1862.] WHITAKER LONDON BASIN. 271 



between those formations. Now, if the thinning should continue 

 (as there is good reason to suppose, from its constancy in the dis- 

 tricts where enough of the beds to show their order and thickness 

 has escaped denudation), still further west the Bagshot Beds would 

 rest directly on the Chalk, all the Lower Eocene strata having thinned 

 out. This will, perhaps, be made clearer by the diagram -section, 

 p. 263, which shows the thickness of the various Lower Eocene beds 

 from Woolwich to Marlborough Forest. 



Part III. Age of the Qreywethers. — Mr. Prestwich has inferred * 

 that the blocks of Grey wether- sandstone scattered over the surface 

 of the Chalk and other formations have once formed part of the 

 Woolwich and Beading Beds. His reasons are, that their distribution 

 is ^' in accordance with the range of the Lower London Tertiaries " 

 [the basement-bed of the London Clay, the Woolwich and Beading 

 Beds, and the Thanet Sand] *' rather than with that of the Bagshot 

 Sands \ " and that, as there is no reason for supposing them to have 

 come from either the basement-bed or the Thanet Sand, they must 

 be referred to the intermediate Beading Beds ; — that this conclusion 

 is borne out by the facts that the occurrence of the greywethers " is 

 exactly coincident with the development and preponderance of the 

 sand-beds of the mottled clay " (that is, the Woolwich and Beading) 

 "series," and that "the lithological structure of each variety is 

 respectively in accordance with the mineral components forming the 



strata in the immediate vicinity i.e. that the concretionary 



stone in each case represents the component parts of some portion 

 of the adjacent Woolwich and Beading series ;" thus, " in the neigh- 

 bourhood of Hatfield, Hertford, and Ware, the sands of the Beading 



Series are often glutted with flint-pebbles ; it is 



over this area more particularly that the Hertfordshire pudding- 

 stones are so abundant." 



Speaking of the gravel- drift around Newbury, which contains many 

 blocks of greywether-sandstone, Mr. Prestwich says, " The course of 

 this drift is towards, and not from, the area of the Bagshot Sands ; 

 and as we have no proof of the extension of this formation over the 

 chalk-downs, whereas we know that detached outliers of the Lower 

 Tertiary sands extend far over those hills, we should expect to find 

 in the drift the debris derived from the latter and from the Chalk, 

 and not from the Bagshot Sands." 



I think, however, that what has been said in the former parts of 

 this paper must lessen the force of Mr. Prestwich's argument, founded 

 as it is on evidence " circumstantial rather than direct," I have not 

 only proved the extension of the Bagshot Sand over the chalk-downs, 

 but have shown that in Marlborough Forest, owing to the dying- out 

 of the London Clay and the thinning of the Beading Beds, that for- 

 mation is but 15 feet or so from the top of the Chalk. If the Beading 

 Beds became still thinner further westward, as is most likely to have 

 been the case (unfortunately there are no outliers of any Tertiary 

 bed on the Chalk in that direction), the Bagshot Sand would gradually 

 * Quart. Journ. Geol. Soc. vol. x. pp. 123-130. 



VOL. XVIII. PART I. T 



