130 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



country also of the Druid Sandstones from beds of the age of the 

 "Woolwich and Heading series*." 



§ G. Conclusion. 



The sectional diagrams, A, B, C, PI. I., which, owing to the want of 

 some connecting links, were not completed until after the preceding 

 pages were written, (ionfirm, in my mind, by the structural fitness of 

 their parts, the conviction, before expressed, and derived from litho- 

 logical and palseontological evidence, of the independence^ of the 

 "Middle division of the Lower London Tertiaries," with regard to 

 the "Thanet Sands," — the latter forming a distinct and underlying 

 marine deposit ; and that, notwithstanding the nearly total diiference 

 of all its characters, the estuarine and freshwater group of fossilifcrous 

 strata at Upnor and Woohvich must be regarded as strictly syn- 

 chronous wath the unfossiliferous mottled clays of Alum Bay, Reading, 

 and Hedgerley. Cases are common where such changes of condition, 

 as those displayed in this " Woolwich and Reading division," take 

 place in particular beds of a series ; in this instance, however, the 

 alteration affects the whole depth of the group, not as a recurring 

 change at different periods, but as a maintained development at 

 different places. It shows an accumulation of materials not within 

 the range of a single river action, howsoever variable, but of con- 

 temporaneous strata deriving, in the same sea, supplies from different 

 and independent sources. Still, notwithstanding the variable cha- 

 racter of the mass as a whole, there are two subordinate featiires, the 

 one mineral and the other palseontological, sufficiently well maintained, 

 although not constant, over both the Reading and the Woolwich 

 areas to afford a common base-line. It has been shown that the 



* I find that M. Passy, in his ' Descrip. Geol. du Dep. de la Seine Inferieure ' 

 (Rouen, 1832), takes the same view of the relation of the " Grey-weathers " of 

 Wiltshire and of the " Ores a silex pyrouiaques" of the Dieppe Cliffs, dra\\-ing 

 his conclusions, however, with respect to the former, simply from their Uthological 

 resemblance to the latter (pages 127-131). 



•f- At all events so far as the central and eastern areas are concerned. The only 

 point about which I feel slightly doubtful, is whether some of the thick pebble 

 beds under and around Shooter's Hill may not belong to the upper part of the 

 Woolwich series, rather than to the Basement of the London Clay, inasmuch as the 

 character of the former is so variable as often to render it lithologically undi- 

 stinguishable from the latter, except when seen in actual superposition, and for this 

 extremely few opportunities occur. For the same reason the beds which at Upnor 

 and Heme Bay I have included in the " Basement-bed " may also possibly belong 

 to the upper section of the Woolwich series, in which case the Basement-bed 

 itself might be considered in this area to merge into the thin seam of sandy clay 

 (with fossils at Upnor and with a few pebbles at Heme) just at the base of the 

 great mass of the London Clay. It is also (hfficult to say positively whether some 

 lower portion of the Reading series may not possibly be synchronous with the 

 Thanet Sands. In the absence of sections which alone could clearly settle this 

 point, I have given the best conclusion I could arrive at upon collateral evidence. 

 I mention these doubts, which, however, do not affect the superposition and 

 grouping of the three divisions here proposed, although it would modify the exact 

 lines of separation, in order to ilirect attention to any new facts which may arise 

 to throw light upon those rjucstions where 1 consider the evidence not quite 

 conclusive. 



