BEACHES, ETC., OF THE SOUTH OP EJ^^QLAND. 



329 



§ 9. Theoretical Consideratio^^-s : The Eubble-Drift due to 



A WIDE SUBMERGEN^CE OP THE LaND. 



The points on which all the geologists who have treated of this 

 drift in any form agree are : — 



1. The angularity and sharpness of the harder constituent debris. 



2. The derivation of all the materials from the higher ground 



behind the K-aised Beaches. 



3. The absence of marine and fluviatile shells. 



4. The occasional presence of mammalian bones and land-shells. 



5. The want of regular stratification. 



It is obvious from the above that we may at once dismiss marine 

 and fluviatile agency, and although subaerial agency has its claims, 

 it is clear that it fails in many essential particulars. We require a 

 cause that will not only account for all the above-named results, but 

 one that also must not involve other consequences at variance with 

 the assumed cause. The cause I would suggest is not free from 

 difficulties, especially in connexion with the subsequent distribution 

 of life. These, however, need separate enquiry, and are not, I am 

 satisfied, incapable of solution. On the other hand, my hj^pothesis 

 will, I think, be found to answer, in the purely geological and 

 physical points, to all the conditions of the case. 



The elevation of the land to a height of 2000 feet or more was 

 a suggestion made in order to account, by the greater cold to which 

 the land would thereby be exposed, for such a surface-disintegration 

 as would supply the large quantity of detritus forming the Head. 

 But that object must have been attained independently by the 

 exposure of the surface during the preceding severe Glacial times. 

 Nor is there anything in the resulting land-conditions to show the 

 need of so great an elevation. Had the land been raised to 

 the great height named, the river-channels in their lower reaches 

 would have been excavated to a depth corresponding with the 

 altitude of the land. Of the actual depth of the river-channels at 

 those times we have opportunities of judging by the depth of 

 the tin-streams in the valleys of Cornwall, and by the excavations 

 for the Tilbury Docks. The Cornish sections show that in no case 

 does the bed of the river-channels underlying the Stream-tin gravel 

 exceed 80 feet below present high-tide level ; while the old bed of 

 the Thames, beneath its gravel and alluvial covering, does not exceed 

 70 feet in depth. If we add to this the difference between the level 

 of the old Beaches and of the present high-water mark — say, 

 20 feet on the average — it would show that the elevation need 

 not have exceeded 100 feet. This, or a little more, would have 

 given quite sufficient height to the land for the growth of the 

 forests on the surface of the Stanniferous gravels in Cornwall and 

 on the surface of the basement-gravels in the Thames Valley. This 

 limit agrees also with the estimate of about 120 feet based on the 

 presence of the old Dunes, whereas at a height of 2000 feet the 



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