74 Proceedings of Royal Society of Ediriburgli. [sess. 
upon calcium sulphide, producing calcium carbonate in the interior 
of the cells, where it replaces the organic matter molecule by 
molecule, sulphuretted hydrogen being set free. The sulphuretted 
hydrogen afterwards unites with iron, forming the compound known 
as iron pyrites.^ 
The reactions may be represented as follows : — 
CaS 04 + 2C = CaS + 2C02. 
CaS + CO2 + H2O = CaCOg + H2S . 
For each equivalent of carbonate of lime deposited we should 
accordingly have one equivalent of carbonic acid passing off as gas, 
provided the reaction was complete ; but, under the circumstances 
no doubt, hydrocarbons were also formed, as we see in our peat 
bogs of the present day, small quantities of these hydrocarbons 
being found in the fossils after treatment with hydrochloric acid. 
Sea water, therefore, accounts for the chief constituents of these 
nodules. The explanation I have above given is contrary to the 
opinion of Mr Binney, as it does not presuppose any interval of 
time to elapse between the processes of decay and petrifaction. 
The decomposition and replacement by carbonate of lime once 
having commenced at a particular place in the plant, the physical 
force known as aggregation would in time lead to the petrifaction 
of a large mass of it in the immediate neighbourhood, and decom- 
position still proceeding, amorphous carbonate of lime would be 
deposited around the petrified portion of the plant, when decay and 
fossilisation would be practically complete, and the nodule in a 
perfect condition. 
I am led to think, from the slow rate of petrifaction, that decay 
and fossilisation proceeded simultaneously; moreover, the perfect 
nature of these enclosures, as may be seen when a thin section is 
prepared and examined microscopically, leads us to the conclusion 
that little or no decay had taken place before the mineral com- 
menced to replace the organic matter ; signs of decay would have 
been readily apparent had the trees lain for only a short time at the 
bottom of the water. 
After a portion of the plant had become fossilised, the decom- 
1 For formation of iron pyrites in deep-sea deposits, see Deep-Sea Deposits^ 
Challenger Report, by Murray and Renard, pp. 254-258. 
