150 THEORY OF ATMOSPHERE CHAP. VII. 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 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 putrefaction 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 atmosphere, 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. Peat-mosses 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. ocean Coral reefs are year after year growing in the 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 152 ATMOSPHERE OF CARBONIC ACID. CHAP. VII. 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. 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. We Dec. 28.-CHARLESTON, South Carolina. 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 154 FACILITIES OF LOCOMOTION. CHAP. VIII. 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. At Charleston I made acquaintance with several persons zealously engaged in the study of natural history, and then went by an excellent railway 136 miles through the endless pine woods to Augusta, in Georgia. This journey, which would formerly have taken a week, was accomplished between sunrise and sunset; and, as we scarcely saw by the way any town or village, or even a clearing, nor any human habitation except the station houses, the spirit of enterprise displayed in such public works filled me with astonishment which increased the farther I went South. Starting from the sea-side, and imagining that we had been on a level the whole way, we were surprised to find in the evening, on reaching the village of Aikin, sixteen miles from Augusta, that we were on a height several hundred feet above the sea, and that we had to descend a steep inclined plane to the valley of the Savannah river. The strata cut through here in making the railway consist of vermilion-coloured earth and clay, and white quartzose sand, with masses of pure white kaolin intermixed. These |