seams are generally near the middle of this series; as if the conditions necessary for the formation of coal had gradually come into existence and as gradually disappeared; that there were two poles and an equator belonging to this time-world-a morn, noon, and evening to this geological day.

We have spoken thus far only of the thickness of coal strata and of coal seams; but it is impossible to form a correct idea of the amount of matter contained in these strata or in these seams without taking into account also their horizontal extent. Coal is very widely distributed over the world, although some countries are more favored than others. England, France, Spain, Portugal, Belgium, Sweden, Poland, and Russia have their beds of coal. It is also found abundantly in Asia, Africa, and South America; but no where is the coal formation more extensively displayed than in the United States, and no where are its beds of greater thickness, more convenient for working, or of more valuable quality. There are within the limits of the United States no less than four coal fields of enormous dimensions. One of these, the Appalachian coal field, commences on the north, in Pennsylvania and Ohio, sweeping south through western Virginia

Fig. 10.

U.S. of America

Coal Area

133500 Sq. Miles

Brit. Amer. Gr. Brit. Spain France Belgium

and eastern Kentucky, Tennessee, extends even into Alabama. Its area is estimated at about 60,000 square miles. A second occupies the greater portion of Illinois and Indiana; in extent almost equal to

* Recent estimates by Marcou and by H. D. Rodgers make the coal area of the United States near 200,000 square miles.

the Appalachian. A third covers the greater portion of Missouri, while a fourth occupies the greater portion of Michigan. Just out of the limits of the United States, in New Brunswick and Nova Scotia, there is still a fifth, occupying, according to Mr. Lyell, an area of 36,000 square miles. Besides these there are several others of less extent.

If we now compare the relative coal areas of the principal coal producing countries, the superiority of our own will be still conspicuous. The following diagrams represent these relative areas in a more intelligible form than could be done by mere figures.

But if, on the other hand, we compare in the same manner the relative annual production of the same countries, we will find the order very different.

It will be seen that the annual production of coal in Great Britain is more than seven times that of the United States, although her coal area is so much less. It is estimated that even at this enormous rate of production the coal fields of Great Britain will yet last for 500 years. There is little danger, then, that ours will fail us shortly.

Now industry, as the basis of the organization of society, forms the distinguishing feature of modern civilization. Coal is the very aliment of industry. The material prosperity of any country may therefore be tolerably accurately estimated by the amount of coal consumed.

According to this method of estimation, Great Britain is superior to all other countries in actual material civilization. But if the consumption of coal is a measure of the actua civilization of a country, the amount of coal area represents its potential civilization. How far are we superior to all other countries in this respect! What a glorious destiny awaits us in the future-a destiny already predetermined in the earliest geological history of the earth.

One more remark and I am done. It is certain that, as manufacturing and productive industry take root and flourish almost exclusively in cool and temperate climates, so also in them do the coal formations prevail in the greatest abundance. Our scientific maps and investigations confirm the one, and national statistics the other. Almost all the true coal of the world is found in the north temperate zone. Thus the climates which are most congenial to laborious occupations, the latitudes best adapted to the vigorous growth of industrial civilization, are precisely those where, fortunately, have been placed the materials of labor, the aliment of industry. Fortunately did I say? No; this has not been the result of blind chance, but of deliberate providential design. We have here a sublime illustration of that all-comprehensive foreknowledge which foresees and designsthe end from the beginning; of that immutability which changes not, but only unfolds its eternal plans; of that unity in the system of time-worlds of which I have already spoken, our own epoch being the sun and centre.

THEORIES OF THE COAL.

There is no point connected with the coal which has been the subject of so much discussion as the manner of its accumulation. At first view, existing nature seems to offer no analogy to guide us in our attempts to account for such enormous accumulations of carbonaceous matter. It is admitted, however, I believe, on all hands, that the deposit must have taken place in water. The perfect preservation of the carbon of the plants, and often of their external forms and structure, which must have suffered complete oxydation and disintegration if exposed to the air, the fact that the plants were most or all of them swamp plants, and, more than all, the alternation of coal seams with sedimentary deposits of clay and sand, all seem to point unmistakably to water as the preserving agent. There is still another evidence which I think has generally been overlooked. In the midst of the more structureless bituminous matter of the coal are often found imbedded wedge-shaped masses of vascular tissue called native carbon. No one who attentively examines these wedges can fail to perceive that they are the wooden wedges of exogens separated by the decomposition of the softer cellular tissue of the intervening medullary rays, while they floated as logs upon the water and finally became imbedded in the carbonaceous mud below.

Thus far I believe all theorists agree. But from this point opinions diverge; some geologists holding that the coal was deposited on the spot where the plants grew, others that the plants were drifted in the

form of rafts to great distances and deposited at the mouths of rivers; the former, that a coal basin is the site of an ancient peat bog, the latter, that it is the position of an ancient estuary or delta. The former opinion is called the "peat bog theory, "the latter the

"estuary theory."

Peat bog theory.It is well known that in many countries, particularly in moist, cool climates, and damp, low grounds, certain plants, such as ferns, mosses, &c., as well as trees which delight in moist places, if allowed to grow undisturbed from generation to generation will, by their decay, accumulate enormous masses of earbonaceous matter. Such a spot is called a peat bog. The theory of this accumulation is as follows: Plants derive all their carbon from the atmosphere. In the annual fall of leaf, and finally their own death, they return to the earth the whole of the matter thus silently extracted from the air. Undisturbed vegetation, therefore, constantly enriches the soil by adding to it what has been taken from the air. Thus worn out lands improve by lying fallow. Thus the rich black vegetable mould found covering the ground in forests continues to increase from year to year. In all ordinary cases, however, there is a limit beyond which this accumulation will not go. By decomposition the organic matter is again returned to the atmosphere as fast as it accumulates. But if by any means this decomposition is prevented the organic matter accumulates indefinitely. This is precisely what takes place in peat bogs. The presence of water in a great measure prevents the oxydation of the carbon. The growth of plants now continually takes carbon from the atmosphere, their death as continually deposits it upon the earth. Each generation rises, phoenix-like, from the ashes of the last, to become in its turn soil for the next. Thus the ancestral accumulation continues to increase, the funeral pile continues to rise, until pure carbonaceous matter may in time accumulate to the depth of thirty or forty feet. Such a mass of carbonaceous matter deprived of its water and compressed to the density of coal, would make a seam of perhaps three or four feet in thickness. Now, according to the peat bog theory, it is under such circumstances that the carbon of a coal seam has been : accumulated.

The arguments in favor of this theory are: 1st. The purity of the coal. It is true that coal is often found largely mixed with earthy matter or mud. As we have already shown, every stage of gradation may be traced between pure coal and pure shale. But by far the larger portion of coal seems to be entirely free from foreign matter. The amount of ash is not greater than five to ten per cent.; that is, not greater than might arise from the earthy matter of the plants from which the coal was derived. This purity of the coal indicates complete absence of sediment in the water in which the coal was originally laid down. Now the water of peat swamps, though discolored by organic matter in solution, is always entirely free from sediment. In fact, this seems a necessary condition of the growth of peat plants-an incursion of water containing mud is fatal to such plants. If, then, a coal seam is the result of carbonaceous matter

slowly accumulated at the bottom of ancient peat swamps, the purity of the coal is completely accounted for. But if, on the contrary, it is formed by the accumulation of timber carried down to the mouths of great rivers during freshets, it should always be largely mixed with mud.

2d. The fine preservation of the tenderest and most delicate parts of plants. We have already spoken of the profusion of finelypreserved leaves and entire fronds of ferns in the black slate overlying à coal seam. So perfect is this preservation that large and complex fronds are often entirely unbroken, and even the minutest variation of the leaves as distinct as in the living fern. This fine preservation of tender parts seems strongly to indicate that these leaves had fallen gently from the parent stem, and been preserved on the spot where they fell. It seems utterly inconsistent with the violent action of currents bearing rafts to great distances.

3d. The position of the finely-preserved leaves, &c., always on the upper surface of the coal seam, (roof of the coal mine.) Precisely. the same is observed in every peat swamp. The perfect leaves are to be found only on top, for the plain reason that these are the last fallen, and therefore not yet disorganized. But in the case of accumulations of vegetable matter at the mouths of rivers, there seems to be no reason why leaves should not be entangled in all parts alike.

4th. Coal, like peat, is composed of completely disorganized carbonaceous matter, containing fragments in which vegetable structure is more distinct. This is not inconsistent with what I have already said in my last lecture of the vegetable origin of even the most structureless coal being detectable by the microscope. Plants are composed entirely of cells. Both in peat and in coal these cells are generally separated from one another. The vegetable structure is completely disorganized, but the separate cells still bear unmistakable marks of their origin; the organic structure is gone, but the organic origin is still visible. But if a coal seam was an imbedded raft, it should be composed almost entirely of fragments of trunks, branches, &c., instead of a structureless mass containing only a few such fragments.

5th. It will be recollected that a seam of coal is overlaid by black slate and underlaid by fire-clay. In the black slate, as already said, are found the finest impressions of leaves and other tender parts; in the fire-clay, which underlies the coal seam, are found imbedded in the greatest abundance the roots of plants, and not unfrequently the stumps of trees with the roots attached, precisely as they grow. And, what is still more remarkable and significant, trunks of trees are not unfrequently found almost entire, standing erect, with their roots still bedded in the fire-clay, their trunks passing through the seam, and far into the overlying strata of shale and limestone. By means of evidence of this kind Lyell and Dawson have been able to make out distinctly nearly 60 planes of successive vegetation in the coal field of Nova Scotia. In many of these, viz: about 20, the trees are still in the position in which they grew, as shown in figure 12; of the rest the evidence consisted in the imbedded stigmaria or roots of sigillaria.