CHAPTER III. 

 THE 'DERIVED' FOSSILS. 



The remains of fossil organisms which have been derived from 

 the destruction of older rocks are very numerous in all the copro- 

 lite pebble-beds of Cambridgeshire and Bedfordshire. Referring 

 to my notes on some pits near Ampthill I find "the coprolite heap 

 looks like one mass of Ammonites biplex, mostly worn and frag- 

 mentary," and in all cases a large proportion of the coprolites shew 

 traces of the outlines of shells. 



The vast majority of the derived fossils are preserved in phos- 

 phate of lime, but the Ammonites of the Oxford clay are composed 

 of limonite and some of the fragments of fossil-wood are silicified. 

 The vertebrate fossil remains are more or less impregnated with 

 oxide of iron or phosphate of lime, but we find some difficulty in 

 deciding exactly to what extent these are derived, and which are 

 native to the deposit. 



The Coral Rag fossils from the neighbouring rock at Upvvare 

 have not been phosphatised at all, but retain their original character 

 although they occur well in the nodule-bed amongst a mass of 

 other phosphatised fossils. 



The proper shells of the derived fossils are usually lost, and 

 the casts themselves have suffered much from trituration on the 

 old sea shore, the burrowing work of boring shells, &c. 



The present condition of these phosphatised fossils will best be 

 understood from a description of a typical example. Two of the 

 commonest are Ammonites biplex and a Myacites. The former 

 occurs as a discoidal mass of oval or elliptical shape, like a mineral 

 concretion, but in it one or more whorls of the Ammonite may 

 here and there be marked in shadowy outline. No fragment of 



