July 18, 1884»] 



♦ KNOWLEDGE ♦ 



51 



which it possesses as a chemical reagent. The densest 

 rock and the stiftest clay are yet so porous as to give it 

 ready access, and so great is the power of capillarity, in 

 virtue of which it descends, that DaubrcJe's experiments* 

 have proved concluNively that it is able to resist the great 

 ettects of counter pressure of vapour and internal heat, and 

 to establish itself in the form of a gas at the roots of 

 volcanoes and far down in the depths of the earth ; liow 

 far has not yet been ascertained. 



But, although stiff clays are permeable, they may be 

 regarded as practically impervious ; they are only stiff 

 clays because they hold, in intimate admixture, a large 

 proportion of water. Suppose, then, that a stratum of clay 

 aup})orts a sandy porous soil as at Fig IT), which represents 

 ■the condition of things which obtain at Ilampstead Heath. 

 The major portion of the rain-water is filtered by the sand, 



somewhat concave, or, it may be, altogether irregular on 

 account of the differences of the rock textures through 

 which the water is forced to pass, or because of their un- 

 conformability. A boring sucli as that shown at v, Fig. 1 6, 

 is called a well ; since t and c are at higher levels than ?'•, 

 the water, in virtue of the law of hydrostatics, rises up the 

 bore of the well to its outlet, and forms the kind of artificial 

 spring called an Artesian well.* 



Thus far we have only taken into account springs and 

 wells such as obtain in comparatively unbroken strata. In 

 other regions, however, where seismic action has been rife, 

 where numerous faults intersect the country, and where 

 the rocks lie more or less out of their strict order of suc- 

 cession, springs are to be soui^'ht for chiefly at escarpments 

 and in the lino of faults. In Fig. 17 the water-bearing 

 strata are shown in shaded bands, whilst the sandy beds are 

 doited ; /,/, are the outlets of powerful springs which rise 

 up the fissures due to faults. 



Fig. 15. — B., Bapshot eand ; I., water-beariog stratum; L., London 

 clay ; s., spring ; ii'., well. 



and collects above the clay in a sort of water-bed. At 

 certain parts the clay is laid bare by a cessation of the 

 overlying sand, l>y a natural depression, as at s (Fig. 15), 

 or by an artificial boring as at lo (Fig. 1.5) ; s gives forth a 

 natural spring, v is called a well. Now, remove all the 

 Bagshot clay and lay bare the London clay, as at Fig. 16, 

 which depicts an hypothetical section across the London 

 basin. The sandy strata, known as the Lower London 

 Tertiaries, underlie the clay, and the whole rests upon 

 the uppermost beds of the mesozoic age, termed the 

 cretaceous or chalk formations. The London clay takes 



Fig. 16. — Hypothetical section across the London basin. L. London 

 clay ; t., porous strata of the Lower London Tertiaries ; 

 c, chalk; I., water-level; tc, well. 



in but little water ; the exposed surface of the sandy ter- 

 tiaries at i and the chalk at c are saturated, and the water 

 finds its way through both until it reaches the so-called 

 water level (indicated by the dotted line in Fig. 16), or 

 underground reservoir, which pervades the crust of the 

 earth irregularly, and at depths which vary with the nature 

 of the siiil, the seasons, and the external configuration of 

 the surface.! Thus, under a plain, it is in reality toler- 

 ably level ; beneath a hill it becomes convex ; in a valley 



* " Geologie Experimentale," p. 274; Tschermak, "Sitz. der 

 Wiener Akademie," March, 1877; Reyer, " Beitr. zur Physik. der 

 Eruptionen," § 1. 



+ In the case of chalk formations, the surface waters which are 

 absorbed by the upper strata find their way by capillary percolation 

 and through fissures into the deeper portions, which then become 

 saturated and simulate retentive clays. It is in this way that water- 

 bearing strata are extemporised in chalk, and, as one would expect, 

 they are often transitory. 



Fig. 17.—/,/, Powerful springs rising at faults; s, porous sandy 

 beds; w, water-bearing strata. 



Thus it appears that the rain-water which penetrates the 

 earth is not permanently removed from the surface, but rises 

 again to well forth as springs, along joints and fissures, 

 charged with substances in solution and suspension which 

 are characteristic of the formations through which it passes. 

 Some of the water, however, is absorbed, and other portions 

 combine chemically with certain constituents in the rocks. 

 Its underground course, moreover, is not confined to 

 capillary percolation, but often assumes the character of 

 subterranean streams and reservoirs,! which are the ex- 

 teuded representatives of former crevices and fissures. 

 These are so pronounced in some cases as to materially 

 affect the water-supply of rivers— to drain or to swell them ; 

 curious facts illustrative of this have been recorded by 

 Desor,* and the occasional presence of plant-stems, leaves, 

 and even of fish, in recently-made Artesian springs, are 

 additional and interesting proofs of the underground circu- 

 lation of water. 



The descent of rain-water into the depths of the earth to 

 regions of peculiar mineral salts, aud their subsequent rise 

 in the form of springs, provides us with the so-called 

 natural mineral waters, for a detailed account of which we 

 must refer the reader to Gairdner's exhaustive " Memoir."§ 

 In like manner, thermal springs are the result of heat 

 derived from the interior of the earth. If we attribute 

 the heat thus gained by the water to the greath depth of 

 its origin, then, by allowing 1° Fahr. for every GO feet of 

 descent, when the surface temperature is about 50° Fahr., 

 the springs of Bath, which average 120° Fahr., ought to 

 come from a depth of at least 4,200 feet. 



Let us now turn to a consideration of the London 



* So-called after the village of Artois, in France. 



t A paper on " The Underground Waters of England and Wales" 

 was read before the Geologists' Association on June 6, last, by Mx. 

 C. E. De Ranee. 



X "Bull. Soc. Sci. Nat.," Neufchatel, 1864. 



§ " Essay on the Nature, History, Drigin, and Medicinal Effect* 

 of Mineral and Thermal Springs," Edinburgh, 1832. 



