396 Notes on the Formation of extensive 



common to find, in coasts composed of both hard and soft ma- 

 terials, taluses of blocks or large indurated concretions, detach- 

 ed from the cliffs, and defending them from that quick destruc- 

 tion that would otherwise ensue. 



The effect of the joint action of the sea and air upon hard rocks 

 is well seen in the Scilly Islands. There the granite decompo- 

 ses into its usual blocky forms, the angles gradually disappear, 

 and eventually the masses fall on the beach, where the tremen- 

 dous breakers of that coast grind them against each other into 

 balls, and often hurl them high up on the shore. 



I know not how Playfair could have imagined that following 

 waves were merely confined to the shore*; for the destruction of 

 coasts of equal hardness almost always bears a proportion to the 

 extent of open sea to which they are exposed, allowance being 

 always made for the force and duration of prevalent winds. 



The power of the sea 10 erect barriers against itself, under 

 other circumstances than those previously noticed, is very ably 

 illustrated by Mr. R. C. Taylor, in his Geology of East Nor- 

 folk. After observing that the land encroachment at Lowestoft 

 Ness had been effected at distinct and distant intervals ; that 

 its form had been influenced by the direction of the currents in 

 the channel, and the position of the adjacent shoals, and that 

 the lines of growth are indicated by a series of concentric ridges 

 or embankments, inclosing certain areas, he observes "a ram- 

 part of heavy materials is first thrown up to an unusual altitude, 

 by some extraordinary tide, attended with a violent gale. Sub- 

 sequent tides extends the base, and heap up lighter substances 

 on its summit. Sand is blown from the beach and fills the in- 

 terstices. The Arundo arenaria and other plants, by degrees 

 obtain a footing ; creep along the ridge, give solidity to the 

 mass, and in some cases forms a matted covering of turf: 

 meanwhile another mound is forming externally, and by the 

 like process rises and gives protection to the first. Occasional" 

 ly the sea forces its way through one of the external and incom- 

 plete mounds, but it is singular to observe how soon the breach 



is repaired!-'' 



In tropical countries the advance of the Mangrove trees out- 

 wards from sheltered situations at the bottom of creeks, bays, 

 or the mouths of rivers, has a great tendency to increase land 

 at the expense of the sea. So long as the sea continues shel- 

 tered, it throws up no barrier against the Mangroves; but 

 when their strange stilt-like roots have advanced the mass of 



* Illustrations of the HultoniaD Theory, p. 432,— Had it been the Professor's 

 fate to have lain in the trough of a 1>< avy following sea in the middle of the At- 

 lantic, or to have rejoiced in the dexterity of the helmsman in avoiding the shock 

 of a far seen heavy wave, he would hardly have supposed that following waves 

 were confined to the shore. 



f On the Geology of East Norfolk, p. 52, and the highly instructive plate 6. 



