\'A4 I'ROCKEDINGS OF THK GKOLOGICAL SOCIETY. 



In my last paper on the Tlianet Sands I showed the probability of 

 the Wealden elevation having commenced at that early Tertiary 

 period ; and of a small island, vrithout any important river, having 

 existed somewhere in the central position of the present Weald. It 

 was further shown that the Thanet Sands present in part of their 

 range a worn and eroded upper surface, on which the "Woolwich 

 series reposes. Now as that surface was one of sand, the edge of 

 which would have been softened even by a small and prolonged 

 ordinary quiet action of the sea currents, which would also have 

 produced an intermixture of the two beds, I conceive that where 

 there is erosion and clear demarcation the change has been sudden 

 and abrupt ; but where on the contrary there is a passage in the 

 beds, as for example at Heme Bay, we must suppose a point more 

 distant from the centre of disturbance, and where the change both 

 in lithological structure and in the fauna only became apparent sub- 

 sequently, as the new order of things gradually prevailed over the 

 preceding one. Where, therefore, we have phsenomena of this de- 

 scription, and a distinct alteration, taken as a whole, in the litholo- 

 gical structure of the beds above and below such divisional lines, I 

 cannot but think that, however slight those alterations may be, they 

 indicate a change in the hydrographical condition of the then exist- 

 ing land and sea caused by movements sudden and powerful in pro- 

 portion to the effects exhibited on the pre-existing sea-bed by the 

 scouring power of the sea during the translation of its mass ; whilst 

 the permanent alterations in mineral composition show that such 

 effects were not transitory or momentary, inasmuch as they led to 

 maintained changes in the nature of the materials carried down 

 from the land or worn from the clifFs, indicating in fact a different 

 arrangement of the rivers and the coast lines. In this case the 

 change is proved also by the alteration in the character of the orga- 

 nic remains, which in the lower sands (Thanet Sands) are marine, 

 whilst the Woolwich series contains a superimposed group of estua- 

 rine and fluviatile shells. 



That the pebbles were rolled from time to time into the position 

 in which they are now found, and that they were not worn there, may 

 be inferred from the fact of their association with delicate and friable 

 shells which have remairted uninjured amongst them ; from iVIolluscs 

 having bored undisturbed into the bed of mixed sand and pebbles ; 

 from Ostrece and Serpulce having so often attached themselves to, and 

 gro^vn upon, some of the pebbles ; and from the circamstance that 

 broken pebbles occasionally occur, the fractured surfaces of which 

 only show worn edges, with no approach to a restoration of the 

 pebble form*. 



* The sand of the ^yoolwich Series I have heard attributed to tlie wear and de- 

 struction of tlie chalk-flints which produced the pel)l)les. That a certain quantity of 

 sand resulted from such an action is inevitable, hut that the whole mass was 

 derived from that source cannot be. The grains of the sand consist usually of 

 pure transparent quartz, worn but not rounded. The presence further of grains 

 of Chlorite and other allied minerals shows that there were other sources of supply 



