﻿Vol. 6 1.] PHOSPHATIC CHALK OE TAPLOW. 483 



IV. Some Facts beaking ok the Mode oe Fokmation oe the 

 Phosphatic Chalk. 



The nature of the conditions under which the Senonian Phosphatic 

 Chalks of North- Western Europe were accumulated is a subject 

 that has been dealt with at length by Continental geologists, and it 

 is one which we, who are but imperfectly acquainted with the 

 English examples of Taplow and Lewes alone, shall not presume to 

 discuss. Some small service may, however, be rendered by calling 

 attention to a few, relevant features of the Taplow section, which 

 have not been recorded hitherto. 



Among these features we note the concretionary phosphate, 

 present in one form or another at almost all horizons. 



In his review of the Cretaceous Phosphates, Dr. Teall l implies 

 that Taplow is not one of those localities where this mineral group 

 occurs ' in the form of detached nodules, or as nodular and more or 

 less conglomeratic deposits.' Contrasted with the vastly-greater 

 bulk of the granular, pseudomorphic variety, the amount of con- 

 cretionary phosphate here is undoubtedly small, but by itself it 

 would form a striking feature of any chalk in which it occurred ; 

 and the nodular and conglomeratic character of the lower parts of 

 the brown chalks is so evident in this pit, that we must suppose 

 Dr. Teall to have been familiar only with small hand-samples of the 

 more homogeneous upper portions of those beds. 



Here, as in the larger French deposits of the Pas-de-Calais, 

 Somme, Aisne, and Oise, the direct precipitation of calcium-phos- 

 phate as a glaze upon the rock- beds seems, in each case, to have 

 preceded the deposition of the main mass of the overlying brown 

 chalk, 2 for the enamelled debris and sponge-casts of the rock-bed are 

 scattered through the latter, together with slightly-altered pieces of 

 white chalk. On the other hand, the formation of the glaze clearly 

 post-dates that of the brown chalk filling those borings in the rock- 

 bed the orifices of which it has sealed up, and the contents of which 

 it has agglutinated; and, as the same glossy substance has been 

 observed by us on the larger fossils (as, for example, Ostrea, 

 Inoceramus, Cidaris) near the base of (B), it would seem that its 

 precipitation accompanied, or alternated with, the deposition of the 

 lowest layers of brown chalk, in that case. 



There is no reason to doubt that the dull, earthy-looking con- 

 cretions, with inclusions of material identical with the surrounding 

 rock, which are common to the white and brown chalks, were 

 formed in situ, and represent phosphatized portions of their softer 

 matrix. They are of all sizes, from 1| inches in diameter down- 

 wards, and may be seen in many stages of development, ranging 

 from brownish or yellowish stains to indurated lumps grating 

 harshly under the knife. 



1 "i'he Natural History of Phosphatic Deposits' Proc. Geol. Assoc. 

 vol. xvi (1900) p. 382. 



2 L. Cayeux, 'Contribution a 1' Etude mierographique des Terrains sedi- 

 jnentaires' Mem. Soc. Greol. du Norrt, vol. iv, 2 (1897) p. 431. 



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