274 A. J. JTJKES-EROWNE ON THE RELATIONS OF 



the strata in Bucks and Bedfordshire. This I interpret to mean, that 

 while sandy material was being deposited on the Gault in the south- 

 west of England, that formation was being gradually denuded over the 

 space occupied by the counties of Cambridge, Herts, and East Bedford ; 

 and the result was the formation of the well-known phosphate-bed 

 when this period of erosion had come to an end. Between these 

 two areas, that of deposition on the one hand and denudation on the 

 other, there seems to have existed a tract where neither the one nor 

 the other obtained ; this extended from Harlington, in Bedfordshire, 

 to the borders of Buckinghamshire ; and from the time of the Upper 

 Gault there seems to have been no deposition of any material over 

 this area until it sank beneath the waters of the Chalk-marl sea. 



The same results may be represented, and perhaps more clearly, 

 in three comparative columns : — 





Cambridge. 



Bedford. 



Buckingham. 



5. 



Chalk-marl. 



Chalk-marl. 



Chalk-marl. 



4. 











Chloritic Marl ? 



3. 











Upper Greensand. 



2. 



(absent by erosion). 



Upper Gault (in part). 



Upper Gault. 



1. 



Lower Gault. 



Lower Gault. 



Lower Gault. 



The above inferences and remarks follow from a consideration of 

 the stratigraphical evidence alone ; and with the object I have in 

 view, viz. that of determining the true position of a given stratum, 

 such evidence is usually of more value than that derived from a 

 comparison of the fossils with those of other more or less distant 

 localities. In the present instance, however, the very fact which is 

 prominently brought out by the above inquiry, viz. that many of 

 the Cambridge fossils are likely to have been derived from under- 

 lying beds, necessitates a thorough and exhaustive study of the 

 fauna ; and in the following pages I have endeavoured to do justice 

 to this part of the subject. 



I desire to record my obligations to Mr. Bonney, and to Messrs. 

 Whitaker and Penning, of the Geological Survey, for suggestions 

 and information relative to the preceding part of my paper ; and I 

 heartily thank them for the interest they have taken in its pre- 

 paration. 



Part II. — Pal^iontological Relations. 



§ 1. Natural Division of the Cambridge Fauna. — The fossil contents 

 of the Cambridge Greensand are readily divisible into two groups or 

 faunas, distinguished by the different states of preservation which 

 their respective components exhibit : the one of these is derivative ; 

 the other is in situ, and belongs to the sandy marl of the formation. 



The fossils composing the one fauna are mineralized by a dark 

 phosphate of lime, either dark brown or nearly black in colour. The 

 bones are nearly always much rolled and worn. The molluscan 

 remains occur usually in the form of more or less waterworn casts, 

 the shell being either entirely removed or else, as is sometimes the 

 case, replaced by calcic phosphate; among the Ostreidae, however, 



