places, was just before the beginning of the bronze-age. Underneath
the moss-peat came two or three feet of silt with sea-shells in it.
Clearly the island of Jersey underwent in those days some sort of
submergence. Below this stratum came a great peat-bed, five to seven
feet thick, with large tree-trunks in it, the remains of a fine forest
that must have needed more or less elevated land on which to grow.
In the peat was a weapon of polished stone, and at the bottom were
two pieces of pottery, one of them decorated with little pitted marks.
These fragments of evidence are enough to show that the foresters
belonged to the early neolithic period, as it is called. Next occurred
about four feet of silt with sea-shells, marking another advance of
the sea. Below that, again, was a mass, six to eight feet deep, of
the characteristic yellow clay with far-carried fragments of rock in
it that is associated with the great floods of the ice-age. The land
must have been above the reach of the tide for the glacial drift to
settle on it. Finally, three or four feet of blue clay resting
immediately on bed-rock were such as might be produced by the sea,
and thus probably betokened its presence at this level in the still
remoter past.

Here the strata are mostly geological. Man only comes in at one point.
I might have taken a far more striking case--the best I know--from
St. Acheul, a suburb of Amiens in the north of France. Here M. Commont
found human implements of distinct types in about eight out of eleven
or twelve successive geological layers. But the story would take too
long to tell. However, it is well to start with an example that is
primarily geological. For it is the geologist who provides the
pre-historic chronometer. Pre-historians have to reckon in geological
time--that is to say, not in years, but in ages of indefinite extent
corresponding to marked changes in the condition of the earth's surface.