﻿Vol. 6 1.] PHOSPHATIC CHALK OF TAPLOW. 469 



The Micrasters and siliceous sponges are from the rock-bed, and 

 from immediately below it. Some of the former and all of the latter 

 are in the form of phosphatized casts. In a few instances, the 

 sponge-stems have been found in rather closely-fitting, conical pits 

 in the upper surface of the rock. 



Porosplicem globular is is of small size, measuring only 6 to 

 12 millimetres in diameter. 



Division (B). 



The lower limit of this, the Lower Brown Chalk, is everywhere 

 clearly defined by the above-described rock-bed ; its upper limit is 

 very ill-marked. We adopt Mr. Strahan's measurement of 4 feet 

 (see p. 465) for the thickness of this division, which seems as good 

 as any that can be made out. 



The lower part, for a distance of 1 to 6 inches above the rock- 

 bed, is usually a brown, or greyish-brown, ferruginous sand, con- 

 sisting mainly of phosphatized foraminifera and prisms of Inoceramus, 

 small coprolites, chips of echinid-tests and oyster-shells, with other 

 more or less minute organic debris, and containing many fish-teeth 

 and a variable quantity of coarser material in which may be 

 distinguished : — 



1. Bounded nodules and casts of Micraster, and of stems of ventriculate 



sponges, consisting of hard greenish- or yellowish-white chalk, en- 

 crusted with Serpulcs and other small organisms of adnascent habit, and 



enveloped in a dark-brown glaze to 4 inches 



in maximum diameter. 



These are evidently rolled and phosphatized pieces of the ruck- 

 bed. 



2. Angular and rounded pieces of the same chalk, speckled with manga- 



nese, within, and either unglazed or but partly glazed, without 



to 2 inches. 



3. Angular and subangular concretions, hard, brown, microgranular or 



homogeneous, and often partly glazed to half an inch. 



4. Irregular, slightly-calcareous concretions of brick-red, or brown iron- 



oxide to a third of an inch. 



Nos. 2 & 3 are also clearly derived from the same source as No. 1, 

 the latter representing portions of the agglutinated contents of the 

 borings. As these, and No. 4, have only a local development, and 

 seem to be confined to the sandy layer, it is probable that they 

 result from the decomposition of the rock-bed at a comparatively 

 recent date. The bed referred to shows, in many places, a tendency 

 to fracture, or scale, along surfaces between J and 1 inch below the 

 glaze, the detached pieces resembling the partly-glazed material of 

 Nos. 2 & 3. 



5. Compact or finely -granular, dull-yellow, brown, or greenish concretions 



of angular or subangular — less often of rounded — form, with perfora- 

 tions, and of a somewhat softer and more earthy character than those 

 of No. 3, from which, however, they are not always to be distin- 

 guished to a third of an inch. 



