CONCERNING THE AGE OF THINGS 133
Now, this discovery does not mean that here we have an
absolute time-keeper within every buried bone : obviously,
some gravels will be less exposed to water permeation than
others, and natural waters differ greatly in their fluorine
content. All one can say is that if there are, for example,
half a dozen really contemporary bones in a deposit, then
they must all have been exposed to the same opportunity for
exchanging fluorine and will give, on analysis, substantially
the same fluorine content'. If, in fact, one piece of bone in
a group were found to have, say, 2 per cent fluorine, while
adjacent pieces all had 0.5 per cent or thereabouts, there
could be no doubt that the abnormal piece had either
recently come into the group from some remote place more
favourable to fluorination (which is unlikely), or that it was
from the same locality but much older, probably four or
five times as old at least, as the other pieces. Since all the
pieces in this example were assumed to have been found
together at the same level, it is also apparent that the older
piece must have been carried into the group, perhaps by
erosion of earlier deposits higher up the gravel-forming
stream, but not necessarily from very far away.
In the reverse case, a more recent bone fragment may
sometimes have been introduced by grave-digging deep into
gravel strata where disturbance is not always easily seen in
later years by the archaeologist, but the fluorine test can often
detect the intrusion.
The value of this ''fluorine in bones'' technique is
admirably illustrated by its solution of the 65-year-old
problem of the Galley Hill Skeleton and also, by its help,
with study of the Piltdown and Swanscombe skulls. In
1888, the Galley Hill Skeleton was found eight feet deep in
middle Pleistocene gravels, perhaps a quarter of a million
years old. Much argument has centred on this skeleton
since it was found, but analysis of the fluorine content of
its bones by the British Government chemist in 1948 showed
0.3 per cent fluorine while known late Paleolithic skeletons
from the same gravel gave 1 per cent and a Saxon skeleton,
also from the same gravel, about 0.1 per cent. Fossil animal
bones (non-human) of early Paleolithic times from the
Galley Hill gravels gave 2 per cent fluorine. Therefore, it