BENEATH THE LONDON CLAY. 159 



chalk (calcium carbonate) or magnesium carbonate may be soft- 

 ened in several ways. One of these is by adding carbonate of 

 soda, which precipitates not only carbonate of lime and magnesia, 

 but also sulphates, forming soluble bicarbonate and sulphate of soda. 



Now, this process seems to be that to which the softening 

 in the passage through the chalk is due. But whence comes the 

 carbonate of soda ? To this I can find no answer. From the fact 

 that soda is found in considerable quantities in all cases of waters 

 from the chalk under the London clay, it would seem to be due 

 either to soda in the chalk itself — though I cannot find this 

 mentioned as a constituent of chalk or as being found in the 

 chalk — or to the influence of water from the overlying water- 

 bearing sands, with respect to which I should be inclined to doubt 

 whether the quantity -is sufficient to produce so widespread an 

 effect, even if it was shown that this would be one consequence of 

 the mingling of' these waters. It would be interesting to examine 

 whether the softening is regularly progressive as the water passes 

 inwards further and further away from the outcrop. Here, then, 

 is a problem which, perhaps at some future day, our Society may 

 assist in solving. 



Other questions of a similar character will suggest themselves 

 with regard to the sulphates and chlorides and to the magnesium, 

 which seems to be a constant, though not in general a large 

 constituent of the solid contents of the waters in question. But 

 leaving these I will pass to the consideration of the waters in 

 Section C in the table. ■ 



These waters will at once be seen to be exceptional in their 

 character, in that, while, like other waters under the London clay, 

 they have a large quantity of soda, they have not thereby lost 

 anything like the same proportion (barely one-third) of the chalk 

 they originally contained, and they have acquired an exceptionally 

 large amount of magnesia. They are all taken from wells in the 

 immediate neighbourhood of Harrow,* and their anomalies are 

 doubtless due to some local cause extending over a limited area. 

 Somewhat similar anomalies appear to present themselves at 



* Measuring from the Harrow waterworks Sheepcote Farm is about | 

 mile east on the opposite side of the hill, Kenton about 2 miles to the 

 north-east, and Sudbury Brewery about \\ to the south-east. 



