Seeley—Rock of the Cambridge Greensand. 305 



similar to some of seladonite, the green earth occurring in the 



cavities of certain traps. 



An analysis of the Cambridge glauconite, kindly placed at my 



service by Professor Liveing, is as follows : — 



Water 10-80 



Silica 51-09 



Alumina 9-00 



Iron (protoxide) 19-54 



Magnesia — 3-37 



Lime 0-30 



Soda 3-56 



Potash 2-47 



100-13 

 I cannot concur with Ehrenberg in regarding these grains as 

 chiefly made up of the silicified shells of PolytJialamice, or with the 

 American Professor Bailey, who also found many grains to resemble 

 casts of the cells of Foraminifera, and therefore concludes that these 

 were the great agents in forming the green particles. No decaying 

 organic matter could furnish the grains with alumina of which they 

 contain from 5 to 13 per cent. ; and until detected by the recent in- 

 vestigations of Professor Forchhammer it was a substance not knwon 

 to occur in the sea. I have many sections of the glauconite grains. 

 The larger ones have a few Foraminifera scattered through them, the 

 cells of which are often filled with a more transparent substance, 

 probably the Eed Silicate. Smaller grains sometimes only just in- 

 vest one polythalamous shell ; while the smaller fragments are often 

 shapeless. In many grains there is no trace of structure. 



Now if, as I have suggested, the mass of the greensand is derived 

 from plutonic rocks, its formation, disintegrating the felspars and 

 micas, would set free all the ingredients of glauconite ; and in the 

 process the new substance might be formed. Or the Silica would 

 be extracted by Diatoms, PolycystincB, etc. Decomposing sea-weeds, 

 etc., would set free the necessary alkali to render them gelatinous, 

 while the particles of alumina falling through the shallow water 

 would serve as nuclei round which these and the other constituents 

 would aggregate. 



The dark phosphatic nodules are usually named coprolites. NoV 

 name could be more unfortunate, for there is no evidence of their 

 coprolitic origin ; and the only coprolites found, those of small 

 fishes, are among the rarest fossils of the bed. The nodules vary 

 much in size up to a diameter of three or four inches. Most often 

 they are irregular concretions without any definite shape, but fre- 

 quently occur as tubes, or halves of tubes, sometimes more than a 

 foot long, and which are made up of successive layers. These 

 seem to have been rolled about the shore, around the stems of 

 sea plants ; and one specimen, collected by the Duke of St. Alban's, 

 appeared to have been formed about a stem where it branched 

 dichotomously. When broken, they are dark brown, having a dull 

 fracture, and sometimes contain scales of fishes and small shells. 



VOL. III. — NO. XXV. 20 



