$B2; 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 Gaulé 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. 
a Amm. inflatus=Upper Gault and Blackdown Beds. 
if 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 
a 
