are recovered from a stratum of gravel or clay which has been
deposited gradually and has fixed them in place safe from the air, a
sure proof that they have been there for a long time.

=Prehistoric Science.=--Scholars have examined the débris and have
asked themselves what men have left them. From their skeletons, they
have tried to construct their physical appearance; from their tools,
the kind of life they led. They have determined that these instruments
resemble those used by certain savages today. The study of all these
objects constitutes a new science, Prehistoric Archæology.[1]

=The Four Ages.=--Prehistoric remains come down to us from very
diverse races of men; they have been deposited in the soil at widely
different epochs since the time when the mammoth lived in western
Europe, a sort of gigantic elephant with woolly hide and curved tusks.
This long lapse of time may be divided into four periods, called Ages:

1. The Rough Stone Age.

2. The Polished Stone Age.

3. The Bronze Age.

4. The Iron Age.

The periods take their names from the materials used in the
manufacture of the tools,--stone, bronze, iron. These epochs, however,
are of very unequal length. It may be that the Rough Stone Age was ten
times as long as the Age of Iron.