572 



ARRANGEMENT OF MATERIALS IN CURRENT [Ch. XXII. 





Suppose strata as destructible as the greater part of the 

 Tertiary, Cretaceous, and Wealden deposits of the British 

 Isles, or the coal-measures or mud-stones of the Silurian 

 period, to be thus slowly upheaved, how readily might they 

 all be swept away by waves and currents in an open sea ! 

 How entirely might each stratum disappear as it was brought 

 up successively and exposed to the breakers ! Shoals of wide 

 extent might be produced, but it is difficult to conceive how 

 any continent could ever be formed under such circumstances. 

 Were it not indeed for the hardness and toughness of some 

 limestones and of many crystalline and volcanic rocks which 

 are often capable of resisting the action of the waves, few 

 lands might ever emerge from the midst of an open sea. 



Arrangement of materials in current deposits and their wide 

 diffusion. — It has been ascertained by soundings in all parts 

 of the world, that where new deposits are taking place in the 

 sea, coarse sand and small pebbles commonly occur near the 

 shore, while farther from land, and in deeper water, finer 

 sand and broken shells are spread out over the bottom. Still 

 farther out, the finest mud and ooze are alone met with. 

 Mr. Austen observes that this rule holds good in every part 

 of the English Channel examined by him. He also informs 

 us, that where the tidal current runs rapidly in what is called 

 ' races/ where surface undulations are perceived in the calmest 

 weather, over deep banks, the discoloration of the water does 



not arise from the power of such a current to disturb the 

 bottom at a depth of 40 or 80 fathoms, as some have sup- 

 posed. In these cases, a column of water sometimes 500 

 feet in height, is moving onwards with the tide clear and 

 transparent above, while the lower portion holds fine sediment 

 in suspension (a fact ascertained by soundings), when sud- 

 denly it impinges upon a bank, and its height is reduced to 

 300 feet. It is thus made to boil up and flow off 1 at the surface, 

 a process which forces up the lower strata of water charged 

 with fine particles of mud, which in their passage from the 

 coast had gradually sunk to a depth of 300 feet or more.* 

 One characteristic effect of the action of currents is, the 



* Robt. A. C. Austen, Quart. Journ. Geol. Soc. vol. vi. p. 76. 





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