on the Geology of the South-East of Surrey. 223 



dental presence of certain minerals, such as oxides or silicates of 

 iron, seams of chert or flints, &c. ; and though they may hold good 

 and prove very useful in the south-eastern parts of England, they fail 

 when applied to the whole area of the chalk and its subjacent beds, 

 and will be found to interfere with the grouping of the remains of 

 the animals which range through this system. Hence it is there 

 have been so many doubts respecting the positions in the series 

 which the deposits of several localities should occupy ; as, for in- 

 stance, those of Blackdown and Haldon in the west of England, and 

 the Speeton clay of Yorkshire. 



Again, the subdivisions founded on mineral character, even when 

 they are sufficiently marked to produce distinct physical features 

 over the surface of a district (and which has been much insisted 

 upon by geologists), will often be found to interfere most inconve- 

 niently with those derived from a consideration of the included or- 

 ganic remains : thus no contrast can be greater than that between the 

 upper and lower chalk ; the latter abounding in huge and varied 

 forms of Ammonites, Scaphites and Turrilites, — which are altogether 

 wanting higher up, where the Cephalopods are represented by one 

 or two species of Belemnite only (passing over the differences which 

 the other classes present) ; so that seven may nearly represent the 

 number of species common to the two. 



In this instance a great change in the conditions of animal life is 

 unaccompanied with any very obvious change in the character of the 

 deposit. 



The grey calcareous beds of the lower chalk are underlaid by cal- 

 careous sands and bright green siliceous strata, forming a well-defined 

 mineralogical group, and which has been formed into the upper 

 greensand : but subordinate to these are beds of firestone and thick 

 bands of limestone, and in these all the Cephalopods of the lower 

 chalk reappear; so that here a change of some sort, sufficiently 

 great to produce very different deposits, was not attended with any 

 sensible change in the form of animal life. 



The topographical arrangement of these several groups in an as- 

 cending order is as follows : — 



The Neocomian group, or the equivalent of that for which the 

 French and Swiss geologists have adopted that name, is found only 

 within the Wealden denudations, and rests everywhere, in the south- 

 east of England, on the blue Wealden clays, which were the central 

 deposits of that ancient estuary. 



The Speeton clay of Phillips has been referred to the gault, be- 

 cause it contains a greater number of species in common with that 

 group than with any other ; but the number is very small. Of the 

 species supposed to be peculiar the Corbula punctum is generally 

 quoted from the lowest beds of the French cretaceous group ; and 

 in addition, M. D'Orbigny has ascertained that the Hamites pli- 

 catilis belongs to the genus Crioceras, whilst the Hamites intermedins 

 and beanii are both species of Ancyloceras, a genus which is cha- 

 racteristic of the lowest beds of the cretaceous series. To these 

 we may add Spatangus retusus ; so that it becomes very probable 



