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.

When vegetable matter is excluded from air and under great pressure, it
decomposes slowly, parting with carbonic acid gas; and is first changed
into lignite or brown coal, and then into bituminous coal, or the soft