166 Burton: The Cleveland Ironstone. 
but as there are differences of both a physical and chemical 
character in various parts of the many seams of ironstone, 
shewing that the conditions prevailing at the time were not 
uniform, may it not also be that these same varying conditons 
may account for the substitution of iron for lime in one case 
and not in another? The time the changes took place is not 
known, and it is quite possible that the time between deposition 
and substitution was long enough to allow other changes, as 
for instance the shells may have crystallized into calcite, which 
is much less soluble than limestone—and “it is remarkable 
that nearly all the shells found in the Cleveland Ironstone 
beds mainly consist of calcite in distinct crystals differing 
entirely from the structure of similar shells of the present day, 
which if crystalline at all are only microscopically so.”"* There 
is the further possibility that some of them may have received 
a silicious coating, thus adding greater resistance to the attack 
by the carbonate of iron solution. It has been shown by 
several writers that the surface of some of the shells has been 
partly or wholly converted into carbonate of iron, while the 
interior still remains calcite, so that it may well be that at the 
infinitely slow rate of many of nature’s processes the time has 
not been sufficient for a complete change, and that the work 
is even now in progress, rather than that it has been arrested. 
Dr. Sorby made some interesting experiments on the action 
of chloride of iron on calcite, which are given in The Natuvralist 
No. 597, 1906, where he says :—‘ Since writing my original 
papers I devoted much attention to the production of artificial 
pseudomorphs, especially including those in which carbonate 
of lime is replaced by carbonate of iron. Crystal of calcite 
or portions of Iceland spar were sealed up in tubes with a neutral 
solution of iron protochloride and heated to various tempera- 
tures. Kept for a few weeks in the boiler of a high pressure 
steam engine, at a temperature somewhat under 300° F. 
replacement was somewhat rapid, and pseudomorphs were 
formed as hard as any similar natural product. Kept much 
longer in a boiler at a temperature varying up to 212° F. the 
replacement was slower, and the pseudomorphs much more 
tender. I sealed up a piece of Iceland spar in a glass tube 
so full of the chloride that there was a mere trace of air left, 
and after keeping for a few years the replacement was so small 
that I came to the conclusion that it did not take place at the 
ordinary temperature, but on re-examining after thirty-six 
years, though the amount of replacement was small, there could 
be no doubt about its having occurred. This shews the 
importance of such long continued experiments, and proves 
that the changes met with in the Cleveland Ironstone may 
have taken place at the ordinary temperature of the rocks.” 
* Dr. J. E. Stead, loc. cit. 
Naturalist, 
