PLEISTOCENE RIVER-DEPOSITS. 121 



CHAPTER VIII. 



RIVER-DEPOSITS OF THE PLEISTOCENE PERIOD. 



M. Boucher de Perthes' discoveries — Professor Prestwich on origin of the ossi- 

 ferous and implement-bearing "drifts" — Fluviatile origin of the so-called 

 "drift"— Erosion of river valleys during Pleistocene times— Time required 

 for excavation of valleys — Professor A. Geikie on modern denudation — 

 Flooded condition of Pleistocene rivers — Professor Prestwich on relation 

 between ancient river-gravels and loams —Absence of well-marked river- 

 terraces accounted for — River ice and ice-fioated erratics — Professor Prest- 

 wich on climatic conditions implied by Pleistocene river-deposits — Com- 

 mingling of different groups of mammals— Sir C. Lyell's views — Mr. Darwin 

 on angular gravels of Southern England. 



The evidence we are now about to consider is in certain re- 

 spects more satisfactory than that derived from the study of 

 cave-deposits. The latter, indeed, teach us in the most impres- 

 sive manner that the Palaeolithic Age is separated from our own 

 by a great interval of time — an interval that may well be 

 measured by hundreds of centuries ; but taken by themselves 

 alone they do not tell us to what particular stage in the geolo- 

 gical record they ought to be referred. We have seen that 

 their fossil contents have enabled geologists to class them as of 

 Pleistocene age. But the term Pleistocene embraces a great 

 variety of accumulations of diverse formation. Besides cave- 

 deposits, there are lacustrine, fluviatile, and marine strata, some 

 of which attain a considerable thickness, and spread over wide 

 tracts of country. Again, there are enormous sheets and heaps 

 of glacial detritus that cover a large part of the British Islands 

 and Northern Europe, and gather abundantly upon the low 

 grounds that sweep out from the base of every mountainous or 



