V 



Notices of Memoirs — Sir W. Dawson — Eozoonal Stntctures. 273 



Although the plates apparently show some of the forms described 

 in the paper as lying parallel to igneous veins, and as their selvages, 

 or as rounded masses like nodules and geodes, on closer inspection 

 essential differences may be observed. The Vesuvian specimens 

 consist of continuous laminse of crystalline igneous matter, including 

 interrupted or lenticular layers of calcite. Eozoon on the contrary, 

 when well preserved, consists of a continuous skeleton of calcite 

 made up of broad layers slightly pitted on their surfaces, and 

 connected at intervals ; while the siliceous material appears as 

 a substance filling wide flattened mammillated chambers more or 

 less limited, and presenting amoeboid lobes at their extreme edges, 

 and passing finally in the upper part into rounded charaberlets. 

 This difference should commend itself to any palaeontologist, but 

 I am aware that it may be overlooked by cursory observers. Scores 

 of specimens have been sent to me of banded rocks, supposed by 

 their finders to resemble Eozoon, though, in arrangement of parts, 

 the converse of it. 



Perfect detached individuals of Eozoon are usually of inverted 

 conical form, springing from a narrow base and widening upward in 

 the manner of some sponges and corals. When close together they 

 often become confluent, and when these confluent masses or layers 

 appear to be hollow or doubled, I believe that this usually results 

 from the folding of the containing bed ;. and the laminse may be 

 observed to be bent and crushed at the flexures. 



In the specimens figured in the paper, the characteristic micro- 

 scopic structures of Eozoon are entirely absent. There is no trace 

 of the beautiful and complicated system of canals ; and the fibrous 

 structures compared with the miaute tubulation are merely prismatic 

 fibrous crystals, like the secondary veins of chrysotile which some- 

 times cross and deteriorate our specimens of Eozoon. With reference 

 to these chrysotile veins, while their filling of minute and often 

 transverse and branching cracks shows that they are merely aqueous 

 deposits of later origin than the structures which they traverse, and 

 while their appearance under high powers is very different from that 

 of the tubuli of the calcite layers, they have no doubt been, when 

 parallel to the layers, and in poor specimens, fertile causes of error. 

 They are absent from the more perfect specimens. I may also 

 explain that while the finely tubulated margin of the calcareous 

 layers can be seen to terminate abruptly against the filling of the 

 chambers, it passes gradually in the interior of the layer into the 

 larger canals when these are present. Naturally also, tlie finely 

 tubulated wall often fails to show its structure, just as anyone who has 

 examined large series of sections of Nummulites may observe in these 

 fossils ; and the tubuli are often filled with dolomite or calcite very 

 difficult to distinguish from the substance of the calcareous lamina. 



The late Dr. Carpenter quite understood the distinction between 

 the veins of asbestiform serpentine and the organic structures, and 

 he hoped to have prepared an exhaustive memoir on the subject, 

 including my material as well as his own. Had this intention been 

 fulfilled many subsequent mistakes might have been avoided. 



DBCADB IT. — TOL. II. HO. VI. 18 



