24 Mr. H. G. Seeley on the Potton Sands. 



With this confession I now proceed to examine Mr. Walker's 

 paper. 



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 oflfers 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 Gryijlicea 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 Atheriield, or in the finding of the eminently 

 Cretaceous Neithea quinquecostata in the Kimmeridge Clay of 

 Weymouth. 



IL 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- 

 creted 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 calcu- 

 lated to mislead ; for, involving the idea of denudation of older 

 beds, these might furnish our author with his would-be extra- 

 neous fossils. 



IIL 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 



