1862.] WHrrAKER — London basin". 259 



paper show that the " thinning " treated of in the second is greater 

 than has been hitherto thought ; and the extent of this latter must 

 much change our notions as to the formation from which have come 

 the greater part of those loose blocks of Greywether-sandstone that, 

 in many places, lie on the surface of Cretaceous and Tertiary beds. 



The age of the sands noticed in the third part may also have some 

 bearing on that of the Grey wethers. 



The data on which a great part of this paper is founded have been 

 in my hands for some time ; and the conclusion that I have come to 

 with regard to the age of the Grey wethers at the western end of the 

 London Basin has been shortly given in the Geological Survey 

 Memoir on Sheet 13 (p. 48). I have great pleasure in knowing 

 that Prof. Eamsay wholly agrees with my views of the beds in that 

 district, to which this paper chiefly refers. The thiuning-out of the 

 London Clay in Marlborough Forest has also been noticed at p. 54 

 of the above-mentioned memoir. 



The new points of this paper, which treats of the London Tertiary 

 District alone, are — the proof of the occurrence of the London Clay 

 and the Lower Bagshot Sand further westward than they have been 

 before noticed* ; the thinning of the Woolwich and Eeading Beds 

 west of Hungerford ; the proof that the London Clay thins much 

 more quickly westward from Heading than has been hitherto thought, 

 and that in Marlborough Forest it has thinned out altogether ; the 

 inference from the above that further westward, where the Grey- 

 wether-blocks abound, the Bagshot Beds probably rested at once on 

 the Chalk ; the natural conclusion that the greater part of those blocks 

 came from that formation, and the further evidence in support of this 

 theory that may perhaps be given by certain sands, as yet of doubtful 

 age, that are foTind here and there on the Chalk of Surrey and Kent. 



I must state, however, that the idea that the Greywethers once 

 formed a part of the Bagshot Beds is not by any means new ; but it 

 has of late years been given up in favour of Mr. Prestwich's theory 

 that they for the most part belonged to the Woolwich and Eeading 

 Beds. With the data that Mr. Prestwich had, I do not see how he could 

 have come to any other conclusion than the one so ably and logically 

 worked out in the latter part of his paper in vol. x. of the Society's 

 Journal (p. 123) ; but I think that the further data given in the first 

 part of the present paper, and the conclusions to which I have shown 

 that they lead, in the second part, must lead us back again to the old 

 doctrine that the greater part (not the whole) of the Greywethers are 

 of Bagshot age. The Hertfordshire " pudding-stone " I agree with 

 Mr. Prestwich in referring to the Woolwich and Beading Beds. 



Paet L — The first part of this paper refers chiefly to the neigh- 

 bourhood of Bedwin and Savernake (or Marlborough) Forest, in Wilt- 

 shire, mapped m the north-eastern comer of Sheet 14 of the Map 



* Except on Sheets 12 and 14 of the Map of the Geological Survey, and in 

 the Memoirs on the former and on Sheet 13. The most western Tertiary outliers 

 in the London Basin (in Sheet 14) have not been hitherto described with any 

 detail. 



