76 Rainfall, Natural Drainage, 



may be sufficient to lift the water towards or above the actual 

 surface from which it is pierced, or at which it is intersected, 

 provided the level of such surface is sufficiently below that of the 

 gathering ground. 



The wells bored through various rocks to reach absorbent and 

 saturated strata at some depth lying between impermeable strata 

 of any kind, are well known under the name Artesian, having 

 been introduced into Europe during the middle ages in the 

 province of Artois, in the north of France. Similar wells have, 

 however, been known, and sunk from time immemorial in various 

 parts of the East, and in the desert of Africa. It would at 

 once be admitted that water is constantly in circulation if, 

 wherever we sunk through permeable beds, we always found a 

 surface below which everything was saturated — and if, whenever 

 we penetrated impermeable beds, and reached absorbent beds 

 below them, these latter always yielded a supply of water rising 

 in the well or boring. But it is known that these results do not 

 always follow, and, although generally the reason of failure in 

 water-sinkings is that the surface of saturation is too deep to be 

 available, or the impermeable beds too thick to be pierced, it is 

 certain that the best expectations founded upon sound knowledge 

 of strata have sometimes been disappointed. Such failures 

 might induce the notion that the circulation of water was only 

 partial, and confined to certain rocks ; and in one sense this is 

 true, for the ready transmission of available quantities of water 

 is no doubt so limited. Many rocks interrupt it, many distur- 

 bances of rock interfere with it, and some rocks and disturb- 

 ances have the effect of checking it altogether. 



But in addition to the perceptible and available circulation, 

 here is another which is not less important, and is quite uni- 

 versal. Clays and granites, and some other rocks, only allow 

 of the free passage of water through cracks and fissures in their 

 mass ; and they certainly prevent the flow of water when they come 

 in the way and are unbroken. But amongst them water is always 

 moving, though this kind of circulation is not to be measured 

 and recognised by the eye. The best proof of it is found in the 

 chemical changes constantly taking place in them, as in all 

 other rocks that form the external crust of the earth. All, with- 

 out a single exception, have been entirely modified since they 

 were deposited, and always by the aid and in the presence of 

 water. The changes are incessant though slow. Crystallisa- 

 tion is one of these results, and no one who examines a 

 crystallised fragment of shell, and compares it with a corre- 

 sponding fragment of its recent analogue, can doubt the extent 

 of the change. The external characters of the shell may be 

 preserved without the smallest alteration ; but within, while 



