THE DATE OF THE GLACIAL PERIOD. 359 



forces in past times, to take notice of the unknown quan- 

 tity, x, although this, in the absence of actual experience, 

 which cannot be had, can only be estimated by the results 

 and by a knowledge of the contemporaneous physical con- 

 ditions. It may be a complicated equation, but it is not 

 to be avoided.* 



" In this country and in the north of France broad val- 

 leys have been excavated to the depth ol from about eighty 

 to a hundred and fifty feet in glacial and postglacial times. 

 Difficult as it is by our present experience to conceive this 

 to have been effected in a comparatively short geological 

 term, it is equally, and to my mind more, difficult to sup- 

 pose that man could have existed eighty thousand years 

 or more, and that existing forms of our fauna and flora 

 should have survived during two hundred and forty thou- 

 sand years without modification or change." f 



The discussion of the age of the high-level river gravels 

 of the Somme and other streams in northwestern Europe 

 is not complete, however, without considering another 

 possibility as to the mode of their deposition. The con- 

 clusion to which Mr. Alfred Tylor arrived, after a pro- 

 longed and careful study of the subject, was that the main 

 valleys of the Somme and other streams in northern France 

 and southern England were preglacial in their origin, and 

 that the accumulations of gravel at high levels along their 

 margin were due to enormous floods which characterised 

 the closing portion of the great ice age, which he denomi- 

 nated the pluvial period. J The credibility of floods large 

 enough to accomplish the results manifest in the valley 

 of the Somme is supported by reference to a flood which 

 occurred on the Mulleer Eiver, in India, in 1856, wdien a 



* Prestwich's Geology, vol. ii, pp. 520, 521. 



f Ibid., p. 533. 



\ Proceedings of the Geological Society, London, Xovember 8, 

 1867, pp. 103-126 : Quarterly Journal of the Geological Society, 

 February 1, 1869, pp. 57-100.' 



