192 THE RIDDLE OF THE BURBOT 



which occurs neither in the Severn, nor in any other 

 English rivers with a westerly or southerly course ; but 

 geological time interposes a bar to such a simple 

 solution. 



Even if Sir Andrew Ramsay's theory were demon- 

 strably in accordance with fact, and the Thames once 

 flowed westward into the Severn valley, the reversal of 

 the flow towards the east must have taken place at a 

 very remote geological period. Sir Andrew himself 

 indicated the late Miocene as the time of the change, 

 and accepted the view that thereafter the Thames 

 became a tributary of the Rhine. But a vast deal has 

 taken place since Miocene times, enough to baflSie con- 

 jecture about the probable course of this river or that. 

 Still, recourse must be had to conjecture in endeavour- 

 ing to get at the secret of the absence of burbot 

 from the Thames, and I venture to submit the follow- 

 ing hypothesis which, I believe, is not inconsistent 

 with the views held by leading geologists on the general 

 effects of ice in Western Europe. 



Many ages after the Miocene period, during which 

 the Thames may be assumed to have flowed into the 

 Rhine, perhaps uniting its waters with the Trent before 

 its junction with the mightier river, there ensued in 

 what is termed the Post-tertiary, Quarternary, or 

 Pleistocene period a gradual refrigeration of the climate 

 of the northern hemisphere, culminating in the Great 

 Ice Age. In what is now Great Britain (at that time 

 forming part of the continent of Europe) the conditions 

 became such as to destroy all life, animal and vegetable, 

 except such rudimentary organisms as it has been 



