PART IT. CHAPTER XVII. 



217 



Peculiar Fossils. 



mud. In some instances we find that, after the parasitic serpu- 

 Ise were full grown, they had become incrusted over with a coral, 

 called Berenicea diluviana ; and many generations of these 

 polyps had succeeded each other in the pure water before they 

 became fossil. 



a. Single vertebra, or articulation of anEncrinite overgrown withserpulse and 



corals. Natural size. Bradford clay. 

 L Portion of the same magnified, showing the coral Berenicea diluviana 



covering one of the serpute. 



We may, therefore, perceive distinctly that, as the pines and 

 cycadeous plants of the ancient Portland Forest were killed by 

 submergence under fresh-water, and soon buried under muddy 

 sediment, so an invasion of argillaceous matter put a sudden stop 

 to the growth of the Bradford Encrinites, and led to their preser- 

 vation in marine strata.* 



Such differences in the fossils as distinguish the calcareous 

 and argillaceous deposits from each other, would be described by 

 naturalists as arising out of a difference in the stations of spe- 

 cies ; but besides these, there are variations in the fossils of the 

 higher, middle, and lower part of the oolitic series, which must 

 be ascribed to that great law of change in organic life by which 

 distinct assemblages of species have been adapted, at successive 

 geological periods, to the varying conditions of the habitable sur- 

 face. In a single district it is difficult to decide how far the 

 limitation of species to certain minor formations has been due to 

 the local influence of stations, or how far it has been caused by 

 time, or the creative and destroying law above alluded to. But 

 we recognize the reality of the last-mentioned influence, when 

 we contrast the whole oolitic series of England with that of parts 



* For a fuller account of these Encrinites, see Buckland's Bridgewater Trea- 

 tise, vol. i. p. 429. 

 T 



Fig. 203. 



