LOWER CILytK — CAMBRIDGE AND SUFFOLK. 195 
The pliosphatic nodules or “coprolites” are thickly crowded in 
the lower part of this glauconitic marl, a few scattered nodules 
occurring in the higher part, but not reaching so high as the green 
grains do. The usual thickness of the nodule-bed, as distinguished 
from the Greensand, is 8 or 9 inches, hut in some of the hollows 
and troughs it reaches a thickness of 12 inches, or even more. 
The nodules themselves are mostly very dark brown or nearly 
black, but are internally of a light brown or buff colour ; amongst 
them, however, are some few of a dull buff colour both inside and 
outside. A good aggregate sample of Cambridge “ coprolites 
contains about 57 or 58 per cent, of tribasic phosphate of lime, 
with 11 or 12 per cent, of carbonate of lime and from 13 to 15 per 
cent, of silicates (clay, glauconite, etc.) ; variable amounts of sul¬ 
phate and fluoride of lime are also present. The following is the 
composition of a sample calculated from an analysis by Professor 
A. Voelcker, assuming that the amount of fluorine present was 
1*50 per cent., for in the analysis it was not separated from the 
sulphuric acid*: — 
Moisture and organic matter - - - - - 4*01 
Phosphate of lime - - -. 58*39 
Carbonate of lime - - - - - - - -11*66 
Sulphate of lime -. 7*65 
Fluoride of lime - - - - 3*07 
Silicates (clay, glauconite, etc.). 15*20 
99*98 
Many of the nodules are simply casts of different kinds of Mol- 
lusca, Echinoderma, Crustacea, and Corals, with more or less of the 
calcareous test adhering. Among the others phosphatised casts of 
siliceous sponges (Ventriculites and others) are common, and many 
of the comparatively amorphous nodules appear to be really phos¬ 
phatised sponges.f True coprolites or the exuviae of fish and reptiles 
occur occasionally, but are rare, and always of a bright brown colour. 
Many of the nodules are covered with the attached valves of small 
Oysters and Plicatulce , showing that they lay for a long time on 
the sea-floor. 
The phosphatised fossils are all of species which occur in the Gault 
of England and France, except a few which have not yet been 
found elsewhere, and the most common and characteristic species, 
such as the Am. [ Schlcenb .] rostratus, Am. [. Hoplites ] auritiis, 
Am. [Hopl.\ splendms , are forffis which specially characterise the 
upper and middle portions of the Gault in those areas where the 
so-called Upper Greensand is slenderly developed. 
Finally, the nodule-bed which occurs at the base of the Upper 
Gault of Bucks (described in Vol. I.) contains similar nodules and 
* Prof. Voelcker states that the amount of calcium fluoride is generally 
3 or 4 per cent., and that in many commercial analyses it is precipitated 
with the phosphate of lime. 
+ See Prof. Sollas in Quart. Journ, Geol. Soc., Vol. xxix. p. 76 (1873). 
4219. O 
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