264 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 8, 



to satisfy myself of the occurrence of the structures of Eozoon in 

 such specimens as I have had the opportunity to examine *. It is 

 perhaps necessary to add that there exists in Canada abundance of 

 Laurentian limestone which shows no indication of the structures of 

 .Eozoon. In some cases it is evident that such structures have not 

 been present. In other cases they may have been obliterated by 

 processes of crystallization. As in the case of other fossils, it is only 

 in certain beds, and in certain parts of those beds, that well-charac- 

 terized specimens can be found. I may also repeat here that in the 

 original examination of Eozoon, in the spring of 1864, I was fur- 

 nished by Sir W. E. Logan with specimens of all these limestones, 

 and also with serpentine-limestones of Silurian age, and that, while 

 all possible care was taken to compare these with the specimens of 

 Eozoon, it was not thought necessary to publish notices of the crys- 

 talline and concretionary forms observed, many of which were very 

 curious and might afford materials for other papers of the nature 

 of that criticised in the above remarks. 



[The examination of a large number of sections of a specimen 

 of Eozoon recently placed in my hands by Sir William Logan, in 

 which the canal-system is extraordinarily well preserved, enables 

 me to supply a most uuexpected confirmation of Dr. Dawson's state- 

 ments in regard to the occurrence of dendritic and other forms of 

 this system, which cannot be accounted for by the intrusion of any 

 foreign mineral; for many parts of the calcareous lamellge in 

 these sections, which, when viewed by ordinary transmitted light, 

 appear quite homogeneous and structureless, are found, when the 

 light is reduced by Collins's " graduating diaphragm," to exhibit a 

 most beautiful development of various forms of canal-system (often 

 resembling those of Dr. Dawson's Madoc specimen represented in 

 PI. XII. figs. 4, 5), which cross the cleavage-planes of the shell-sub- 

 stance in every direction. Now these parts, when subjected to decal- 

 cification, show no trace of canal-system ; so that it is obvious, both 

 from their optical and from their chemical reactions, that the sub- 

 stance filling the canals must have been carbonate of lime, which 

 has thus completely solidified the shell layer, having been de- 

 posited in the canals previously excavated in its interior, just as 

 crystaUine carbonate of lime fills up the reticular spaces of the 

 skeleton of Echinodermata fossilized in a calcareous matrix. This 

 fact affords conclusive evidence of organic structure, since no con- 

 ceivable process of crystallization could give origin to dendritic 

 extensions of carbonate of lime disposed on exactly the same crys- 

 talline system with the calcite which includes it, the trs'o substances 

 being mineralogically homogeneous, and only structurally distin- 

 guishable by the effect of their junction-surfaces on the course of 

 faint rays of light transmitted through them. — W. B. C] 



* Such Irish specimens of serpentine limestone as I have seen, appear much 

 more highly crystalline than the beds in Canada which contain Eozoon. 



