﻿Yol. 6 1.] phosphatic chalk: op taplow. 489 



(a) Though the zone of Micraster cor-anguinum is, of all the sub- 

 divisions of the Upper Chalk in the South and South-East of England, 

 the least liable to great fluctuation in thickness ; though the mono- 

 tonous character of its fauna argues a stability in the physical 

 conditions during the deposition of its constituent beds which 

 would be entirely in keeping with this uniformity, yet the a-priori 

 probabilities of its attenuation in this neighbourhood are but 

 little smaller than those of an exceptionally-marked shrinkage 

 in the succeeding zone — such as we know to occur. But, if the 

 Marsupites-Beds, by their small development at the Lodge section, 

 serve as a precedent for zonal thinning in the Taplow area, 

 they do not serve as a precedent for rapid zonal thinning, for 

 their apparent isolation leaves us entirely in doubt as to the 

 rate of lateral attenuation in their case ; and the possibility of the 

 Micraster cor-anguinum-Zone suffering so large a reduction as that 

 above indicated within so small a space — a reduction, moreover, 

 unsignalized by appropriate changes in the character of the higher 

 beds of that zone in the vicinity — is one which, we imagine, will 

 recommend itself to few who are familiar with the Chalk. 



(b) The alternative supposition under this heading, namely, that 

 the trough was formed by submarine erosion towards the close of cor- 

 anguinum times, and refilled with the latest beds of that zone and 

 the more richly-phosphatici¥«r5^i^s-sediments, finds some support 

 in the mode of occurrence of certain bodies of Phosphatic Chalk 

 in Northern France, for instance, Hardivillers (Oise), Senercy, and 

 Hem-Monacu(Somme), which have been shown, or inferred, to occupy 

 eroded hollows. The evidence of the erosive origin of the phosphate- 

 basins in that country is not always as complete as one could wish ; 

 and some of the phenomena attributed to intra-Cretaceous ' ravine - 

 ment' seem equally explicable by other processes. Setting aside 

 the doubtful cases, however, there remain a number of others in 

 which the basin almost certainly owes its existence to the scouring 

 action of currents upon the Upper Cretaceous sea-bed. 



Now, in the French phosphate- workings the bored and hardened 

 floor ('craie blanche durcie, a surface vernissee') of the 

 lowest bed of rich Phosphatic Chalk is usually regarded as the down- 

 ward limit of the erosion referred to, that is to say, as the bottom 

 of the phosphate-trough ; and, during the earlier stages of our exa- 

 mination, we were much disposed to view the corresponding feature 

 at the top of the Micraster coi*-anguinum-Bed.s (A) of the Taplow- 

 Court section in the same light. "When, however, it became apparent 

 that the greater part of the (A) beds were not only distinct from 

 those visible at a similar, or at any other, depth below the base of the 

 Eocene deposits in the adjoining country, but in addition so intimately 

 allied to the overlying Chalk as fully to warrant their inclusion in 

 the Phosphatic Series, that idea was abandoned. As already pointed 

 out, the occurrence of erosion-phenomena within the Phosphatic 

 Series has no direct bearing on the problem of the mode of occur- 

 rence of the latter as a whole ; and if that Series, as we define it, 

 lies in an eroded hollow, the unconformity marking the bottom of 



