THE 'derived' fossils. 39 



fossilized. Then by a series of changes which are commonly 

 interpreted as first an upheaval and then a depression of the 

 land, the rock formations were removed, so that the imbedded 

 fossil shells were exposed and carried down to the shore, to be 

 battered by the waves and otherwise injured by destroying agents, 

 the whole process requiring a long period of time. 



But just as a rock unconformity is often simulated by and 

 may be confounded with false bedding, so also I believe the story 

 of a derived shell may be greatly exaggerated by such an account 

 as that given above. Many of the cases of derived fossils, including 

 those we have under consideration, seem scarcely to admit of so 

 prolonged a history as this ; otherwise we should be obliged to 

 refer our Midland Neocomians to at least as late an age as the 

 gault, which is impossible. 



I think we must admit that a very thorough fossilization 

 may go on far more rapidly than is generally supposed. The rapid 

 metamorphism of the interior of coral reefs is a well-known fact, 

 and many other very modern deposits are quickly hardened into 

 'rock' especially in the presence of iron or carbonate of lime 1 . 

 Rolled nodules of coal occur in the coal measures, and concretions 

 are well formed in modern bogs. In the ' Challenger ' dredgings 

 concretions were found spread over the bottom of the deep sea. 

 Just so, I believe, was the case with our Neocomian rocks, which 

 contain derived Neocomian fossils. The ferruginous and phos- 

 phatic concretes were formed quickly after the formation of the 

 rock, and, afterwards, some such alteration as a change of level 

 or a variation in the strength or direction of the ocean currents, 

 caused the sediments to be torn away again, and the older or- 

 ganisms were redistributed as 'derived' fossils in immediately 

 subsequent deposits. 



Looking to the geographical distribution of these derived 

 fossils in the Neocomian rocks, we find that the Cephalopoda are 

 essentially Southern species belonging to the Anglo-Gallic area, 

 whereas we have found in the grit boulders evidence of a northern 

 origin. This mixture is not surprising when we reflect that at 

 this period the advancing southern sea had overwhelmed the 

 Wealden estuary and spread itself northwards over the Eastern 



1 I have seen glacial moraine matter hardened into a strong breccia on mountain 

 bog lands. 



