118

ORIGIN OF COAL.

CHAP. VII.

ering the mud. The water is transparent, though tinged of a pale brown-colour, like that of our peatmosses, and contains abundance of fish. This sheet of water is usually even with its banks, on which a thick and tall forest grows. There is no beach, for the bank sinks perpendicularly, so that if the waters are lowered several feet it makes no alteration in the breadth of the lake.

Much timber has been cut down and carried out from the swamp by means of canals, which are perfectly straight for long distances, with the trees on each side arching over and almost joining their branches across, so that they throw a dark shade on the water, which of itself looks black, being coloured as before mentioned. When the boats emerge from the gloom of these avenues into the lake, the scene is said to be "as beautiful as fairy land."

The bears inhabiting the swamp climb trees in search of acorns and gum berries, breaking off large boughs of the oaks in order to draw the acorns near to them. These same bears are said to kill hogs and even cows. There are also wild cats, and occasionally a solitary wolf, in the morass.

That the ancient seams of coal were produced for the most part by terrestrial plants of all sizes, not drifted, but growing on the spot, is a theory more and more generally adopted in modern times, and the growth of what is called sponge in such a swamp, and in such a climate as the Great Dismal, already covering so many square miles of a low level region bordering the sea, and capable of spreading itself indefinitely over the adjacent country, helps us greatly to conceive the manner in which the coal of the ancient Carboniferous

rocks may have been formed. The heat, perhaps, may not have been excessive when the coal-measures originated, but the entire absence of frost, with a warm and damp atmosphere, may have enabled tropical forms to flourish in latitudes far distant from the line. Huge swamps in a rainy climate, standing above the level of the surrounding firm land, and supporting a dense forest, may have spread far and wide, invading the plains, like some European peat-mosses when they burst; and the frequent submergence of these masses of vegetable matter beneath seas or estuaries, as often as the land sunk down during subterranean movements, may have given rise to the deposition of strata of mud, sand, or limestone, immediately upon the vegetable matter. The conversion of successive surfaces into dry land, where other swamps supporting trees may have formed, might give origin to a continued series of coal-measures of great thickness. In some kinds of coal, the vegetable texture is apparent throughout under the microscope; in others, it has only partially disappeared; but even in this coal the flattened trunks of trees of the genera Lepidodendron, Sigillaria, and others, converted into pure coal, are occasionally met with, and erect fossil trees are observed in the overlying strata, terminating downwards in seams of coal. The chemical processes by which vegetable matter buried in the earth is gradually turned into coal and anthracite has been already explained (see above, p. 72.).

Before concluding the remarks which are naturally suggested by a visit to the Great Dismal, I shall say a few words on a popular doctrine, favoured by some geologists, respecting an atmosphere highly charged with

120

THEORY OF ATMOSPHERE

CHAP. VII.

carbonic acid, in which the coal plants are supposed to have flourished. Some imagine the air to have been so full of choke-damp during the ancient era alluded to, that it was unfitted for the respiration of warm-blooded quadrupeds and birds, or even reptiles, which require a more rapid oxygenation of their blood than creatures lower in the scale of organization, such as have alone been met with hitherto in the Carboniferous and older strata. It is assumed that an excess of oxygen was set free when the plants which elaborated the coal subtracted many hundred million tons of carbon from the carbonic acid gas which previously loaded the air. All this carbon was then permanently locked up in solid seams of coal, and the chemical composition of the earth's atmosphere essentially altered.

But they who reason thus are bound to inform us what may have been the duration of the period in the course of which so much carbon was secreted by the powers of vegetable life, and, secondly, what accession of fresh carbonic acid did the air receive in the same. We know that in the present state of the globe, the air is continually supplied with carbonic acid from several sources, of which the three principal are, first, the daily putrefactien of dead animal and vegetable substances; secondly, the disintegration of rocks charged with carbonic acid and organic matter; and, thirdly, the copious evolution of this gas from mineral springs and the earth, especially in volcanic countries. By that law which causes two gases of different specific gravity, when brought into contact, to become uniformly diffused and mutually absorbed through the whole space which they occupy, the heavy carbonic acid finds its way upwards through all parts of the at

mosphere, and the solid materials of large forests are given out from the earth in an invisible form, or in bubbles rising through the water of springs. Peatmosses of no slight depth, and covering thousands of square miles, are thus fed with their mineral constituents without materially deranging the constituents of the atmosphere breathed by man. Thousands of trees grow up, float down to the delta of the Mississippi, and other rivers, and are buried, and yet the air, at the end of many centuries, may be as much impregnated with carbonic acid as before.

Coral reefs are year after year growing in the ocean -springs and rivers feed the same ocean with carbonic acid and lime; but we have no reason to infer that when mountain masses of calcareous rock have thus been gradually formed in the sea, any essential change in the chemical composition of its waters has been brought about. We have no accurate data as yet for measuring whether in our own time, or at any remote geological era, the relative supply and consumption of carbon in the air or the ocean causes the amount of those elements to vary greatly; but the variation, if admitted, would not have caused an excess, but rather a deficit of carbon in the periods most productive of coal or peat, as compared to any subsequent or antecedent epochs. In fact, a climate favouring the rank and luxurious growth of plants, and at the same time checking their decay, and giving rise to peat or accumulations of vegetable matter, might, for the time, diminish the average amount of carbonic acid in the atmosphere

-a state of things precisely the reverse of that assumed by those to whose views I am now objecting.

122

TOUR TO CHARLESTON.

CHAP. VIN

CHAPTER VIII.

Tour to Charleston, South Carolina.—Facilities of Locomotion.Augusta.-Voyage down the Savannah River.-Shell Bluff.Slave-labour.-Fever and Ague.-Millhaven.—Pine Forests of Georgia. Alligators and Land-Tortoises.--Warmth of Climate in January. Tertiary Strata on the Savannah.-Fossil Remains of Mastodon and Mylodon near Savannah.-Passports required of Slaves.-Cheerfulness of the Negroes.

Dec. 28.-CHARLESTON, South Carolina. We arrived here after a journey of 160 miles through the pine forests of North Carolina, between Weldon and Wilmington, and a voyage of about 17 hours, in a steam ship, chiefly in the night between Wilmington and this place. Here we find ourselves in a genial climate, where the snow is rarely seen, and never lies above an hour or two upon the ground. The rose, the narcissus, and other flowers, are still lingering in the gardens, the woods still verdant with the magnolia, live oak, and long-leaved pine, while the dwarf fan palm or palmetto, frequent among the underwood, marks a more southern region. In less than four weeks since we left Boston, we have passed from the 43d to the 33d degree of latitude, carried often by the power of steam for several hundred miles together through thinly peopled wildernesses, yet sleeping every night at good inns, and contrasting the facilities of locomotion in this new country with the difficulties we had contended with the year before when travelling in Europe, through populous parts of Touraine, Brittany, and other provinces of France.