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The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from The Underground City by Jules Verne: the elements of rocks which were to superpose the coal strata.
In course of time, periods of which include millions of years,
these earths hardened in layers, and enclosed under a thick
carapace of pudding-stone, schist, compact or friable sandstone,
gravel and stones, the whole of the massive forests.
And what went on in this gigantic crucible, where all this
vegetable matter had accumulated, sunk to various depths?
A regular chemical operation, a sort of distillation.
All the carbon contained in these vegetables had agglomerated,
and little by little coal was forming under the double influence
of enormous pressure and the high temperature maintained by
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