Thus in the lowest layers of the coal ground appears the anthracite, which, being almost destitute of volatile matter, contains the greatest quantity of carbon.

In the higher beds are found, on the contrary, lignite and fossil wood, substances in which the quantity of carbon is infinitely less.

Between these two beds, according to the degree of pressure to which they have been subjected, are found veins of graphite and rich or poor coal.

It may be asserted that it is for want of sufficient pressure that beds of peaty bog have not been completely changed into coal.

So then, the origin of coal mines, in whatever part of the globe they have been discovered, is this: the absorption through the terrestrial crust of the great forests of the geological period; then, the mineralization of the vegetables obtained in the course of time, under the influence of pressure and heat, and under the action of carbonic acid.
Now, at the time when the events related in this story took place, some of the most important mines of the Scottish coal beds had been exhausted by too rapid working.