SUBMARINE DENUDATION. 
107 
surface never having been able to emerge and become land. 
It is by no means improbable that the annual removal of an 
average thickness of half an inch of rock might counteract 
the ordinary upheaval which large submarine areas are un¬ 
dergoing; and the real enigma which the geologist has to 
solve is not the extensive denudation of the white chalk or 
of our tertiary sands and clays, but the fact that such inco¬ 
herent materials have ever succeeded in lifting up their 
heads above water in an open sea. Why were they not 
swept away during storms into some adjoining abysses, the 
highest parts of each shoal being always planed oif down to 
the depth of a few fathoms? The hardness and toughness 
of some rocks already exposed to windward and acting as 
breakwaters may perhaps have assisted; nor must wo for¬ 
get the protection afforded by a dense and unbroken cover¬ 
ing of barnacles, limpets, and other creatures which flourish 
most between high and low water and shelter some newly 
risen coasts from the waves. 
