144 INTRODUCTION TO THE STUDY OF [LECT. 



by any sudden cataclysm, both on account of their 

 regularity, and also of the fact, already mentioned, that 

 the materials of one river- system are never mixed with 

 those of another. To take an instance. The gravel of 

 the Somme valley is entirely formed of debris from the 

 chalk and tertiary strata occupying that area ; but at 

 a right angle to, and within a very few miles of, the 

 headwaters of the Somme, comes the valley of the Oise. 

 In this valley are other older strata, no fragments of 

 which have found their way into the Somme valley, 

 though they could not have failed to do so, had the 

 gravels in question been the result of any great cata- 

 clysm, or had the Somme then drained a larger area 

 than at present. The beds in question are found in 

 some cases 200 feet above the present water-level, and 

 the bottom of the valley is occupied by a bed of peat, 

 which in some places is as much as 30 feet in thickness. 

 We have no means of making an accurate calculation ; 

 but even if we allow, as we must, a good deal for the 

 floods which would be produced by the melting of the 

 snow, still it is evident that for the excavation of the 

 valley by the river to a depth of more than 200 feet, 1 

 and then for the formation of so thick a bed of peat, 

 much time must have been required. If, moreover, we 

 consider the alteration which has taken place in the 

 climate, as well as in the fauna ; and, finally, remember 

 also that the last eighteen hundred years have produced 

 scarcely any perceptible change, we cannot but come to 

 the conclusion that 'many, very many, centuries have 



1 Many persons find a difficulty in understanding how the river 

 could have deposited gravel at so great a height, forgetting that the 

 valley was not then excavated to anything like its present depth. 



