60 GEOLOGICAL NOTES OX THE ISLE OF PORTLAND. 



Lyell attributes it to the strong currents or movements of the 

 sea during storms, when a gale from the south-west co-operates 

 with the tide, and acts more powerfully in the open channel or 

 the part furthest from the head of the bay, within which the land 

 affords more shelter from the wind and waves ; in other words 

 the force of the sea increases southward, and as the direction of 

 the Bank is from north-west to south-east, the size of the 

 pebbles coming form the westward and thrown ashore, must 

 always be largest where the motion of the waves and currents 

 are most violent. In November, 1853, during a gale, the south- 

 west wind threw in upon the Bank during one night and part of 

 the following day, a mass of shingle amounting to no less than 

 three and a half million of tons. Mr. Godwin Austen agrees 

 with Sir John Ooode as to the direction of the moving power 

 being from west to east. Mr. Gr. H. Kinahan accounts for the 

 arrangements and sorting of the pebbles to the flow-tide 

 currents, and the powerful prevailing winds acting together in 

 the same direction. As Lyme Bay is open and unobstructed, 

 the Bank is exposed to the full force, not only of 

 the tidal current, but also of the most prevailing and effective 

 winds; the current also increases in velocity as it passes on east- 

 ward, carrying with it large fragments which are driven ashore 

 and accumulated on the Bank. The inshore currents branching 

 away from the main stream, increase in velocity from west to 

 east, each carrying different sized pebbles, which, when 

 thrown ashore, cause their peculiar arrangements and sorting. 

 Professor Prestwich on the other hand considers the materials 

 of the Bank to be derived from an ancient beach, of which the 

 raised beach at Portland Bill is a remnant, which contains 

 besides chalk-flint with the Devonshire pebbles, Greensand chert- 

 pebbles, and others of Portland- stone flint, together 

 with a few pebbles of the harder limestones and oolites 

 of Portland. Although he does not doubt that originally, 

 many of the pebbles are derived from Devonshire rocks, 

 he does not consider their transport to Portland due to 



