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 



