SLIPS ARREST AND THROW UP THE WATER. 



315 



scends along the inclined beds towards C from the country beyond A, 

 and thus to arrest its further progress. But the pressure of the water 

 behind forces that which has reached the fault B D to seek, a way up- 

 wards, and, as spaces not unfrequently exist between the wall of clay 

 and the rocks between which it stands, the water jfinds a more or less 

 ready outlet at the surface B, and either gushes forth as a living and 

 welcome spring, or oozes out unseen among the soil, rendering it cold, 

 wet, and unproductive. Thus from 6 the water accumulated in the bed 

 (2) may rise to the surface, or from/ that which exists in (4), or from 

 any other bed in which water exists, and from almost any depth. 



2. But even where no such wall of clay exists, the waters may still 

 find their way to the surface along lines of fault, and from great depths. 

 Thus suppose the thin bed (2) to be full of water, and that it is covered 

 by an impervious bed (1), then the water which tends downwards from 

 a to b will be arrested at the fault, and dammed back by the impervious 

 extremity of (1) against which it now rests. If an outlet can be found, 

 it will therefore rise towards the surface. And as the rocks incline up- 

 wards in the direction of A, the pressure from behind may easily cause 

 the water to ascend to the summit of the hill at B, and to gush out in a 

 more or less copioas spring. 



5°. Where no natural outlets of the kind above described exist in^a 

 district, there may be a great scarcity of water on the surface, while 

 abundance, as we have already seen (2°), may exist in the rocks be- 

 neath, ready and willing to rise if a passage be opened for it. Such is 

 the case with the site of the city of London, represented below : — 



St. Alban's. 



Hampstead. London. Thames. Sydenham. 



Knockholt. 



SECTION ACROSS THE LONDON BASIN FROM ST. ALBAN'S TO KNOCKHOLT. 



iBiickland's Bridgewater Treatise, plate 69.) 

 1. Marine Sand. 2. London Clay (almost impermeable). 3. Plastic Clay and Sand. 

 4. Chalk, both full of water. 



The rain-water which falls between a and A on the one hand, and 

 upon the plastic clay and chalk between d and B on the other, sinks into 

 these two beds and rests in them till it finds an escape. It cannot rise 

 through the great thickness of impervious clay on which London and its 

 neighborhood stands, unless where wells are sunk, as above represented 

 at a, 6, c, d, either into the plastic clay (3), or into the chalk (4), when 

 the water ascends copiously till it reaches the general level of the country 

 about St. Alban's, the lowest part of the basin where the permeable beds 

 form the surface. Hence in the vale of the Thames at &, it rises above 

 the surface, and forms a living spring, while at other places, as at a, c, </, 

 it has still to be pumped up from a greater or less depth.* It is the ex- 



* In January 1840, there were stated to be in the London clay upwards of 200 such wells, 

 of which 174 were in London, and of which latter 30 taken together were known to yield 30 

 14 



