In the same way currents which flow over the Newfoundland bank a great part of the year at the rate of two miles an hour, and are known to retain a considerable velocity to near the bottom, may carry away all loose sand and mud, and make the emergence of the shoal impossible, in spite of the accessions of mud, sand, and boulders derived occasionally from melting icebergs which, coming from the northern glaciers, are frequently stranded on various parts of the bank. They must often leave at the bottom large erratic blocks which the marine currents may be incapable of moving, but the same rocky fragments may be made to sink by the undermining of beds consisting of finer matter on which the blocks and gravel repose. In this way gravel and boulders may continue to overspread a submarine bottom after the latter has been lowered for hundreds of feet, the 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 undergoing; 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 incoherent 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 off 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 we forget the protection afforded by a dense and unbroken covering of barnacles, limpets, and other creatures which flourish most between high and low water and shelter some newly risen coasts from the waves. [1] Principles of Geology 7th ed., p. 506; 10th ed., vol. ii, p. 196. [2] Second Visit to the United States, vol. i, chap. xxxiv. [3] Physical Geography and Geology of Great Britain, p. 78, 1864. [4] Principles, 10th ed., vol. i, p. 569. CHAPTER VII. JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE EARTH’S CRUST. How we obtain an Insight at the Surface, of the Arrangement of Rocks at great Depths. — Why the Height of the successive Strata in a given Region is so disproportionate to their Thickness. — Computation of the average annual Amount of subaërial Denudation. — Antagonism of Volcanic Force to the Levelling Power of running Water. — How far the Transfer of Sediment from the Land to a neighbouring Sea-bottom may affect Subterranean Movements. — Permanence of Continental and Oceanic Areas.