38 INVERTEBRATE PALAEONTOLOGY 



representing a fauna long past may be reinterred among 

 shells of a later period. In the palaeontological record, 

 as in human manuscripts, " anachronisms " may occur ; 

 but they can only be retrospective in character, and so 

 would be more accurately termed " Catachronisms." 

 Fossils derived from denudation of older rocks are un- 

 likely to be found in any but littoral deposits, and even 

 there can be confidently ascribed to local origin. During 

 degradation of a sea-cliff, many fossils will fall to the 

 beach, but the scour of waves will destroy all that are 

 not speedily buried in sand. Of the innumerable fossils 

 " weathered out " on a land surface, few, if any, survive 

 the dangers of travel to a place of permanent sedimen- 

 tation. Glaciation may, however, transport shells un- 

 harmed for great distances, as is shown by the frequent 

 occurrence of Gryphaea and other massive fossils in the 

 Boulder Clay. 



Conglomerates, such as the Budleigh Salterton Pebble 

 bed, may include pebbles that are fossiliferous, but under 

 such circumstances there is little risk of confusion. 

 However, many layers in the Dorsetshire Inferior Oolite 

 and East Anglian Pliocene are remanie' beds made up 

 of fossils separated from their matrix. Again, the 

 Lower Greensand of Faringdon and Cambridgeshire 

 contains many Belemnites and other durable fossils that 

 are not Aptian, but Kimmeridgian. The normally 

 broken and water- worn nature of such truly " extraneous " 

 fossils is not a reliable criterion of derived origin ; wave- 

 action can produce such conditions in a few hours. 

 Only by careful identification and comparison can the 

 true fossil evidence be sifted from the false. But derived 

 fossils, though deceptive in some respects, are of great 

 geological interest, since they indicate the age of the 

 rocks locally exposed to denudation at the period in 

 whose sediments they occur. 



