132 THE ESSEX NATURALIST
statistically estimated, the prevalence or absence of certain
species will enable a fairly accurate dating of the deposit to
be made.
Another dating technique is applicable to Scandinavian
and North German lands, which were completely covered
with ice until the retreat of the last Ice-age. Analagous to
the lines of seaweed left on a beach by the retreating tide
are the lines of coarse mud which were left by the annual
summer retreat of the ice since the maximum of the last
Ice-age. The mud left during the winter deposition was
much finer in texture, and the resultant banding in
Scandinavian clay is clearly marked. Professor de Geer has
been able to establish a sequence of as many as 5,000 such
lines in the clay of Sweden, and then another, later,
sequence of 9,000 such lines, or "varves", as the ice-mud
lines are called, in a lake recently drained for hydro-electric
engineering purposes. An estimated gap of 6,000 more lines
between these two sequences has been partly closed since
Gerhard de Geer's first observations, thus making a
continuous series of twenty thousand varves. The great
value of the ''varve-technique'' is that it enables us to date
the peak of the last Ice-age to 20,000 years ago with an
uncertainty of less than 1,000 years.
Varve-counting is the last of the older dating methods
with which I shall deal; there are three techniques so new
as to be truly ''post-war'' and yet already so promising
that I must deal rather more fully with them.
Firstly, we have the discovery that the calcium phosphate
in animal bones can effect a slow exchange of at least some
of its calcium atoms for ionised atoms of fluorine where this
element is present in water to which the bones have access
over great periods of time. If, for example, bones lie in a
porous gravel deposit through which water is seeping, and
this water has even one part in a million of fluorine com-
pounds in solution, then gradually the bones will acquire
fluorine—perhaps 0.2 per cent, 0.5 per cent, even as much
as 11/2 per cent or 2 per cent. More than 3 per cent has not
been found, and this value probably represents a saturation
level beyond which, no matter how long the bones were
exposed to fluoriferous water, no further increase would
take place in their fluorine content.