le* Mr. De la Beche's Notes on the Formation 



for each end of the bank (See fig. 1.). It also appears that the 

 shingles do not travel from the bank (see fig. 1.) ; for Portland 

 Roads have a bottom of clay, the continuation of the Kim- 

 meridge clay of the base of Portland and the Ferry Point, 

 affording one of the best holding grounds for vessels in the 

 Channel; and the bottom to the S.W. of the bank is sand, 

 fine gravel with shells, or rock*. 



Shingle beaches are generally formed on the sea shores under 

 consideration, from the harder parts of the neighbouring 

 coasts, destroyed by the joint action of atmospheric agency, 

 land springs, and the sea. The softer portions ai'e soon washed 

 away, and even the harder, first forming the shingles, are 

 eventually ground down into sand. It is, however, by no means 

 uncommon to find, in coasts composed of both hard and soft 

 materials, taluses of blocks or large indurated concretions, de- 

 tached from the cliffs, and defending them from that quick 

 destruction 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 

 decomposes into its usual blocky forms, the angles gradually 

 disappear, and eventually the masses fall on the beach, where 

 the tremendous 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 follow- 

 ing waves were merely confined to the shore f, for the de- 

 struction of coasts of equal hardness almost always bears a 

 proportion to the extent of open sea to which they are ex- 

 posed, allowance being always made for the force and dura- 

 tion of prevalent winds. 



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

 other circumstances than those previously noticed, is very ably 



• This bank also possesses considerable interest in another point of view. 

 The hills behind the bank are composed of clays and loose rubbly or slaty 

 limestones (Forest marble, Cornbrash, Oxford clay, Oxford oolite, and 

 Kimmeridge clay), which, if not protected by this mass of shingles, would 

 soon be swept away before the heavy seas rolling in from the Atlantic, and 

 breaking with so much fury on this coast. That they have not been thus 

 attacked is evident, for the large rounded forms ol the hills and dales are 

 only here and there marked by little cliff's, cut by the water intervening be- 

 tween the bank and main-land ; it therefore seems fair to conclude, that 

 since the existing order of things the Chcsil Bank has existed, and that the 

 main-land behind it has not, since it acquired its gentle undulatory form, 

 been attacked by the furious waves from the Atlantic. 



f Illustrations of the Iluttonian Theory, p. 432. — Had it been the Pro- 

 fessor's fate to have lain in the trough of a heavy following sea in the mid- 

 dle of the Atlantic, or to have rejoiced in the dexterity of the helmsman in 

 avoiding the shock of a far seen heavy wave, he would hardly have sup- 

 posed that following waves were confined to the shore. 



illustrated 



