534 RECENT, 



Well-water at moderate depths, loo to 200 feet, is usually colder 

 in summer and warmer in winter than surface-water. In very deep 

 wells the temperature of the water at some distance from the surface 

 rises at about the same rate as that of the atmosphere in mines — 1° 

 for every 50 or 60 feet. Thermal springs no doubt arise from great 

 depths, and in non-volcanic districts, according to Prof. Prestwich, 

 they may generally be considered as natural artesian wells whose 

 temperature is proportionate to the depth from which they rise. 



Water penetrating the rocks often becomes charged with sub- 

 stances which are contained in them, such as salts of iron, lime, 

 soda, magnesia, and sometimes sulphur, which is derived from the 

 decomposition of iron-pyrites. Warm waters naturally take up 

 larger proportions of mineral matter than do the cold springs. 



The water of sandy and clayey districts is 'soft' like rain-water, 

 but in limestone areas it becomes impregnated with carbonate of 

 lime, and is called ' hard.' Hardness of water is also caused by 

 sulphate of lime. Thus soft-water is usually obtained from the 

 Bagshot Beds, Lower Greensand, Wealden Beds, Upper Carbon- 

 iferous strata, from the Devonian rocks, Old Red Sandstone, 

 Silurian and Cambrian gritty and slaty beds, as well as from the 

 Igneous and Metamorphic Rocks. Whereas the water obtained 

 from the Chalk, Oolites, Lias, and Carboniferous Limestone is 

 hard : that from the IVIagnesian Limestone is very hard, and so also 

 is that from the New Red Sandstone, which frequently contains 

 gypsum. The waters that supply the fountains in Trafalgar Square, 

 London, are derived from two wells sunk (300 and 395 feet) into the 

 Chalk, and they contain carbonate of soda, sulphate of potash, etc.^ 



The relation between health and geology is a point which has in recent years 

 received a good deal of attention. It is well known that gravelly, sandy, or chalky 

 soils are more healthy than a clay foundation, because the former are pervious to 

 water, and the latter is impervious. On the former there is less consumption than 

 on the latter, as Dr. Buchanan and Mr. Whitaker have demonstrated ; the artificial 

 removal of sub-soil water has, however, done much to equalize the conditions. ^ 

 Again, the water-supply is a most important subject, for in some villages and small 

 towns the wells are liable to contamination. Situated, perhaps, on elevated 

 ground, with a porous soil, they yet suffer because of the state of the drainage, the 

 wells being shallow and the sewage from dead-wells or cess-pits, even the church- 

 yards, draining into the water-bearing strata. 



Tufa. — This is a deposit of carbonate of lime, known also as 

 Travertin or Calcareous sinter, which is often formed by springs 

 that issue from limestone strata. It frequently contains im- 

 pressions of leaves and blades of grass, and specimens of the 

 common land-snails, which have become as it were ' petrified.' 



The rain absorbing carbonic acid from the atmosphere and from 

 the soil, with the help also of vegetable acids, as humic and ulmic, 

 dissolves portions of the limestone, which it parts with again when, 

 on coming to the surface, it loses a portion of the carbonic acid. 

 The amount of carbonic acid gas is generally less than 2 cubic 

 inches per pint. 



^ De la Beche, Address to Geol. Soc. 1S49, p. Ixv ; and Journ. Chem. Soc. i. 97, 

 - G. Mag. 1S69, p. 499. 



