APPENDIX. 
THE CROYDON BOURNE.* 
The ‘‘ Nailbournes”’ or intermittent springs of the Chalk-tracts 
are of some interest, and their origin has been ascribed to various 
causes, siphons in the chalk and what not; but they are easily 
explained in a very simple way, and without calling in the aid of 
any wonderful agency, as being the simple overflow of the chalk 
after seasons of heavy rain. The question has been well worked 
out by Mr. C. W. Johnson in a short essay on the Bourne of 
Croydon,+ from which I quote :— 
All the rainfall upon the chalk either evaporates or sinks into the earth, 
—there is litile or no surface drainage. In the swmmer months the rain 
which it receives is almost entirely evaporated ; there is little or no overplus 
to feed its springs. But in the autumn and winter months the case is 
reversed ; the smaller portion of the rainfall then evaporates ; the larger por- 
tion descends beneath it. 
Whenever the rainfall considerably exceeds the average annual amount, 
the old streams or bournes are unable to discharge the unusual amount of 
water with which the chalk hills are saturated ; and this surface water, in 
obedience to the law of gravitation, makes for itself a new channel. A large 
portion of the water which flows from the Godstone and Merstham hills 
usually percolates through the thick beds of porous gravel and sand upon 
which the town of Croydon is built, and thus finds its way by slow degrees 
and through various springs into the Wandle..... When, however, these 
beds of sand and gravel, after a very rainy season, are so saturated with this 
water that they can take no mode, that which they cannot receive overflows 
the usual subterranean channel, and appears on the surface:.....: the 
subterranean watercourse is choked and filled up with water, and that as 
effectually as if it were filled in with pitch or metal, or any impermeable 
material. 
My colleague Mr. Topley has favoured me with the following 
extract from an old book which shows that this view was held long 
ago, but like many others was hardly grand enough for those more 
modern philosophers who could not see the efficacy of existing 
causes :— 
The rising of a bourn or stream near Croydon (as the common people 
hold) presageth death or the plague and it hath been observed to fall out so. 
The rising of bourns in places where they run not always, we have before 
proved to be caused by great wet years, which (according to Hippocrates’ 
observation) are generally the most sickly ; and if they prove hot as well as 
* Extracted from Mr. Whitaker's ‘‘Geology of the London Basin.”—Mem. Geol, 
Surv., vol. iv. p. 391. 
+ Dr. Westall’s, “‘ Advantages to be derived from the Adoption of the Local Govern- 
ment Acts, as exemplified in Croydon,” pp. 39, 40. 
