W. J, Sollas — Green Grains in the Cambridge Greensand. 539 



11. — On the Glauconitic Granules of the Cambridge 



Greensand. 



By W. J. Sollas, B.A., F.G.S. 



(PLATE XXI.) 



rilHE Coprolites or phosphatio nodules of the Cambridge Greensand 

 1 frequently contain amongst other included bodies a number of 

 Foraminifera and glauconite granules, of the same kind as those 

 found scattered loosely throughout the Greensand bed. 



When a transparent slice is prepared of one of these coprolites, it 

 of course reveals a number of these included bodies in section, and 

 so offers a favourable opportunity for the examination of their internal 

 structure, an opportunity which I have made nse of, especially with 

 a view to test the conclusions which I had published in 1872 on the 

 origin of the Greensand glauconitic grains. 



Position of the Green Granules and Foraminifera in the Coprolites. 

 — These, like most of the other foreign bodies contained in the phos- 

 phatic nodules, occur most usually and in greatest abundance in the 

 secondary deposit of coprolite, which has, subsequently to the forma- 

 tion of the original nodule, incrusted its surface or filled in the 

 cracks that fissured it as it contracted on fossilization ; while in 

 those nodules which owe their origin to sponges these included 

 bodies are found, in addition to the former positions, in the material 

 which has entered into and filled up what were once the canals and 

 ' cloaca ' of the sponge. 



Foraminifera. — These are arenaceous and perforate vitreous forms, 

 like those which occur in the Greensand bed ; their chambers are 

 filled with different materials in different cases, sometimes crystal- 

 line calcite, and occasionally glauconite has infiltrated them, but 

 most commonly phosphatic material similar to that in which they 

 are contained. These three minerals are found in the interior in- 

 differently of both arenaceous and perforate foraminifera ; and when 

 glauconite fills the latter, it may be traced from the cast of the 

 chambers passing into the fine tubulations which perforate the walls. 



Green Grains. External Form. — Some of the green grains are of the 

 regular shape which characterizes casts of perforate Foraminifera ; but 

 the majority of the sections are of such various and irregular out- 

 line, that from it alone no clue to their mode of origin can be 

 obtained ; of the^e irregular forms a few exhibit attached to their 

 exterior fragments of arenaceous foraminiferal tests, the remnants 

 probably of the moulds in which they were formed ; but the greater 

 number are free from any such incrustation, and are simply granules 

 of glauconite of very irregular form. What the origin of these may 

 be is very doubtful ; judging by analogy it seems probable that many 

 of them may be casts of arenaceous Foraminifera; but how far this 

 is actually the case we have no means of saying. 



Internal Structure. — The thin sections of the glauconite granules 

 are of a light green colour, deeper in tint in some places than others, 

 and transj)arent except where rendered opaque by the presence of 

 included particles. 



