﻿488 MESSES. WHITE AND TEEACHEE ON THE [Aug. 1905, 



Eocene plain of erosion marking the upper surface of the surrounding 

 flinty cor-anguinum-Chalk, or occupy a depression below that plain. 



Throughout the western part of the London Basin the observed 

 inequalities at the junction of the Chalk and the Eocene, other 

 than those due to ' piping ' and post-Cretaceous folding and faulting, 

 are of so slight and gentle a nature, that the former interpretation 

 seems highly improbable ; and an examination of the contours of the 

 base of the Heading Beds around Taplow makes it practically certain 

 that the Phosphatic Series lies in a depression — its top being planed 

 flush with that of the enclosing older Chalk. 



What is the nature of this depression? Is it (1) an inherent 

 feature of the Chalk, or (2) a result of earth-movements occurring 

 in the interval between the deposition of the highest beds of that 

 formation and the lowest member of the Eocene System now existing 

 in the district? If the contained beds were of the normal lithological 

 character of the zones that they represent, their appearance in such 

 a manner in a country where only lower beds are otherwise known 

 to occur would, in most cases, be unhesitatingly ascribed to folding 

 or faulting. In view of their exceptional features, however, so 

 simple an explanation should not be lightly accepted. The data 

 which are at present available seem hardly sufficient for the 

 solution of this problem, and we have little more than general 

 probabilities to guide us. 



(1) On the assumption that the hollow or basin in which the 

 Phosphatic Series of Taplow lie is a congenital feature of the Upper 

 Chalk, it may reasonably be ascribed, either to (a) irregular depo- 

 sition, or to (b) intra-formational erosion — each of a kind involving 

 a sudden and local decrease in the thickness of one group of beds, 

 with a corresponding downward deflection of the base of the suc- 

 ceeding group. In the Phosphatic Series, as exposed at Taplow 

 Court, there is, as we have seen, some indication of the operation 

 of both processes ; but no loss which that Series has thereby sus- 

 tained, however great, will account for its presence apparently in 

 the midst of other strata. It is a local reduction in the thickness 

 of these older rocks (namely, the main mass of the flinty Micraster 

 cor-anguinum-heds), by thinning or by erosion antecedent to the 

 deposition of the Phosphatic Group, that is demanded on the present 

 assumption. 



The amount of the supposed loss cannot, of course, be adequately 

 gauged until the maximum depth of the trough is known, but it is 

 clearly not less than the thickness of the beds exposed at the Lodge- 

 Pit section plus the few feet of chalk lying between the Eocene 

 base and the top of the pit, that is, about 65 feet. The maximum 

 possible distance in which this loss takes place in the rocks to the 

 south-east of the Lodge section is fixed by the Eail way-Station pit, 

 which shows the Eocene deposits in contact with the Micraster cor- 

 anguinum-Beds, at five- eighths of a mile ; while, so far as can be 

 judged from the less complete evidence of the exposures in the river- 

 scarp, the loss is incurred in an even smaller distance on the north. 



