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.
It takes the plain man a long time to find out that it is no use asking the pre-historian, who is proudly displaying a skull or a stone implement, "Please, how many years ago exactly did its owner live ?" I remember hearing such a question put to the great savant, M.
Cartailhac, when he was lecturing upon the pre-historic drawings found in the French and Spanish caves; and he replied, "Perhaps not less than 6,000 years ago and not more than 250,000." The backbone of our present system of determining the series of pre-historic epochs is the geological theory of an ice-age comprising a succession of periods of extreme glaciation punctuated by milder intervals.