Vol. 52.] EOCENE DEPOSITS OF DORSET. 495 



seems to have continued till early Pliocene times. It would thus 

 appear that the district between Dorchester and Wej mouth is one 

 of those areas of weakness which are affected again and again by 

 similar disturbances. 



The discovery of the peculiar composition of the Eocene gravels 

 of Dorset has thrown an unexpected light on the source of the 

 material in the Pleistocene series. These gravels, from Brighton 

 westward into Dorset, always contain a considerable proportion of 

 Greensand chert of marked character, besides other foreign stones. 

 Of these stones a considerable number have been shown to be 

 glacial erratics brought by floating ice, though the glacial erratics 

 are entirely confined to low levels and to the area between Brighton 

 and Southampton Water. * Above that level and beyond that district 

 the Plateau-gravels contain, however, large quantities of chert and 

 also of Palaeozoic grits and quartz. The cherts have usually been 

 considered to point to derivation from a central Wealden axis. But 

 when it is found that the masses become larger and more abundant 

 westward, and that they are always associated with Palaeozoic grits 

 that could not be derived from the Wealden area, it is evident that 

 they can have no connexion with the Weald, but clearly must have 

 been derived from Devon and Dorset. Every kind of rock found in 

 the Plateau-gravels of Sussex, Hampshire, and Dorset above the 

 level of the glacial action has now been traced to the Bagshot 

 gravels of Dorset, and through them to districts still farther west. 



Discussion. 



Mr. Steaban said that he had had occasion to trace part of the 

 Tertiary base-line while mapping the Secondary rocks in South 

 Dorset, and that he had speculated on the age of the gravel-outliers 

 near Dorchester. No clue, however, was forthcoming until the 

 Tertiary deposits were mapped by Mr. Eeid from Hampshire con- 

 tinuously westwards, and the overlap of the Bagshot Beds traced 

 step by step. It had seemed to him that there was a marked dis- 

 cordance in the extreme west between the Eocene and Cretaceous ; 

 in fact, the Chalk seemed to have been carved into hill and dale 

 before it was overspread by the Eocene. Moreover, in drawing a 

 section through Bincombe, he had found that there was not room 

 for the whole of the Chalk below the Tertiary outlier, and had been 

 obliged to show a marked overlap by the latter, as had been previously 

 done by Sir Joseph Prestwich. The Author's observations tended 

 to confirm this conclusion. He considered that the Author had made 

 a material advance towards solving an important problem in Tertiary 

 geology. 



Mr. Monckton remarked that if pebbles from the Permian of the 

 West of England were found in the gravels described by the Author, 

 it was rather surprising that the liver-coloured quartzites of the 

 Triassic pebble-beds should not also occur. He asked whether there 



1 See Quart. Journ. Geol. Soc. voL xlviii. (1892) p. 344. 



