With regard to this vegetation, it would imply the existence of sunlight, though some of the lower orders would require but little. The atmosphere was still dense and loaded with vapor. The lower orders of flowerless plants were succeeded by tree-like ferns, some of which grew to the enormous hight of forty or fifty feet. The careful observer will frequently find traces of this early vegetation in the lumps of common coal. Sometimes the galleries of coal mines are overhung with beautiful proportions of extinct vegetable forms. Thus a wise Providence has preserved the remains of primeval vegetation for perhaps millions of years, to tell us the story of those ancient forests, and reveal to us the various steps of creation. "The vegetation of the coal period presents a remarkable character, being composed almost entirely of {183} the highest class of flowerless plants, along with a few of the lowest class of those that flower." (_Dawson's Chain of Life, p_. 96.) Concerning the climatic condition of the earth during the coal-forming, (carboniferous) period, as it is called, the description given by Hugh Miller, the eminent geologist, may not be out of place: "From the circumstance that no dew is deposited in our Summer evenings, save under a clear sky, it is now ascertained that even a thin covering of cloud--serving as a robe to keep the earth warm--prevents the surface heat of the planet from radiating into the space beyond. And such a cloud, thick and continuous, as must have wrapped round the earth, as with a mantle, during the earlier geologic periods, would have served to retard, for many ages, the radiation, and consequently the reduction, of that internal heat of which it was itself a consequence. Nor would a planet, covered over for ages with a thick screen of vapor, be a novelty yet in the universe. It is doubtful whether astronomers have ever yet looked on the face of Mercury. It is, at least, very generally held that, hitherto, only his clouds have been seen. Even Jupiter, though it is thought his mountains have been occasionally detected raising their peaks through openings in his cloudy atmosphere, is known chiefly by the dark, shifting bands that, flecking his surface in the line of his trade-winds, belong not to his body, but to his thick, dark covering. Nor, yet further, would a warm, steaming atmosphere, muffled in clouds, have been {184} unfavorable to a rank, flowerless vegetation like that of the coal measures.