Rev. A. Irving—Water Supply from Bayshot Sands. 409 
and greenish sands of the Lower Bagshot prevail for nearly 100 feet, © 
when they give place to clays of the same character as those 
above. Roughly then say out of 200 feet of Middle and Lower 
Bagshot strata, there are nearly 140 feet of sands, stained of various 
shades of green. This green colour is, as I have shown, due to the 
products of the decomposition of vegetable matter; and it follows 
therefore that in so far as the presence of such vegetable pollution 
renders water unwholesome for drinking and culinary purposes, the 
Middle and Lower Bagshots must be abandoned as a source of water- 
supply. The light which is thus thrown upon the origin of these 
Middle and Lower Bagshot strata is also interesting. In the almost 
entire absence of fossils, we have now distinct evidence of the 
conditions under which they were for the most part deposited. 
Slow and probably intermittent as this process of deposition was, it 
is clear that dense swamps and marshes covered for ages many 
portions of what is now called the London Basin; since the strata 
themselves are evidently a series of marsh- shore- and lagoon- 
deposits. And J prefer to look upon the vegetable matter as originating 
in this way rather than from marine alge, because, among other 
reasons, there is a remarkably low per-centage of chloride of sodium 
found in the waters which are derived from these strata. Taking 
the proportions of crenic and apocrenic acids as a general index of the 
extent to which the products of vegetable decomposition are present 
in these sands, I find from analyses of separate samples of the sandy 
strata which make up the series for the most part, that the propor- 
tion of vegetable matter present in the sands increases with the depth 
of the green colour, which characterizes the whole series more or 
less. ‘These green sands, as well as the green sands of the valley- 
heads, which I have described above as occurring beneath modern 
peaty layers, turn black on heating, and give off water with in 
some cases a distinctly alkaline reaction to test-paper, with in 
some instances traces of ammonia; on heating more strongly in the 
air, they lose their organic matter entirely, and are converted into a 
bright red sand, by the complete oxidation of the iron which is 
present. The clays (of which there are three principal strata, 
each of them pretty persistent through the country) assume when 
dry various shades of black-brown; and these also, when calcined 
in the air, part with the whole of their carbonaceous matter and 
assume a light brick-red colour. 
The bearing of all this upon water-supply is obvious. The clay- 
seams prevent the free passage downwards of the water which freely 
penetrates the Upper Bagshot Sands from the surface; so that a well, 
which passes through the uppermost or the middle clay-seam, must 
be supplied with water, which, though perhaps abundant, is necessarily 
impure, from the fact that it has travelled nearly horizontally, for a 
distance perhaps of several miles, from the outcrop of the sands to 
the well. Of course, where wells are dug for a few feet only into 
these Middle and Lower sands at their outcrop, the water may be pure 
enough ; for their proximity to the surface has, through a long period 
of time, led to the destruction, by the action of free atmospheric 
