EXCURSION TO WALTON AND FRINTON. 219
be partly due to inaccuracy in the survey of 1838, or the opera-
tions for protection of the coast, although somewhat inadequate,
may have retarded the waste, as far as is perceptible on the
small-scale map, one inch to the mile. The difference indeed
are chiefly in the opposite direction, that of increase of the land.
Considerable areas are shown in the new map as enclosed from
tidal access in the interesting area of Hamford or Hanford
Water, west and northwest of the Naze. Some of these have
since been regained by the sea, either through accident or
neglect. But outside, and apart from human action, deposition
has been proceeding, and the new map shows a projecting spit
of sand (the Pye Sand), nearly a mile in length beyond the low-
water line of 1838. This, coupled with the fact, informally
mentioned by Mr. Mothersole at the excursion meeting, of the
presence of quantities of Crag shells on the beach in that
direction, far beyond the outcrop on the Naze, points to a
northerly shore-current thereabouts, as a backward eddy from
the general southward tendency, so fully evidenced at the groins
and breakwaters along the Naze and towards Frinton. These
are banked up to the top with sand on the north side, whilst to
the south of each the bare London-clay forms a black slippery
surface, full of water-holes.
Seawater, apart from its motive force, tends to preserve the
London-clay, by excluding atmospheric air, and maintaining a
state of saturation, ft is the alternation of drought and wet
that breaks up the fairly-solid clay, converting its abundant iron-
pyrites into oxide of iron and free sulphuric acid, the latter in
its turn attacking the carbonate of lime generally present. Both
the ferric oxide and the sulphate of lime occupy greater bulk
than in their previous combinations, apart from that of the
carbonic acid gas set free by the later re-action. Some change
possibly takes place also in the elements of the clay itself, for it
passes from a rather tough mass, but one which will break into
fragments, to a plastic dough, which can only be torn or
squeezed apart. Perhaps in the absence of sufficient carbonate
of lime, the sulphuric acid attacks the alumina of the clay, or
the change may be purely molecular. At any rate, the modifi-
cation proceeds inwards from every crevice, and each expansion
produces further Assuring, until the mass, even at an early stage
of the process, is traversed in every direction by cracks with
slimy surfaces. A little excess of rain, and these surfaces are