are stalled and fed in side-rooms excavated in the coal and superincumbent rocks. The requisite circulation of pure air is maintained through the mine by the consumption of refuse coal at some suitable place, the smoke and heated air from which ascend through a separate shaft. The escape of heated air through this shaft causes a descent of external air to take place through the main shaft. Communication between the two shafts is effected only through the remote portions of the mine, so that the pure air is made to permeate all the passages. Still there must always be side-rooms through which no circulation can be effected, and here not unfrequently collects that explosive "fire-damp," or light carbide of hydrogen, so often evolved. spontaneously from the coal, and so often the cause of fatal accidents to the miners (Fig. 62). When the seam of coal is less than five feet thick, it becomes necessary to remove some of the superincumbent rock, to render the roofs of the main passages sufficiently high for the mules to travel under them.

Thus entire square miles of a coal-seam, hundreds of feet beneath the surface, are perforated in all directions by the hand of the miner (Fig. 63), as ship-timber is riddled by the depredations of the Teredo.

By the feeble light of our miner's lamp we enter one of these dusky aisles. The substratum beneath our feet has been ground to dust. The whole thickness of the coalseam is exposed along the lateral walls. Occasionally it presents gentle undulations instead of lying in a rigidly plane position, and not unfrequently a huge bulge of the underlying rocks completely cuts off the seam. Overhead a black, bituminous shale forms the ceiling. Perhaps here and there the white shell of a univalve or a bivalve projects from the surface-the products of the sea buried in their native sediments, and suspended above our heads.

G

What a change in the condition of things since those little animals lived in the shallow surface-waters in which those sediments accumulated! Lo! here above us is a mirrorsurface gleaming in the light reflected from our lamps. Its polish is like that of jet, and yet it is wrought upon the face of the solid rock. Some slight movement of the

earth's crust has cracked the shaly roof; the opposite sides of the fissure have been moved to and fro over each other, and under the mighty pressure the two opposing faces have been beautifully polished.

But probably different sights will greet our eyes. The rocky ceiling is ornamented every where by the most exquisite tracery-inimitable representations of the delicate fronds of ferns (Fig. 64). We remove a scale of the rock, and behind is still another picture. Remove a second, and from the dark black rock gleams forth another form of grace and beauty. The whole mass of the

Fig. 64. Coal Ferns.

shaly roof is a port- g

folio of inimitable sketches. The sharpest outlines and minutest serratures of the leaves are clearly traced. The very nerves, with their characteristic bifurcations, are accurately depicted on this wonderful lithograph. Petioles, and buds, and woody stems, and

cones, and fruits, slender grass-leaves, striated rushes, the

fluted stems of gigantic club-mosses, the scarred and pitted trunks of extinct tree-ferns, diversify, by turns, the crayon sketchings of the dusky ceiling. Prostrate, all! They have stood erect; the soil has held them by their spreading roots, the genial sunlight has warmed them, the vital breeze has fanned their verdant foliage; change, which transforms all things, has swept over them, and graceful fern and giant club-moss, slender reed and arrogant conifer, have laid down together in their couch of sediment, and the old sexton, Time, has piled upon them the accumulated ashes of a hundred succeeding generations of trees, and herbs, and perished populations. What a store-house of suggestions is here! The dusty "Catacombs" are less eloquent in their inscriptions; the vaults of the Pyramids recite a history less full of meaning. To the soul that holds communion with the visible ideas that dwell about him, these rocky walls are vocal with narratives of earthquake and flood, of nodding verdure and of desolating surge; these shales are the tombstones of generations, on which are inscribed chronologies whose minutes are the cycles of the Hindoo. Here is the populous abode of world-ideas. Through these dim avenues flit spectres of the ancient thoughts which were once the acting energies of our planet. Here is the real Acherontian realm. He who has descended to these subterranean halls, and held converse with the forms which here abide, has visited a world and communed with intelligences of which Anchisiades had only dreamed.

Shall we venture to translate the histories recorded upon these rocky leaves? What were the scenes and events of that epoch of the world when these buried vegetable forms were living, growing organisms, and Nature was storing away for the human race these magazines of fuel?

IT

CHAPTER XIV.

THE SCENERY OF THE COAL PERIOD.

T was in the middle ages of the history of the world. The growing continents had lifted their brows above the surface of the all-embracing sea; but their spreading plains and long-extended shores were still the empire of the garpikes, and the nursery of illimitable beds of encrinites and polyps. The Gulf of Mexico jutted northward to Middle Iowa, and rolled its widening waters northwest far toward the sources of the Missouri River. There are good reasons for believing that it stretched through the entire length of the continent to the Frozen Ocean. The shoreline of the Atlantic reached from Connecticut through Southern New York and Northern Ohio, Indiana, and Illinois, to the valley of the future Mississippi. All the centre of Michigan was a sea-bottom, and not unlikely a gulf projected northward over the peninsula now inclosed by the great lakes. There was never, however, any free communication between the Michigan Gulf and the ocean after the later portion of the Devonian Age. Hudson's Bay stretched far toward the site of Lake Superior, as the Arctic Sea pushed down from the north to fall into the warm embrace of the waters of the Mexican Gulf. The great lakes were not-save, perhaps, Lake Superior-nor the mighty Mississippi, nor the thunder-voiced Niagara. The youthful continent was yet unclothed with soil, save the rocky detritus which nourished the lean vegetation which began to garnish the land during the period of the Chemung and Marshall. The skeleton rocks protruded every