so 



THE GEOLOGIST. 



characteristic differences in the fossils. But more than this, 

 where similar rocks have been subjected to different physical 

 conditions, there also such differences will be manifested in the 

 conservation of the fossil itself. 



Any one who regards the aspect of a fossil will at once perceive 

 that, although he cannot fail to recognise it as the remains of some 

 organised being, a fragment or portion, it may be, of some fish, 

 animal, plant, or shell, yet it is not really the actual bone, 

 nor wood, nor shell which he sees, but these substances 

 greatly altered from their former state, each having mider- 

 gone a process of transformation in its intimate structure or its 

 composition, which, while the resemblance of the former shape has 

 been perfectly retained, has in reality converted it into a something 

 very different. Fossilization is not the mere impregnation of an 

 object with mineral matter by its long continued entombment in 

 the earth, but it is a process, or rather a collection of processes 

 of far more wonderful character. Particle by particle of the ori- 

 ginal organism may have been removed by the percolating waters — 

 — slowly, and in the lapse of ages ; and as slowly and as constantly, 

 particle by particle has been replaced by particle after particle of 

 some other=i^ and very different element ; thus the lime of the calca- 

 reous shell has been often replaced by flint, or ordinary bone by car- 

 bonate of lime. Or, the dissolving waters may have absorbed the 

 shells and have left only a hollow cavity to mark their vacant 



1 2 3 



LiGN. 1. Terebra Portlandica from the Portland Oolite. 

 1, Cast of Interior of tlie Shell. 2. Mould or Impression of the Exterior of the Shell. 

 3. The Shell itself. 



* The scientific reader will be aware that in some rare instances the original condition 



of tlic organic substances appears to be retained. 



