1872.] SOLLAS CAMBRIDGE TTPPEE GEEENSAND. 397 



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. 



3. Some Observations on the Uppeb Greensand Formation of Cam- 

 bridge. By W. Johnson Sollas, Esq., Scholar of St. John's College, 

 Cambridge ; Associate of the Royal School of Mines, London, 



(Communicated by the Rev. T. G. Bonney, M.A., F.Gr.S.) 



[Abridged.] 



The so-called Upper Greensand formation around Cambridge con- 

 sists of a Chalk-marl, with various inclusions disseminated through- 

 out it. These inclusions are separated for manufacturing-purposes 

 from the Chalk-marl by levigation, and sorted by a process of sifting 

 into (i) larger bodies, which remain in the sieve, and (ii) smaller 

 ones, which pass through it. The larger bodice consist almost 

 entirely of the so-called " Coprolites ; " and the smaller ones form the 

 so-called " Greensand," which gives its name to the formation. 



The Coprolites. — In aU cases the coprolites are the result of 

 the fossihzation of organic matter or of the immediate products of 

 its decomposition. The connexion between animal matter and the 

 formation of coprolite is one of the most obvious facts of the 

 whole subject. The bones of reptiles and fishes are incrusted with 

 coprolite precisely on those parts where cartilage was most 

 abundant. The palates of fish such as Pycnodus are not incrusted 

 on their free surfaces ; but their attached surfaces, once covered with 

 cartilaginous tissue, are now quite imbedded in coprolite. In 

 Crustacea such as Palceocorystes the under surface of the body, where 

 animal matter could easily escape, is often one mass of coprolite; 

 while the back of the carapace, which was not covered with animal 



2f2 



