Burton: The Cleveland Ironstone. 165 
Dr. Sorby, Hudleston, Tate and Blake, and other authori- 
ties, all support the replacement theory. The substitution of 
iron for lime is quite a practical operation in nature’s work- 
shop, where the conditions are favourable. As already pointed 
out, a solution of Carbonic acid will take up a portion of iron 
from the rocks through which it passes. Bicarbonate of iron 
in solution, in open water, will, on coming to the surface, 
take up a molecule of oxygen, freeing the Carbonic acid, and 
forming Fe,.O,. On this taking place it immediately combines 
with water and sinks to the bottom, where, if there happens 
to be any decomposing vegetable matter, the carbon it contains 
abstracts a molecule of oxygen from the Fe,O., the abstracted 
oxygen molecule going to make up Carbonic acid and leaving 
the iron as FeO, and so the fight goes on indefinitely between 
the iron salts and carbon, with the one molecule of oxygen 
as the bone of contention. In this way are many lake deposits 
of ferruginous material formed. Some of the iron formed in 
Permian and later times may have been mechanically trans- 
ported and deposited simultaneously with the limestone, 
but probably the greater portion of the iron found was carried 
down with the clays which now form the shale bands above 
the ironstone. These shale bands have been elevated, and 
have at some period of their existence, and perhaps, for con- 
siderable periods, been swampy estuarine beds, on which, in 
the warm Liassic seas, plant life would quickly form and decay. 
The iron in the shales would probably ‘be a ferric oxide 
(Fe.O.), and as the carbonaceous matter produced Carbonic 
acid and acted upon the ferric salts, a bicarbonate of iron 
would be formed, which would percolate through the soft 
strata of the period. According to Dr. Irving, the calcium 
of the limestone rock would take up the strong acid of the 
iron salts, leaving Carbonic acid available for union with the 
iron as ferrous carbonate (FeCo,, or Siderite). The change. 
produced by percolations of ferrous carbonate through lime- 
stone is thus simply put by Dr. Stead, following Dr. Sorby :— 
CaCO, solid + FeCO,, in solution = FeCO, solid + CaCO, in 
solution. 
Dr. Stead gives the constitutional anaylsis of the Cleve- 
land Main Seam as 44.97% of Siderite (FeCO,) by weight 
(FeO 27-91 ) 
(CO, 17:06 44-97) 
The principal difficulty in the replacement theory is the 
fact that fossil shells occur which have not been converted 
into carbonate of iron and still remain carbonate of lime, and 
the question naturally arises, why, if the replacement theory 
is correct, were these shells not also converted into carbonate 
of iron ? 
No entirely satisfactory answer seems to be forthcoming, 
1913 April r. 
