﻿Vol. 6 1.] PHOSPHATIC CHALK OP TAPLOW. 481 



and, having regard to the mineralogical condition of the Marsupites- 

 and Uintacrinus-besxring brown chalks, and to the concentration of 

 fish-remains, belemnoids, oysters, and other fossils within them, it 

 seems rather more probable that their small thickness is chiefly 

 due to an initial defect of sediment. The currents which scoured 

 and pitted the rock-beds may very well have hindered or prevented 

 the deposition of the finer calcareous matter; and the closely- 

 packed debris of the larger fossils (e.g., Inoceramus, Echinocorys, 

 and Micraster) in the lower parts of the brown bands is in harmony 

 with this idea. The presence of fine chalky paste in every part of 

 the Brown Chalks (excepting, of course, the sandy decomposition- 

 seams) at the present day does not constitute a grave objection, for 

 even where the coarser material was washed clean, the chalk-mud 

 of the succeeding whiter beds would inevitably have penetrated 

 into their interstices — the mixing process being hastened by the 

 boring organisms whose work is here so much in evidence. 



The reduction in the total thickness of the zone of Marsupites at 

 Taplow Court below that which was normal in this part of the 

 country, arising from the small development of those characteristic 

 stages that figure so prominently elsewhere, may have been con- 

 siderable, but cannot now be even approximately gauged, as the 

 erosion which preceded the deposition of the earliest Eocene 

 sediments has left no accessible Chalk of that age within a distance 

 of nearly 20 miles. 1 



Assuming the base of the zone of Actinoeamax quadratus to have 

 come in 10 feet above the highest beds of the Lodge section — and 

 the evidence from the form of Echinocorys and the presence of 

 Ostrea lateralis var. striata in the upper beds does not, we think, 

 warrant a higher estimate — the total measurement of the Marsupites- 

 Zone at this spot may be placed at about 55 feet. In East Kent, 

 and in the Hampshire Basin generally, the published measurements 

 and more trustworthy estimates of the thickness of this zone are 

 much greater : usually exceeding, and rarely or never falling far 

 below, 100 feet. 



It would, however, be unsafe to assume that the shrinkage at 

 Taplow which this discrepancy implies was due entirely to local 

 causes, for, as we hope to show in a future communication, the zone 

 did not attain so great a development over the western part of the 

 London Basin as in the adjoining areas to the south and east. 



When taken in connection with the exceptionally large number 

 of individuals by which some other species are represented, the 

 comparative scarcity of Uintacrinus, Marsupites, and PorospTioura 

 and the absence, or great rarity, of many of their associates, which 

 abound in the lower and middle divisions of the Marsupites-ZoiiQ in 

 other districts, will admit of but one interpretation, namely, that 

 the local conditions of existence were unsuited to these forms of 

 life. It is evident that we have not here to deal with a merely- 



1 We deem it highly probable that Marsupites-Beds occur at a much less 

 distance beneath the Tertiary formations to the south. 



Q. J. G. S. No. 243. 2 m 



