174 NOTES ON THE GEOLOGY OF
the upper. There are also pieces of bituminous shale in the former
which I have never known to occur in the latter, although the two
fossils Gryphaea dilatata and G. incurva are common in both.
Again, if the colour of the top stratum is due to weather agencies,
I venture to suggest that the percolation of water, or any other
process, would have extended to a nearly uniform depth; but we
find that the blue clay at Margaret Roothing comes nearly to the
surface (within 12 feet), while at Little Boyton, three miles off,
it is only reached at 28 feet. This seems to me to be a strong piece
of evidence in favour of the view that the two clays are of somewhat
different origin.
Again, the sections of two wells about half-a-mile apart, viz., at the
"Hill Farm and at "Chalk End Farm," show a very great differ-
ence in the depth of the white clay. At the "Hill Farm" it is 30
feet deep (including the top soil), but at "Chalk End" it is only
12 feet deep, while the blue clay under it remains practically of the
same thickness.
These four sections are, roughly speaking, in a line from N.W. to
S.E., and three of them are, according to the Ordnance map, within
a few feet of the same level, viz., 185, 190, and 191 feet above the
level of the sea. The altitude of the fourth, at Margaret Roothing,
I have not ascertained.
This question is of interest other than from a purely scientific
point of view, for by well-diggers and others ignorant of the nature
of the formation, the lower stratum is often confounded with the
London Clay or "blue clay" as it is generally called. Opinions
were strongly expressed by the diggers of a well at the "Hill Farm,"
Roxwell, that water would never be obtained there, because they
had got into the "blue clay." On my recommendation, however,
they persevered, and they were rewarded with a plentiful supply of
good water on reaching the Mid-Glacial Gravel beneath.
It is extremely difficult to collect any fresh information on the
subject, on account of new wells being dug so rarely. There are
no other means available of obtaining a section, and experience
tells me that the memories of well-borers for measurements and par-
ticulars of old wells are very little to be depended upon.
There seems to me no difficulty in supposing that the currents
which brought the floating ice, bearing its load of detritus, from the
northward, should have changed during the long period which must
have elapsed while our clay was being deposited, and that, with the