THE FOUNDATIONS OF PREHISTORY. 93
completely protected from any breakage or flaking. But where
movement, however small it may be, takes place, active forces
are released and the flints are fractured or flaked along the
planes of such movement, while a fragile shell, no less than a
flint, which may lie within a short distance of the movement,
will remain unbroken. This is not merely a matter of abstract
reasoning, but a fact that I have been able to observe in geological
sections. Moreover, the argument from the unbroken shell
further overlooks the fact that naturally flaked flints are often-
derived, or at least carried away from the original site of their
flaking.
I have found large numbers of flaked flints underground
which remained in the position in which the flaking took place,
with the flakes still fitting against the parent block. Not in-
frequently I have seen a whole row of flakes all along an edge,
but that cannot always be so. In the larger number of cases
one must expect that the flaked flint will be moved away from
the flaking site, and the pieces separated.
Underground movements are due to a variety of causes,
such as compression of the surrounding rock, contraction or
expansion arising from changes of temperature or moisture
content, freezing, and thawing, and subterranean erosion of
which the evidence is seen in the large bulk of solid matter
brought up to the surface by springs. Closely associated with
these causes there is an extensive group of phenomena such as
soil-creep, land-slips, subsidences, or other slipping and founder-
ing of the deposits. Direct evidence of these movements may
sometimes be seen very clearly in disturbance and distortion
of the stratification. At other times "slickensides" may be
observed: these are planes of sliding that are not unlike the cut
made by an axe with a jagged edge. Above all, and most durable
of all, are the scratches or striations upon the surfaces of the
flints themselves: these are the hall-mark of movement-under-
pressure.
It would appear an almost inevitable assumption that earth-
quakes would also be a cause, or accessory cause, of the under-
ground agencies that we have been considering, but it is not
easy to obtain specific evidence. So far as my own experiences
go there is usually evidence of other causes, such as the solution