HOW TCHE "HEAD" WAS FORMED 27 



consideration until after the description of it on the 

 Continent of Europe, where it covers extensive tracts. 

 In the meantime we can consider the theoretical ques- 

 tions suggested by the phenomena already noticed, 

 especially with respect to the Head which is better 

 developed in England than on the Continent. 



5. THE ORIGIN OF THE ANGULAR RUBBLE AND 



"HEAD." 



Various explanations have been suggested, 1 but 

 though they might account for some features of the 

 Head, no one suffices to account for its several varied 

 forms of the Rubble-drift, and they all especially fail 

 to meet the consequences which would follow on their 

 assumption. We must look therefore for some other 

 explanation, and the one to which I have been led, 

 taking into consideration the collateral phenomena, is 

 that of a temporary submergence of the land and its 

 subsequent re-elevation. 



The late Mr. Hopkins 2 of Cambridge has shown 

 that if a considerable area at the bottom of the sea 

 were suddenly elevated, a wave, accompanied by a 

 current, the velocity of which would depend prin- 

 cipally upon the depth of the sea, would diverge in 

 all directions from the centre of disturbance. Cal- 

 culations, he says, "prove beyond all doubt that 

 paroxysmal elevations, beneath the sea, varying from 

 50 to 100 feet in height, may produce currents of 



1 For these see Appendix C. 



2 Quart. Journ. GeoL Soc., vol. iv. p. 90. 



