CHAPTER IV
CHRONOLOGICAL PROBLEMS AND LAND OSCILLATIONS
As the units of absolute time, like the elements of history, are inapplicable to prehistoric materials, especially those dating back to the Palæolithic period, anthropologists have from time to time suggested various natural phenomena on which chronological sequences may be founded, with the view of furnishing more or less precise ideas regarding the antiquity of man. The most common data thus improvised consist of some selected physical phenomena which can be shown to have been concurrent, or to have some incidental connection, with relics of man. Thus, the rate at which the Falls of Niagara are receding; the growth of peat and increase in stalagmitic deposits ; the deposition of sedimentary strata of river deltas, such as the cone of Tinière and the Nile delta; the correlation of astronomical phenomena with climatal changes in the environment, etc., have been largely discussed with the view of abstracting from their details some definite points which can be correlated with the developmental stages of man's career on the globe. But of all the evidential materials hitherto advanced from this standpoint the most reliable are to be found among glacial phenomena.
We have already adverted to the fact, that the implements of the river-gravels of England are precisely similar, both in type and technique, to those of analogous deposits at Chelles and the valley of the Somme; and that consequently they must be regarded as belonging to the same period and same civilisation. The oldest deposits in Kent's Cavern, the Grotte de Spy,
