332 



The Geology of Devizes. 



formation, but merely the accumulation of a given condition of deep 

 sea, synchronous as a whole with that portion of the Cretaceous 

 deposits which we call " Upper Greensand/" 



The case, therefore, is reduced to this, that Gault and Greensand 

 may be excellent names for certain kinds of rock-material, which is 

 in fact their primary and original signification, but they cannot be 

 appropriately used in a chronological classification. I do not hesitate 

 to say that the ordinary use of them in text books has conveyed a 

 totally wrong conception of the facts of Nature. Nine students 

 out of ten, and a great many teachers of geology, if asked to describe 

 the Gault and Upper Greensand, would place the former below the 

 latter, in the belief that it was entirely an older formation. This 

 is an error, for though there are localities (Devizes, for instance,) 

 where there is a Gault surmounted by a Greensand, neither of them 

 represents the whole of the Gault or the whole of the Greensand as 

 separately developed elsewhere. 



Again, the compilers of text books and of stratigraphical tables 

 have always found a difficulty in giving a list of the characteristic 

 fossils of the Upper Greensand ; and this is not surprising, because 

 most of the fossils which occur in these sands are also characteristic 

 of the Gault. Hence many have been driven to regard the fossils of 

 the Warminster Greensand as specially characteristic. This selection 

 of the Warminster fossils obliges me to say a few words on that 

 deposit. Dr. Barrois long ago demonstrated that in the compound 

 formation which comprises the Gault and Upper Greensand of 

 English geologists there were three well-marked zones of life ; 

 these he termed respectively the zones of Ammonites interruptus, 

 Amm, inflatus, and of Pecten asper ; and he showed that the equiva- 

 lence of these zones was as follows : — . 



Zone of Pecten asper= Warminster Beds. 



3f Amm. inflatus=Upper Gault and Blackdown Beds. 

 3J Amm. interruptus=Lower Gault. 



Now the zone of P. asper is very variable in thickness ; we have 

 seen that north of Devizes it is very thin, at Urchfont it is thick, 

 while at Warminster and in the Vale of Wardour it is a prominent 



