24 Mr. H. G. Seeley on the Potton Sands. 
With this confession I now proceed to examine Mr. Walker’s 
aper. 
I. The deposit in which the phosphate-bed occurs he names 
the Lower Greensand. The Shanklin (or Lower Green) Sand, 
as I understand it, is the series of beds between the Weald Clay 
and the Gault. But these sands at Potton are between the 
Gault and the Oxford Clay; and, so far as I remember, the 
only fossil previously recorded from the beds in this district is 
Ammonites biplex, mentioned in my paper on the Cretaceous 
beds at Ely,—neither of which facts offers any presumptive 
evidence of the deposit being Shanklin Sands. 
To assume the age is, no doubt, an easy way of settling an — 
exceedingly complicated problem, and at the same time enables 
us to assert with confidence that all fossils except those 
previously found in similar deposits must be extraneous fossils, 
derived from the denudation of older beds, or, if need be, of 
newer ones. But even if the Potton Sands had been Lower 
Greensand, for which there is not an atom of evidence published, 
I am not aware that there would be anything more wonderful in 
the occurrence of Gryphea dilatata in such a bed than there is 
in the occurrence of Ammonites Lamberti, a lower Oxfordian 
species, in the Kimmeridge Clay of Ely and in the Lower 
Greensand of Atherfield, or in the finding of the eminently 
Cretaceous Neithea quinquecostata in the Kimmeridge Clay of 
Weymouth. 
II. A paragraph further on, Mr. Walker calls the phosphatic 
deposit a conglomerate. The idea conveyed by the term to 
most men who have seen conglomerates is a deposit formed by 
the wearing up of older strata into rounded masses, which 
have often become cemented together. But this Potton bed is 
a quantity of rolled concretions of tolerably pure phosphate of 
lime with a quantity of rolled masses of sand, sometimes con- 
-ereted with phosphate of lime, sometimes with iron, rarely with 
silex, and a small proportion of old rocks: these are oftenest 
loose in sand, but sometimes bound into a hard mass by oxide 
of iron. The term conglomerate applied to this bed is caleu- 
lated to mislead ; for, involving the idea of denudation of older 
beds, these might furnish our author with his would-be extra- 
neous fossils. 
III. The author then questions my reference of this stratum 
to the Carstone. That name I have since proposed to restrict to 
the sands of Yorkshire, Lincolnshire, and Norfolk which occur 
between the Upper Greensand (Hunstanton Limestone) and the 
Kimmeridge Clay. But though I abandon the term, I do not 
abandon the idea; for what I wanted to express may be shown 
