GEOLOGY the neighbourhood of Mitcham, 1 and portions of an elephant's tooth and tusk at Croydon, in the valley of the Wandle ; mammoth, rhinoceros, horse, reindeer, etc., with a Palaeolithic implement, from the Caterham Valley 2 ; and many palsoliths from West Wickham, in a branch of the Ravensbourne. In its tributary valleys we find the phenomena of the Thames Valley repeated on a smaller scale, strips of gravel, sand or loam of ancient date occurring at varying levels above the streams, sometimes sharply defined and sometimes more or less coalescent down the slopes to the present valley-floors, all telling the same story of a continuous, though now abated, wearing away of the land and deepening of the drainage-hollows. With the more recent deposits or alluvia of the rivers the sand, loam and mud of the lowest levels, which in some cases are still receiving additions in times of flood the work of the geologist closes and that of the historian commences. The mammoth, rhinoceros and its companions disappeared, and the makers of the rude implements of the gravels gave place to a more advanced race of workers in stone, whose finely chipped tools and weapons lie scattered here and there over the surface of the land. The time-interval from the Palaeolithic or Older Stone Age to the Neolithic or Newer Stone Age and thence through the Age of Bronze and the Age of Iron to the dawn of history is, as measured by our human standards, of vast duration, but as compared with the asons of geological time it is indeed but as yesterday. SUMMARY OF GEOLOGICAL HISTORY In the foregoing pages we have dealt with periods of time that it is beyond our grasp to estimate. To follow the sequence of events is all that we can attempt in the present state of our knowledge, without venturing to guess at their absolute time-value. Before concluding the chapter let us briefly rehearse this sequence. Our earliest glimpse was of a land of ancient rocks, now hidden deep below the surface : a land planed down by erosion, and afterwards buried under the slowly-accumulated deposits of Jurassic seas. These deposits were in turn hidden by the sediments of the mighty Wealden river flowing from a continent whose confines we cannot trace. By renewed submergence this river, after a protracted existence, was obliterated, and its site covered with the sandbanks of the shallow current-swept Lower Greensand sea. Then, with the gradual deepening of the ocean there followed an accumulation of clay and of siliceous silt, forming the Gault and Upper Greensand, until the shore-line had receded so far that scarcely any waste from the land could reach our tract, and only a gentle 1 See G. J. Hinde, ' Notes on the Gravels of Croydon,' Trans. Croydon Microscop. and Nat. Hist. Club (1896-97). 2 See J. P. Johnson, ' Palaeolithic Man in the Valley of the Wandle,' Science Gossip, vol. vii. (1900) p. 75. 27
