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Page 2
In the United States there are about 120,000 square miles underlaid by
known workable coal-beds, besides what yet remains to be discovered;
while on the cliffs of Nova Scotia the coal-seams can be seen one over
the other for many hundred feet, and showing how the coal was originally
formed. With this immense stock of fuel in the cellars of the earth, it
seems evident that we need not trouble our minds or be anxious as to the
duration of our coal supply. Besides, the conversion of vegetable matter
into coal seems to be going on even now. In the United States there are
peat-bogs of considerable extent, in which a substance exactly
resembling cannel coal has been found; and in some of the Irish
peat-beds, as also in the North of Scotland, a similar substance has
been discovered, of a very inflammable nature, resembling coal.
Yes! what could have produced this singular-looking, black, inflammable
rock? How many times was this question asked before Science could return
an answer? This she can now do with confidence. Coal was once growing
vegetable matter. On the surface of the shale, immediately above the
coal, you will find innumerable impressions of leaves and branches, as
perfect as artist ever drew. But how could this vegetable matter ever
accumulate in such masses as to make beds of coal of such vast extent,
some not less than 30 feet thick? It would take 10 or 12 feet of green
vegetable matter to make 1 foot of solid coal. Let us transport
ourselves to the carboniferous times, and see the condition of the
earth, and this may assist us to answer the question. Stand on this
rocky eminence and behold that sea of verdure, whose gigantic waves roll
in the greenest of billows to the verge of the horizon--that is a
carboniferous forest. Mark that steamy cloud floating over it, an
indication of the great evaporation constantly proceeding. The scent of
the morning air is like that of a greenhouse; and well it may be, for
the land of the globe is a mighty hothouse--the crust of the earth is
still thin, and its internal heat makes a tropical climate everywhere,
unchecked by winter's cold, thus forcing plants to a most luxurious
growth.
Descend, and let us wander through this forest and examine it more
closely. What strange trees are here! No oaks, no elms, or ash, or
chestnut--no trees that we ever saw before. It looks as if the plants of
a boggy meadow had shot up in a single night to a height of 60 or 70
feet, and we were walking among the stalks--a gigantic meadow of ferns,
reeds, grasses, and club-mosses. A million columns rise, so thick at the
top that they make twilight at mid-day, and their trunks are so close
together we can scarcely edge our way between them, whilst the ground is
carpeted with trailing plants completely interwoven. What strange trees
they are! Beneath us lies an accumulation of vegetable matter more than
200 feet in thickness--the result of the growth and decay of plants in
this swamp for centuries. All things are here favourable for the growth
of vegetation--the great heat of the ground causes water to rise rapidly
in vapour, and this again descends in showers, supplying the plants with
moisture continuously. The air contains a large proportion of carbonic
acid gas, poison to animals but food to plants, which, by means of its
aid, build up their woody structure. Winds at times level these gigantic
plants, for their hold on the earth is feeble, and thus the mass goes on
increasing.
We are now on the edge of a lake abounding with fish, whose bony scales
glitter in the water as they pursue their prey. Lying along the shore
are shells cast up by the waves, and there are also seen the tracks of
some large animals. How like the impression of a man's hand some of
these tracks are! The hind-feet are evidently much larger than the
fore-feet. There is the frog-like animal which made them, and what a
size! It must be six feet long, and its head looks like that of a
crocodile, for its jaws are furnished with formidable rows of long,
strong, sharp, conical teeth.
The continued growth and decomposition of the vegetation during long
ages must have produced beds like the peat-deposits of America and Great
Britain. In the Dismal Swamp of Virginia there is said to be a mass of
vegetable matter 40 feet in thickness, and on the banks of the Shannon
in Ireland is a peat-bog 3 miles broad and 50 feet deep. When conditions
were so much more favourable for these deposits, beds 400 feet in
thickness may easily have been produced. This accumulated mass of
vegetable matter must be buried, however, before we can have a coal-bed.
How was this accomplished? The very weight of it may have caused the
crust of the earth to sink, forming a basin into which rivers, sweeping
down from the surrounding higher country, and carrying down mud in their
waters, the weight of which, deposited upon the vegetable matter,
pressed and squeezed it into half its original compass. Sand carried
down subsequently in a similar manner, and deposited upon the mud,
pressed it into shale, and the vegetable matter, still more reduced in
volume by this additional pressure, is prepared for its final conversion
into shale. In time the basin becomes shallow from the decomposition of
sediment on its bottom, and then we have another marsh with its myriad
plants; another accumulation of vegetable matter takes place, which by
similar processes is also buried. Where thirty or forty seams of coal
have been found one below another, we have evidence of land and water
thus changing places many times.
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