80 DEPOSITS OF PHOSPHITE OF LIME. [bull. 46. 
The first two beds of the clialk are very poor in phosphoric acid, 
rarely containing over twelve one-hundredths of 1 per cent., but the 
underlying marl is much richer and sometimes contains almost 2 per 
cent, of phosphoric acid. The marl is of a grayish color and contains 
specks of Greensand, which become more and more numerous until the 
marl gradually merges into the underlying Greensand bed containing 
the phospbatic nodules. The passage from the Upper Greensand to the 
Gauit is often very abrupt, especially in Cambridgeshire and Bedford- 
shire, where the nodule bed often lies on the eroded surface of the 
Gault. The Lower Cretaceous deposits of Cambridgeshire and Bed- 
fordshire differ from those of the south and west of England in the 
fact that the former lack that great thickness of Upper Greensand which 
exists in the southern counties. At the same time the lower beds of 
the Chalk are the same in both areas. In Hampshire and Dorsetshire 
there is a thin stratum, similar to the Cambridge nodule beds, which, 
like it, passes up gradually into the Chalk marl. But the difference is 
that in the case of the southern counties the arenaceous deposit, which 
sometimes reaches the thickness of many feet, comes between the phos- 
phate bed and the Gault, while in the case of the Cambridge and Bed- 
ford deposits the nodule bed, which rarely exceeds one foot in thickness, 
rests immediately on the Gault. Concerning the absence of this arena- 
ceous deposit in Cambridgeshire, Mr. O. Fisher 1 says: "It is probably 
due to the ridge of old rocks beneath the London area, which shut off 
the early Cretaceous sea, to the north of it, from those southwestern 
lands which yielded the sandy spoils." He also suspects a similar cause 
to have "produced the marked change between the Lower Cretaceous 
rocks in Cambridgeshire and the corresponding beds in Norfolk and 
Lincolnshire." Mr. Seeley 2 thinks that this increased thickness of 
Greensand to the south and southwest is due partly to the shelving of 
the sea bottom towards the south and partly to a current piling it up 
in the hollow, but principally because the southern area was nearer to 
the old plutonic rocks, whence, he thinks, the necessary ingredients of 
the Greensand came. 
The principal phosphate diggings have been in the Upper Greensand 
of Cambridgeshire and Bedfordshire. The nodules in this deposit are 
buried in the Greensand, which varies from one inch to a foot in thick- 
ness and which is all that is left in these districts to represent the im- 
mense thickness of Greensand in the southern counties. In fact, it is a 
matter of serious doubt with some geologists whether the two deposits 
really rex>resent one and the same geologic horizon. 
The matrix of the nodules is not pure Greensand, but is composed 
partly of siliceous and calcareous matter and partly of glauconitic and 
phosphatic grains. The siliceous matter consists mostly of colored 
1 Quart. Jour. Geol. Soc. London, vol. 29, 1873, p. 62. 
2 Geol. Mag., vol. 3, London, 1866. 
(560) 
