528 Correspondence — R. M. Brydone. 



I do not know with whom ProCessor Bonney is arguing that it is 

 "more probable " that the thin slab is a separate boulder. I clearly 

 stated this as my view (and see my fig. 17). 



As to the grey chalk, I hold it to be of Cretaceous age because the 

 soft matrix contains a pure Cretaceous fauna very abundant both in 

 species and individuals, many of the perfect or well-preserved 

 specimens being so delicate that the presetice of one in a reinanie 

 deposit would be very remarkable, and the })resence of two almost 

 miraculous. Apart from the basement bed and the intimate mixture 

 of very fine clay which causes the greyness, the grey chalk is 

 absolutely pure throughout the thickness (maximum eleven feet) 

 which has been exposed at the North Bluff. 'I'his makes a strong 

 "prima facie case as yet unanswered. I have also good ground to 

 believe that Clement Keid's sandy bed (which I have already found 

 lying unconformably on Ostrea lunnta chalk) is the basement bed of 

 the grey beds which on the foreshore crop out from under Oatrea 

 luiiaia chalk, and are not only identical with the grey chalk of the 

 bluff in appearance, fossil contents, and peculiar flints, but are also, as 

 I can now say, the only other beds in which I have found Terebratula 

 obesa, Ostrea iucsquicostnta, or Ostrea canal iculata. The significance 

 of this is obvious. 



Professor Bonney ignores the direct evidence as to the North Bluff 

 and presumptive evidence as to the South Bluff that they are in 

 direct physical connection with large masses of the foreshore chalk, 

 and abstains from discussing any palaeontological evidence or the 

 behaviour of the foreshore chalk. This of course simplifies the 

 criticism of a theory based almost wholly on those three classes of 

 evidence, but which when formed proved capable of application to 

 the special case of the North Bluff. 



It may be well to take this opportunity to point out (to the general 

 public) that the arch sketched by Professor Bonney was formed in 

 March last, and is not the arch of Ostrea htnaia chalk and grey chalk 

 over a pinnacle of clay referred to in my paper, and which was 

 broken through on the Ist October, 1905. His line g is the coarse 

 basement bed of the grey chalk, the continuation of which on the 

 opposite side of the arch, where it is less coarse, he has missed. 



Mr. Jukes-Browne's letters leave untouched my original proposition 

 that Terebratnlina striata is the best zone fossil. 1 am also still 

 unready to admit that it is logical to combine several beds easily 

 distinguishable palfeontologically or lithologically, often in both 

 ways, in one zone on the strength of a common peculiar fauna, and 

 then to name the zone from a fossil which is most capriciously 

 restricted to some only of these beds. The characteristic assemblage 

 of fossils he quotes in the Survey Memoir, vol. iii, p. 12, can only be 

 obtained in the Ostrea lunata bands, if in all of them. 



R. M. Brydone. 



16, South Audley Street, W. 

 Wi October, 1906. 



