198 AXNITEESAEY ADDEESS 



posited Avhicli now form the bottom of our valleys. Such a change 

 in the fauna must be significant of a great lapse of time. 



And there is another feature in the case. In nearly all instances 

 there is a great similarity in the deposits in which the imple- 

 ments occur, and in the localities in which they are found. 

 The beds cannot be due to any great cataclysm or wave traversing 

 the country, because the materials of which the gravels are formed 

 are confined to the valleys through which the rivers now pass. 

 For instance, if there were such beds in the valley of the Colne, you 

 would only in these gravels get pebbles derived from the Chalk or 

 Tertiary beds above, including, however, the Glacial beds, in which 

 the older rocks occur. In France, in the gravels of rivers which 

 have passed through granitic districts, there are found granitic 

 pebbles; whereas where the rivers do not traverse granitic districts, 

 no such pebbles are to be seen. Other and conclusive evidence 

 of the gravels having been deposited by the action of rivers is 

 afforded by the presence of fresh-water shells. "We are therefore 

 driven to the conclusion that they were deposited by rivers flowing 

 from much the same watersheds as at present, but at a very 

 different elevation. One theory was that the valleys had already 

 been excavated in pre-glacial times and subsequently re-excavated. 

 But in this there is a difficulty, because it is very doubtful whether 

 in certain soils it was not easier for the rivers to cut out new 

 valleys than for them to excavate the beds which had been deposited 

 in glacial times. I think, therefore, that this was not the case, 

 but that the valleys, even when in existence in a shallower form in 

 pre-glacial times, have been cut much deeper in post-glacial times 

 by the rivers which have flowed through them. We can hardly 

 form an estimate of what the old rivers were like when they 

 received a much greater amount of rainfall than at present, and 

 were left in a state of nature. Indeed, if the rainfall were increased 

 to a not immoderate extent, all the conditions of the case would be 

 altered, especially in Chalk districts, in which these implements 

 have been mainly found. If, for instance, instead of the level of 

 saturation being 70 or 80 feet below the surface, as in this district, 

 the rainfall saturated the Chalk to the top, — which would not 

 require a very excessive amount, — the rain falling on the saturated 

 surface would act in the same manner as if falling in a clay 

 country, and there would be enormous floods in districts where 

 they are now unknown. I think this is one of the reasons whiili 

 conduced to the formation of valleys of such depths by streams 

 which at present appear so inadec^uate to the woik which we have 

 every reason to believe they performed. 



