242 J. nopKiNSON — eecent discovery of 



central area or the rising of the strata forming or supporting the 

 sides of the basin, most probably from lateral pressure, assumed 

 their present basin-shaped form. 



These strata vary much in their pervious nature, some of the 

 beds, as the London Clay, being almost impermeable, and others, 

 as the Lower Greensand, containing bands of loose porous sands 

 through which water can easily percolate. It will be at once seen 

 that if we have, for instance, a bed of sand between two beds of 

 clay in a basin-shaped form, the rain falling on the surface (ex- 

 cepting the portion which evaporates or is absorbed by vegetation, 

 etc.) will run off the clay on which it falls, but will percolate into 

 the sand, which will form a reservoir in which a great part of the 

 rain falling over the entire area will accumulate. If now we sink 

 a well through the top bed of clay into the sand, the water will 

 rise to the height of the edge of the bottom bed of clay. If, how- 

 ever, we suppose that the sand does not form a continuous layer 

 between the two beds of clay, which at some part of the basin are 

 in juxtaposition, at such part it would be iiseless to sink a well ; 

 but this circumstance would not interfere with the water-bearing 

 value of the bed of sand where present, except by diminishing its 

 area and thus reducing its capacity as a reservoir. The knowledge 

 of the position and extent of the underground area devoid of the 

 bed of sand must therefore be of the utmost importance, for all 

 attempts to obtain water in this area would be futile. The 

 problem is but a little more complicated, if we imagine that, 

 where the sand is absent, the lower bed of clay is also wanting, 

 and that in that part of the basin there is some other rock present 

 not of a water-bearing nature. The bed of sand has still the 

 same value as an underground reservoir, and the problem is now 

 to determine the position and extent of this rock which takes 

 the place which would have been occupied by the bed of sand, 

 and its bottom bed of clay, had these beds been continuous. 



8uch is, in its simplest form, the problem which is gradually 

 being solved by these deep borings, so far as the search in the 

 London area for a large supply of pure water is concerned. 



The bed of clay on which London is situated, here and there 

 capped by beds of gravel and sand, from which a limited supply 

 of water was at one time obtained (now contaminated by surface 

 drainage), reposes on a series of beds of sands and clays from 

 which some amount of water is even now here and there derived ; 

 these two series of strata being known as the London Clay and the 

 AVoolwich and Heading Beds. The Chalk then follows in regular 

 succession, and from its somewhat porous nature and its numerous 

 fissures and hollows usually filled with water, forms a valuable 

 water-bearing formation ; but its area is limited, and the water it 

 holds is what is called "hard," holding in solution a considerable 

 per-centage of calcareous matter. The Chalk passes almost im- 

 perceptibly into a softer bed called the Chalk Marl, and this again 

 reposes on a bed called the Upper Greensand or Chloritic j\Jarl, 

 below which there is another bed of clay, the Gault, which from 



