258 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dt'C. 1 6, 



These several considerations have distinct bearings. 



The marginal sea-belt is under all circumstances both well-defined 

 and distinctly marked. It commences from the lowest range of 

 wave-oscillation, where that meets the line of sea-bed, and extends 

 upwards to the utmost reach of breakers at high water. The power 

 exerted over this space commences at the point where the wave 

 becomes one of translation, and increases progressively to where it 

 breaks ; from this its moving power decreases, being that only which 

 is exerted by the upper portion of the wave as it is dashed forward. 

 In all seas, therefore, the breadth of this zone will be primarily 

 dependent on the slope of the sea-bed, and next on the range of the 

 local tides. 



The amount of power thus exerted is very variable ; and it is applied 

 under very different combinations of the foregoing conditions ; so that 

 it happens that the materials of the marginal zone are unceasingly 

 being transferred one way or another upwards and outwards, at 

 intervals corresponding to those when the forces are brought to bear 

 in those directions. It is in this way that in tidal seas we have two 

 distinct divisions of the marginal zone, — an upper, which is constantly 

 disturbed, and composed of clean shingle ; a lower, consisting of like 

 materials as to size, and which is only occasionally broken up. In 

 the intervals the constituents of this lower marginal zone become 

 covered with its characteristic animals and plants. The conditions 

 under which portions of the submarginal zone are transferred upwards, 

 though variable, are yet to be easily observed from any part of a 

 coast-line. On our own shores abundant materials will always be 

 met with in the upper track, which bear marks about them of having 

 belonged for some previous time to the submarginal zone. 



These marks or characters have a bearing on the history of some 

 of the pebbles found in the Chalk. To this zone belongs the great 

 belt of marine vegetation which attaches itself to the shingle and 

 gravel, as also do certain species of Balanus, Serpula, Anomia, and 

 small Oysters ; and such pebbles, even long after they have been 

 washed up from the lower to the higher zone, retain the traces or 

 some remains of these incrusting forms. Of the smaller specimens 

 of shingle which have been taken from the White Chalk, some are 

 in the condition of clean pebbles, whilst others have still adhering to 

 them some portions of such shells. So far as the evidences of the 

 zone of origin of all the extraneous materials of the Chalk can be 

 indicated from such characters as we have here been considering, 

 we may feei sure that they all have belonged to the upper marginal 

 zone, — that, though in some cases they may have travelled down, 

 and found a temporary resting-place in the submarginal region, yet 

 their subsequent place was at the upper level, before they started 

 away to find their ultimate position in the deposits of calcareous mud. 



Ordinary wave-disturbance along the marginal sea-line cannot dis- 



sinoothly rounded stones, which are occasionally found in the fine-grained oolitic 

 strata of Normandy, may have heen voided by Crocodilians of that period. I am 

 indebted to Mr. Bowerbank for the information that Sharks also swallow small 

 stones ; hence another agency by which the shingle of the White Chalk period may 

 have been transferred to areas of deep sea. 



