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they were found, were unlike those occurring at the present 
period ; whereas they may, perhaps, have been exactly the 
same, and the difference in the aspect of the rock brought 
about by a subsequent change of the same nature as must 
also now take place in many localities. The case before us 
is an example of this. The Cleveland Hill Ironstone, now 
so extensively worked at Eston, near Middlesbro', is com- 
posed, to a very great extent, of carbonate of iron, and yet 
it can scarcely be supposed that such a deposit could be 
formed in any modern sea ; because, owing to the strong 
affinity of the protoxide for oxygen, it would be accumulated 
as the peroxide. Besides this, I am not aware that there is 
any sea in which any great amount, even of this, is now 
deposited, except with a very considerable quantity of other 
substances mixed with it ; its chief source being the decom- 
position of such silicates as the augite and hornblende of 
various traps and hornblendic schists, which, in some cases, 
would yield a clay containing one-third of its weight of this 
oxide. 
If the stone be carefully examined, it may be seen that it 
contains more or less entire portions of shells. In some 
cases these are still of their original composition, and consist 
of carbonate of lime, but in others they are changed to 
carbonate of iron ; the difference being apparently due, in 
some instances at least, to the kind of shell. The micro- 
scopical investigation of a thin transparent section of the 
stone shows far more clearly that the minute fragments of 
shell have been similarly altered ; the replacing carbonate of 
iron extending, as yellowish obtuse rhombic crystals, from 
the outside to a variable distance inwards, often leaving the 
centre in its original condition, as clear colourless carbonate 
of lime, though in many instances the whole is changed. 
The oolitic grains likewise, have such peculiarities as indicate 
that they were altered after deposition. 
