham, but as. 
his elaborate series of observations has not yet been completed, 
the result of them, necessarily, has not been made known. 
With him the problem, or rather the problems to be solved are 
in very able hands, and the results of his labours are looked 
forward to with great interest in the scientific world. One 
thing, however, I believe Mr. Latham has established, and that 
is that, contrary to what is usually the case, the underground 
