224 PEOOEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 10, 



W. Logan, that some of the Laurentian marbles when struck give 

 forth the same overpowering smell of carburetted hydrogen, as is well 

 known to be given off from many beds of Carboniferous Limestone 

 whose organic origin is most distinct. 



Another probability is afforded by the striking resemblance ob- 

 served by Sir W. Logan in the general arrangement of the masses 

 of Eozoon in Foraminiferal reefs, to the reefs formed by the stony. 

 Corals ; and the evidence of successional building-up of these, as 

 shown by the disintegration of some parts of what would have been 

 once the living surface by currents and eddies, which have left cavities 

 and recesses that were subsequently filled by a new overgrowth. 



The alternating succession of calcareous and siliceous lamellae, of 

 which as many as fifty may be counted in a single mass of Eozoon, 

 is a feature which if not absolutely without parallel in the inorganic 

 world, is a very singular one ; and the difficulty of accounting for it 

 is greatly increased when we look upon the vast multitude and the 

 singular uniformity of its repetitions in the separate masses of Eozoon. 

 The difficulty is yet further augmented when it is taken into account 

 that the alternating minerals, though invariably calcareous and sili- 

 ceous, are not constantly the same ; the serpentine being often re- 

 placed either by pyroxene or by loganite, and the carbonate of lime 

 by dolomite. Now on the hypothesis of organic origin, nothing is 

 easier than to account for the regular alternation of calcareous and 

 siliceous layers, — the former representing the original skeleton formed 

 by successional growth ; and the latter representing its chambers, 

 originally filled with a living sarcode-body, which was subsequently 

 replaced by siliceous minerals by a process which (whatever its 

 nature) has been repeated upon the bodies of Foraminifera in every 

 great geological epoch from the Silurian to the present. And it was 

 the coexistence of this uniformity in structural arrangement with 

 diversity in the component minerals, which first impressed upon the 

 mind of Sir "W. Logan, the idea of the organic nature of these masses. 



The greater variability in the thickness of the calcareous lamellae 

 than in that of the siliceous, is a point of minor importance, but not 

 without its relevancy; for it is exactly paralleled among recent 

 Foraminifera. As Unios and other freshwater mollusks will form 

 thick or thin shells according to the abundance of Carbonate of Lime 

 in the water they inhabit, so do we find among the Nummulites of 

 different localities extraordinary diversities of form produced by 

 variations in the thickness of their shelly layers ; the same species 

 having sometimes a thickness equal to only one third of its diameter, 

 whilst in other specimens the thickness equals three-fourths of the 

 diameter, — the dimensions of the chambers being nearly the same 

 in both cases. The difference in the thickness of the calcareous 

 layers of Eozoon, on the hypothesis of its organic origin, depends 

 upon the amount of superficial addition made in the form of ^' sup- 

 plemental "or "intermediate skeleton " to the proper walls of the 

 chambers, the cavities of which are represented by the siliceous 

 layers ; and wherever the interval between two siliceous layers is 

 unnsually wide, an unusual manifestation of the canal-system is 



