By A. J. Jukes- Browne, B.A., F.G.S. 



331 



marly portion of the Gault. Everyone who has described the 

 Folkestone Gault has divided it into two portions, characterised by 

 different species of Ammonites; now of the five Ammonites which 

 occur in the Devizes Sandstone two (Am. rostratus and Am, 

 varicosus) are specially characteristic of the Upper Gault, two range 

 throughout the Gault, and one occurs only in the Lower Gault at 

 Folkestone, though elsewhere it is also found in the Upper. 



Again, if the Malmstone and Sandstone of Devizes formed a 

 separate zone later than the Upper Gault, this Upper Gault should 

 occur beneath the Malmstone ; but we have seen that the fossils 

 found in the old brickyard at Dunkirk were Lower Gault species. 



It is clear, therefore, that the sandstones which contain the Devizes 

 fauna are merely a sandy facies of the upper portion of the Gault, 

 or conversely that the Upper Gault of Folkestone is the argillaceous 

 representative of what is elsewhere called Upper Greensand. 



It follows from this that the Gault and Greensand are not distinct 

 subdivisions, as generally supposed, but that the mass of the Green- 

 sand was formed contemporaneously with the upper part of the 

 Gault, and is sandy because it was formed nearer the shore of the 

 western Cretaceous land. It is, indeed, very probable that at Black- 

 down, in Devonshire, the whole of the Gault is represented in what 

 is there called Upper Greensand ; for the Blackdown fossils are 

 almost identical with those of Devizes, and at the base there is 

 some dark argillaceous sand. 



So long ago as 1850 Mr. Godwin-Austen was fully aware not 

 only of the distinctness of the Lower Greensand for which he 

 endeavoured to introduce the name Neocomian, but also of the fact 

 that the Gault and Upper Greensand are merely different facies of 

 one formation. Thus he writes (Quart. Journ. Geol. Soc, vol. 6, 

 p. 461; ■—« In applying the names of Gault and Upper Greensand 

 to the beds which underlie the Chalk along the line here described 

 it is not intended to convey the notion that any separation can be 

 traced between two well-defined groups, or that even any true sandy 

 beds occur. Indeed there is no name in the whole series of geological 

 formations so purely conventional as that of Upper Greensand." 

 Again, on p. 472 :— " The Gault, moreover, is not an independent 



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