28 INVERTEBRATE PALAEONTOLOGY 



liable to metamorphism, but the contortion and cleavage 

 that have confused such systems as the Longmyndian 

 or Manx Slates seem insufficient reasons for their un- 

 fossiliferous nature. Other greywackes, in much the 

 same condition, contain traces of organisms, obscure 

 though they may be. The barrenness of the vast piles 

 of argillaceous Pre-Cambrian strata demands a further 

 explanation. 



Typical limestones are largely built of organic remains, 

 but this does not prevent them from often disappointing 

 collectors of macroscopic fossils. Not only does the 

 similarity in mineral composition between matrix and 

 shell make their separation difficult and fortuitous, but 

 often large fossils are wholly wanting. Unless the lime- 

 stone has undergone marmarosis or dolomitization, a 

 microscopic section will usually reveal foraminifera or 

 shell -fragments, but even such apparently promising 

 strata as the Coralline Oolite or Upper Chalk may 

 prove destitute of separable fossils through a weary 

 succession of exposures. Limestones of relatively lit- 

 toral origin are usually more prolific than oozes, so that 

 the best fossil-hunting is to be found in such deposits 

 as the Silurian, Carboniferous and Jurassic Limestones. 

 Among siliceous rocks, radiolarian chert rarely includes 

 many fossils apart from the Radiolaria themselves, while 

 chert derived from dissolution of sponge-spicules may 

 contain abundant remains of larger organisms, usually 

 as casts or siliceous petrifactions. Carboniferous and 

 Portlandian cherts are familiar examples of such rocks. 



Peat is often crowded with shells of fresh -water 

 mollusca, but these rarely remain in carbonaceous rocks 

 approaching coal in constitution. The organic acids 

 produced by plant-decay speedily remove calcareous 

 structures. Although molluscs and other Invertebrates 

 are occasionally found in cannel, they are almost always 



