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.