5 
Thus the several local groups of strata belong to one or other 
of these formations, but they are not so extensively distributed as 
in other districts. 
Making a rough diagram from south to north through 
Croydon, we should find, as shown in the diagram (exhibited), . 
copied from a section by Mr. A. Tylor, in Mr. Smee’s 
interesting book, ‘‘ My Garden,” Ist, The Chalk; 2nd, Thanet 
Sands; 8rd, Woolwich Beds; 4th, Oldhaven Beds ; 5th, London 
Clay; and if continued across London, the chalk of Hertford 
would be found again overlain on its southern side by strata nearly 
similar to those which overlie the northern side of the chalk 
of this district, thus proving a basin-shaped arrangement of the 
strata forming the London basin. This arrangement I hope to 
show is favourable for obtaining water by means of deep Artesian 
wells, while the gravel which covers the surface, from its geological 
position, is another source of water-supply ; hence, therefore, in 
sinking anywhere within the London area, we should find the chalk 
at various depths. But if the section was continued southwards, 
from Merstham to Brighton, although the chalk would be found 
again in the South Downs, no deep sinking would reach the chalk in 
the intermediate area, in consequence of its having once covered 
that area, and having been subsequently removed. This arching over 
and former continuity of the chalk of the North and South Downs, 
previous to denudation, being due to a general folding, or undulation 
which the chalk and some of the subsequent strata have 
undergone, and by which the London and Paris troughs or basins were 
formed. For broken through as the chalk is by the Channel, it is 
continued in France, where the folding is again found, so that it 
underlies Paris, forming the Paris basin, which contains. some 
beds of the same age as those of London, and are also syn- 
chronous with those near Croydon. 
The Chalk which forms the last of the great series of secondary 
deposits—althongh not fully represented in the British area—is a 
soft white earthy limestone, attaining a thickness in some localities, 
as at Mr. Colman’s well, at Norwich, 1,000 feet; and in this neigh- 
bourhood it is caleulated to be from three to 500 feet thick, by Mr. 
Caleb Evans, whose published section of the chalk from Oxted to 
Purley is of much interest. 
Mr. GC. Evans, in his paper * “On some Sections of Chalk 
between Croydon and Oxted,’’ on the Surrey and Sussex Railway, 
shews that the following zones appear to be well marked, the 
highest part of the chalk (about 50 feet) not being included :— 
1.—Purtry Bens: Chalk with nodules and bands of flint. 
Zone with Micraster cor-anguinum in upper part and Inoceramus 
Cuvieri below. 
* Proc. Geol. Association, January, 1870. 
