184 ADDRESS. [LECT. 



district drained by that river being entirely a chalk area. 

 But if the river during the Palseolithie period had ex- 

 tended only six miles further inland, it would have 

 entered upon an area containing rocks of earlier periods, 

 fragments of which must in such a case have formed a 

 constituent part of its gravels. This consideration is 

 very important, because it shows that the valleys must 

 have been excavated by the present rivers ; even ad- 

 mitting that from the then condition of the climate, and 

 from other considerations, floods of that period may have 

 been both more frequent and more violent. Still the 

 excavation of the valleys must have been due to the 

 rainfall of each respective area, and thus not ascribable 

 either to one great cataclysm or to the fact of the 

 rivers having drained larger areas than at present. In 

 many cases, the excavation of the valley is even greater 

 than might at first be supposed. The valley of the 

 Somme, for instance, is forty feet deeper in reality than 

 its present form would indicate, the river having filled it 

 up again to that extent. 



The valley itself is from 200 to 250 feet in depth, and 

 although this affords us no means of making even an 

 approximate calculation as to time, still it is obvious that 

 to excavate a valley, such as that of the 'Somme, to a 

 depth of 250 feet, and to fill it up to the extent of thirty 

 or forty feet with sand, silt and peat, must have required 

 a very considerable lapse of time. 



Passing on now to the question of climate, it will be 

 observed that the assemblage of mammalia to which I 

 have already referred, is remarkable in several ways. It 

 is interesting to find that man coexisted in our woods 

 and valleys on Salisbury Plain, and on the banks of the 



