Cu. XX.] 



PECULIAR FOSSILS. 



3or 



posit in which some now he prostrate. These appearances are lepre- 

 sented in the section 6, %. 365, where the darker strata represent the 

 Bradford clay, which some geologists class with the Forest marble, oth- 

 ers with the Great Oolite. The upper surface of the calcareous stone 

 below is completely incrusted over with a continuous pavement, formed 

 by the stony roots or attachments of the Crinoidea ; and besides this evi- 

 dence of the length of time they had lived on the spot, we find great 

 numbers of single joints, or circular plates ot ihe stem and body of the 

 encrinite, covered over with serjndce. jSTow these serpuloe could only 

 have begun to grow after the death of some of the stone-lilies, parts of 

 whose skeletons had been strewed over the floor of the ocean before the 

 irruption of argillaceous mud. In some instances we find that, after the 

 parasitic serpulce were full grown, they had become incrusted ovei* wdth 

 a bryozoan, called Berenicca diluviana ; and many generations of these 

 mollusks had succeeded each other in the pure water before they became 

 fossil. 



Fig. 866. 



a. Single plate or articulation of an Encrinite OYergrown with serpulcB and hryonoa. Natural 



size. Bradford clay. 

 &. Portion of the same magnified, showing the bryozoan Berenicea diluviana covering one of 



the serpulcR. 



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

 deous plants of the ancient " dirt bed," or fossil forest, of the Lower Pur- 

 beck were killed by submergence under fresh water, and soon buried 

 beneath muddy sediment, so an invasion of argillaceous matter put a 

 sudden stop to the growth of the Bradford Encrinites, and led to their 

 preservation in marine strata.* 



Such difi"erences in the fossils as distinguish the calcareous and argil- 

 laceous deposits from each other, would be described by naturalists as 

 arising out of a difference in the stations of species ; 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 surface. In a sinHe district it is difficult to decide how 



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

 vol. i. p. 429. 



