200 PEOF. H. A. NICHOLSON AND DE. J, MUEIE ON THE 



skeleton, with its delicate layer of calcite-crystals, remaining un- 

 changed, whilst the siliceous infilling of the chambers is brilliantly- 

 coloured. Specimens of this kind, infiltrated with silica, but 

 having their actual skeleton unchanged, are very common in the 

 Corniferous Limestone of North America. 



These specimens, moreover, prove the important fact that the 

 silica filling the chambers and interspaces of the fossil is not 

 amorphous organic silica, but crystalline doubly-refractive silica ; 

 for the use of crossed prisms produces the most brilliant play of 

 colours in the siliceous infilling of the chambers, the variety and 

 intensity of tints being fully equal to what we observe in crystals 

 of quartz. 



(c) In the third place, we find specimens in which the actual 

 skeleton of the fossil is m.ore or less completely converted into 

 silica (often faintly granular under the microscope), whilst the 

 chambers of the organism are filled with calcite. In these speci- 

 mens decalcification, whether natural or artificial, results in the 

 .production of a reticulated siliceous framework, representing the 

 true skeleton, the interspaces being hollow. Specimens of this 

 kind are not unknown in the Corniferous Limestone of North 

 America; but they lend no support whatever to the view that 

 Stromatopora was originally siliceous, for this is precisely the state 

 of preservation in which almost all the fossils of the Corniferous 

 Limestone occur, such as Corals, Brachiopods, &c., which were 

 undoubtedly calcareous to begin with. All these fossils, namely, 

 have their primitively calcareous structure accurately replaced by 

 silica (often of a granular appearance), whilst their cavities are 

 filled with carbonate of lime. It should also be noticed that small 

 specimens in this state of preservation, especially specimens 

 mounted for microscopic examination, are not easily discriminated 

 from similar fragments of specimens of the second group, where 

 the skeleton is unaltered and the chambers have been filled in 

 with silica. This is especially the case when decalcification, na- 

 tural or artificial, has been produced, as the resemblance of the 

 true silicified skeleton to the siliceous network representing the 

 successive tiers of sarcode-chambers is much closer than might be 

 imagined. It may safely be asserted, indeed, that it would be 

 very difficult for any observer to discriminate with certainty be- 

 tween microscopic preparations belonging to these two groups 

 unless he had previously seen the actual specimens from which 

 they were taken, if it were not absolutely impossible. Against 



