1892 - 93 .] H. B. Stocks on the Lower Coal Measures, 
73 
Geological Society, vol. xviii. p. 107) that the nodules in the coal 
are always associated with fossil shells in the roof or shale, the 
destruction of which has formed materials for the calcification of the 
nodules, by the solution and subsequent deposition of the carbonate 
of lime from the shells above around portions of the plants in the 
coal below. With this exception no theory has been ojBfered to 
explain their formation. 
An objection to this explanation lies in the fact, that the rate of 
deposition of strata is exceedingly slow, and that a considerable 
time would elapse before any appreciable thickness of mud with its 
shells could be formed j consequently in the interval the underlying 
plant substance must have undergone a considerable change and lost 
nearly all its original structure, whereas the plants enclosed in the 
nodules are as perfect now as when they grew in carboniferous 
times. 
The plants forming the coal were deposited where they grew and 
have not been carried from a distance ; the presence of the underclay 
containing rootlets of Stigmaria proves this; the overlying strata 
of shale containing marine remains also shows that the deposits 
were laid down in sea-water; and therefore we may adopt the 
general opinion that this particular bed of coal, like many others, 
has been formed in shallow water around a sea coast, as in the 
mangrove swamps of America at the present day. We may there- 
fore trace the origin of the nodules to the matter dissolved in the 
sea water. 
As in the mangrove swamps, owing to the matted state of the 
vegetation, there would be little or no interchange of the sea water, 
which would become loaded with dissolved organic matter to the 
exclusion of oxygen. When, therefore, a portion of a plant com- 
menced to decay, the oxidation could only proceed at the expense of 
the soluble sulphates. Sea water contains only a comparatively 
small quantity of carbonate of lime, *012 per cent, according to 
Dittmar, whereas sulphate of lime is present to the extent of T26 
per cent. Water containing calcium sulphate and other salts in 
solution easily passes through the vegetable membranes by osmosis, 
and coming in contact with the decaying organic matter of the 
cells, the calcium sulphate has been reduced to calcium sulphide 
with the liberation of carbonic acid. The latter immediately reacts 
