Rev. 0. Fisher on the Origin of Phosphatic Nodules. 543 



May 22, 1872.— Prof. Morris, V.P., in the Chair. 



The following communicatiorL was read : — ■ 



'- On the Phosphatic IsTodules of the Cretaceous Eock of Cam- 

 bridgeshire." By the Key. 0. Pisher, M.A., P.G.S. 



This paper contained an attempt to explain the origin of the 

 phosphatic nodules which lie in a thin bed at the base of the Chalk 

 in Cambridgeshire and are largely extracted, by washing the 

 stratum, for the purpose of making superphosphate of lime. Two 

 hundred and seventy tons per acre, at the rate of fifty shillings a 

 ton, represents the valuable yield of the deposit, which is followed 

 to the depth of about 18 feet. The nodules and other fossils of the 

 bed are chiefly derivative, forming a concentrated accumulation 

 from a deposit belonging to the Lower Cretaceous period. Some of 

 the fossils, however, are believed to be indigenous to the deposit. 

 Plicatulce are attached to all the derivative fossils and nodules ; and 

 the sharp broken surfaces of the latter, with Plicatulce on them, 

 show that they were mineralized before they were deposited in their 

 present gisement. The green grains of chlorite have been drifted 

 into patches. Certain calcareous organisms are preserved ; but many 

 genera of moUusks only occur as casts in phosphate of lime. The 

 deposition of the phosphatic matter has been determined by animal 

 substances. There are two chief varieties of the " ordinary " no- 

 dules. The first are amorphous, or else finger-shaped ; the second 

 formed like a long cake rolled, partially or wholly, upon a stick. The 

 surface of these two kinds of nodules is coriaceous and wrinkled ; 

 and they usually show marks of attachment to some foreign body. 

 Certain species, clearly zoophytes, are converted into phosphatic 

 nodules ; and when sections are made of these, they are found to 

 show under the microscope structures and spicula allied to those of 

 Alcyonaria. Slices of the common nodules show similar spicula, and 

 occasionally reticular structure. "When casts in plaster are made 

 from Alcyonium digitatum, and coloured to resemble the nodules, 

 the similarity in general form and structure of surface is very 

 striking. The phosphate was probably segregated by the animal 

 matter from its solution in water charged with carbonic acid, which 

 is a known solvent of the phosphate; an analysis of the matrix 

 has proved that phosphate of lime is appreciably present in it. 

 The author doubted the derivation of the nodules from the denuda- 

 tion of the subjacent Gault, and exhibited a collection of these to 

 show that they were distinguished by more stunted growth. 



The deposit was on the whole considered to represent the thin 

 band with similar fossils at the base of the Chloritic Marl, as seen in 

 the west of England, in which district it is underlain by the true 

 arenaceous Greensand. The absence of the true Greensand was 

 attributed to the intervention of the old palaeozoic axis of the London 

 area ; and it was finally suggested that a similar axis might stretch 

 from Leicestershire to Harwich, causing the change in character of 

 the Lower Cretaceous beds between Cambridgeshire and Norfolk. 



