459 
In order to illustrate this fact of the replacement of 
carbonate of lime by carbonate of iron, I subjoin an analysis 
of a shell from the inferior oolite of Robin Hood's Bay, the 
composition of which, as dried at the ordinary temperature, 
I found to be : — 
Carbonate of protoxide of iron 78.0 
lime 5.2 
„ magnesia. 3.1 
Peroxide of iron 10.9 
Water 2.1 
Carbonaceous matter 0.1 
Quartz substances 0.6 
100.0 
Originally, such a shell would be composed almost entirely 
of carbonate of lime, with only a small quantity of other 
mineral substances, and no iron ; but, as will be seen, it is 
now almost entirely carbonate of iron, with some hydrous 
peroxide, no doubt due to the action of the atmosphere. 
The peculiarities in microscopical structure, already 
described, prove that the same change has occurred in the 
case of a large proportion of the constituents of the Cleve- 
land Hill Ironstone ; and, according to the view I propose to 
explain its general constitution, all that is assumed is that it 
did also in a similar manner affect the more finely grained 
particles, which do not, and could not present such facts as 
actually prove it to have been so, and yet would be more 
perfectly exposed to such a process of alteration. The 
general appearance of the stone agrees remarkably well with 
this supposition ; for, when examined with the microscope, 
it is seen to be extremely like many limestones in all such 
particulars as are compatible with the subsequent alteration, 
being oolite, with small fragments of shells and patches of 
finer granular matter, as is the case in many oolitic lime- 
stones. 
oo2 
