Nodules of the Trimmingham Chalk. 449 



several other kinds of sponges, Lithistid, Choristid, and Hex- 

 actinellid, none of which now retains a siliceous composition ; 

 and considering that not a single instance is yet known to us 

 of any tender small-spiculed sponge existing in the fossil 

 state, although such must have been present in the ancient 

 seas, it would appear certain that Pharetrosponfj-ia formed but 

 an insignificant fragment of the sponge-fauna which existed 

 both in the beds from which the Greensand was derived and 

 in the Greensand proper itself. But if this sponge-fauna once 

 existed and has disappeared, what has become of the silica 

 which must have been produced by it ? It certainly is not to 

 be found as a deposit of free silica anywhere in the Greensand 

 bed. On the other hand, what is the origin of the abundant 

 glauconitic granules scattered through this bed ? The answer 

 that suggests itself is as follows : — The chalk marl which 

 forms the greater part of the Cambridge Greensand still con- 

 tains a good deal of argillaceous impurities, together with 

 traces of ferruginous matter ; and once it contained much more ; 

 the silica set free from decaying sponges combined with the 

 alumina, iron, and alkalies present, to the entire exclusion of 

 lime, and formed glauconite, which was deposited in green 

 granules enclosing coccoliths and Foraminifera, some of which 

 had probably been previously replaced by silica, since when 

 examined in thin slices of the glauconite they are without a 

 tinge of green and quite colourless. In the greensand of 

 Devonshire a simple deposition of silica has followed the for- 

 mation of the green grains, and cemented them and the other 

 materials of certain beds into compact chert. 



We appear to have travelled rather wide of the subject of 

 the Trimmingham flints ; but though we have not confined 

 ourselves to this subject it has been steadily kept in view 

 throughout, and we are now in a position to take up the 

 scattered threads and to frame a consistent explanation of the 

 flints, complete in all respects so far as it goes, save one, since 

 it does not include an account of how they acquired their 

 external form. 



Briefly to sum up, a deposit of sponge-spicules accumu- 

 lated in the chalk ooze, and in the presence of sea-water under 

 pressure entered into solution. Replacement of the calcareous 

 material of the ooze then ensued, small shells, and many large 

 ones too, being converted into silex ; and siliceous chalk, nut 

 flint, was the result. The chambers of the Foraminifera and 

 the interstices of the chalk were now filled up by a simple 

 deposition of silica, and the siliceous chalk became converted 

 into black flint, an incompletely silicified layer of chalk remain- 



