M. B. Newton — Cretaceous Zones in Dorset. 199 



In tabular order the beds : may be thus arranged : — 



Feet. 

 Subsoil 1 



Yellow clay with chert 3 



Brown clay without chert 5 



Hovlites ( ^ark-grey coloured, micaceous, and sandy clay \ 



incerrwtus \ w ^ P nos P na *ic nodules; fossiliferous in the [ 15 



Zone ' ) lower 4 feet ' 



\ Brown sandy rock, with fossils in the upper part 4 



Acanthomas ( Ar g illaceous saud y beds, micaceous and of a 



mammillatum b ™ wn ' «**> "J! 8 " ™ 11 ?° lour I ferruginous 



Zone / a °°htic 5 siliceous pebbles interspersed ; 



\ fossiliferous 



PAptian. Pure sand 3 



Kimeridgian. Stiff blue clay 8 or 10 



? Corallian. Sandy rock. 



Among the great group of the Ammonites characterizing the 

 Cretaceous formation, none is perhaps of more importance than that 

 of Acanthoceras mammillatum. As a zonal species it is of the 

 highest value to the geologist, being restricted in its distribution to 

 the topmost stratum of the Folkestone beds, and never occurring 

 either above or below this position in the series. 2 The present 

 discovery, therefore, of the Acanthoceras mammillatum zone in Dorset- 

 shire is of interest as establishing its extension more westerly than 

 hitherto recognized. A few years ago Mr. Jukes-Browne called 

 attention (Geological Magazine, 1891, p. 456) to the occurrence 

 of some clays and sands beneath the Gault in Dorsetshire, which 

 were traced for a distance of four or five miles between Twyford 

 and Childe Okeford, and determined as of Lower Greensand or 

 '■ Vectian " age. The stratigraphical particulars given of these 

 beds appear to correspond so closely with those observed at Okeford 

 Fitzpaine that it is highly probable they refer to the same series 

 of deposits, though, as no palasontological evidence was referred to 

 by Mr. Jukes-Browne, it must remain a little doubtful until a future 

 comparison can take place on the spot. There is, of course, no 

 reason why the beds on the eastern side of the Stour valley should 

 not be continued across the river in a south-westerly direction to 

 Okeford Fitzpaine, and even to other localities further on that are 

 situated near the junction line of the Kimeridge with the Cretaceous 

 rocks (see Geological Survey map of this district, No. xviii). 



Opinions were long divided as to whether the Ac. mammillatum 

 zone represented the top of the Lower Greensand (Aptian) or the 

 base of the Gault (Albian). The latter view is that generally 

 adopted now, on account of so many species passing up into the true 

 Gault series. The present fossils contain only two forms which 



1 I am greatly indebted to Miss Forbes and Miss Lowndes for supplying me with 

 the details of this section. Unfortunately, only the Moplites interruptus beds are 

 now exposed, the lower part of the section having been tilled in. 



2 In Beid and Strahan's " Geology of the Isle of Wight" (Mem. Geol. Survey, 

 1889, p. 279), Ac. mammillatum is stated to have been found in the Chloritic 

 Marl of Mottistone Down, which is probably either a misdeterminatiou or some 

 error has crept in as to the horizon. 



