Mr Black's Remarks on the Chessil Bank, Weymouth. 125 



The following remarks are chiefly founded upon reductions 

 to diagrammatic representation of the geological and mechani- 

 cal features of the bank, according to natural scale, taken from 

 the surveys of Sir J. Coode and the Ordnance Survey, supple- 

 mented by personal inspection of the site : 



1st, That the j)ebbles are assumed to be driven from the 

 west, at Bridport, by west-south-west waves running along 

 the beach obliquely, and also another set from the south, at 

 Portland, by south-south-west waves, according to the respec- 

 tive prevalence and force of these winds. 



2d, That the highest point of the bank on the Isthmus of 

 Portland may probably mark the meeting of these two sets 

 of wind waves' actions from west and south, and the respective 

 lengths of the bank west and south of this may be commen- 

 surate with the greater or less prevalence and force of each 

 of these winds. 



od, That the pebbles are not driven by the tides from the 

 bottom of Lyme Bay, as there is nothing there but mud and 

 sand in the soundings, but that they are derived from the 

 disintegration of the chalk, gTeensand, and red sandstone cliffs 

 of the coast farther west from Exmouth, and from the marl 

 cliffs at Chessil Town at Portland. 



4:th, That a line drawn perpendicular to the axis of the 

 bank lies in a south-west direction, and that lines drawn 

 perpendicular to the two ends of the bank will meet together 

 about forty-five nautical miles out in the channel to the south- 

 west, so that the bank may represent the arc of a great circle 

 of that radius. 



^th, That a line drawn parallel to the Portland perpen- 

 dicular will be found to lie between Start Point and Bridport, 

 and that the intervening space may be supposed to represent 

 the area of the west-south-west waves, driving the pebbles 

 along the bank, to the south-east, to Portland. 



Q)th, That a line drawn parallel to the Bridport perpen- 

 dicular will be found to correspond to one drawn between 

 Downend Point and Otterton Point, and the area between 

 them may be taken to represent the S.S.W. waves, driving 

 the pebbles from the west along the Dorsetshire coast. 



1th, That by the aid of the latter forces the pebbles are 



