PRESTWICH BRITISH AND FOREIGN TERTIARIES. 229 



up on the area, and which result in some measure from those very in- 

 termediate geological aberrations to which allusion has been made, and 

 which may give a sort of local stamp even to wide-spread deposits. 



If, therefore, the Calcaire grossier and the Bracklesham sands are 

 to be considered as synchronous, we may take them as furnishing a 

 common geological horizon, to which, in each case, other and more 

 independent local phsenomena in the several areas may be referred. 

 The point we have now to inquire into, is what proportion of the 

 separate faunas of these two deposits is to be traced downwards in 

 each respective centre. Taking a common measure, out of every 100 

 Mollusca living at the latest -named period, the following numbers 

 approximately express the proportion living at the several earlier 

 periods in each central area. 



French area. English area. 



Calcaire Grossier 100 100 Bracklesham Sands. 



Lits Coquilliers 29 



15 London Clay. 



Sables de Bracheux 5 6 Woolwich and Reading Series. 



If, instead of taking this upper horizon, we take the lower one of the 

 Woolwich and Reading Series in this country, and the Sables de 

 Bracheux in France, excluding in both the freshwater and fluviatile 

 fauna, the following are the results ; — 



Calcaire Grossier 30 31 Bracklesham Sands. 



Lits Coquilliers 38 



58 London Clay. 



Sables de Bracheux 100 100 Woolwich and Reading Series. 



Therefore, howsoever distant the relation between the Lits Coquilliers, 

 the Bracheux Sands, and the London Clay may seem, when viewed 

 with regard to space only, yet it becomes evident that, when viewed 

 in relation to time within their own centres, these strata occupy 

 certain definitely related and parallel planes ; that on such deductions 

 the London Clay holds a position intermediate between the Lits Co- 

 quilliers and the Bracheux Sands. This evidence by itself affords 

 presumptive proof of each area having one fossiliferous zone peculiar 

 to itself, and wanting in the other ; of each having a link in the 

 sequence which the other has not*. These calculations also afford 

 a singular corroboration of the interval of time assigned upon other 

 grounds to the top and bottom French and English zones in the 

 above tables. 



§ 8. The London Clay ; Systeme Ypresien Inferieur, or Glaise 

 Tpresien ; not represented in the Paris Tertiary district. 



In following the Lits Coquilliers and the London Clay as they 

 respectively range, the one towards the London, and the other 

 towards the Paris Tertiary district, there is no appearance of any suf- 

 ficient change in mineral character that would tend to assimilate them 

 to each other. The London Clay retains, with the slight exception 

 mentioned further on, the same well-marked characters in the Isle of 

 Wight and Hampshire as it does around London. It has not hitherto 



* The unfossiliferous Lower Bagshot sands may probably occupy the 2nd 

 English parallel. 



