« AnteriorContinuar »
In the cases in which these trunks and roots,. in situ, are found, and they are by 10 means uncommon,) the evidence is conclusive that the coal was formed on the spot where the trees grew; in other words, that the growth of the trees and the deposit of the coal took place simultaneously on the same spot. This is clearly impossible in an estuary, but is known to take
place in every peat swamp. To recapitulate the whole argument: If we examine a peat bog which has been for many years thickly overgrown with ferns, mosses, and water plants of various kinds, and shaded by many large trees, we find the soil composed entirely of black carbonaceous matter, wholly destitute of structure but revealing its vegetable origin to the microscope, containing fragments of trunks and branches of trees lying in all possible positions, some prostrate, some inclined at all angles, many, both living and dead, still erect, their roots firmly fixed in the clay at the bottom of the bog, below the peaty matter which has slowly gathered about their lower parts, and over the whole lie thickly strewn the freshly fallen leaves. Now suppose such a peat bog to be deeply buried beneath the surface of water and overwhelmed with sediment of clay and sand, and again, after ages, elevated and exposed by section to the scrutiny of the geologist, and we shall have a complete reproduction of the phenomena of a coal seam.
The great, and almost theonly, objection which has been urged against this theory is to be found, not in the phenomena of an individual coal seam, but rather in the general phenomena of coal basins, in the repeated alternation in the same locality of coal seams with marine and fresh water strata. We have already seen that there are in the same coal basin sometimes as many as an hundred coal seams, one above the other; now, according to this theory, when the coal seam was forming the spot must have been above the surface of the sea, but when the interstratified limestones and shales were being deposited the same spot must have been beneath the sea-level. Thus, argues the objector, we are driven to the enormous assumption that the same spot has been successively upheaved above and depressed beneath the sea-level one hundred times during the carboniferous period, and, what is still more remarkable, that every time it rose above the sea it became a peat swamp; or if the intervening strata are fresh water instead of marine, the difficulty seems only to be increased.
Estuary theory.-It is to meet this very difficulty, to account for this remarkable alternation of strata, that the rival theory has been proposed. An estuary is the wide open mouth of a river emptying into a tidal sea; it is occupied sometimes by fresh and sometimes by salt water. The deposit at the bottom of an estuary, in suitable positions, is, therefore, an alternation of fresh water and marine strata. In seasons of freshets the river water, loaded with sediment and
perhaps bearing rafts of drift timber, forces back the sea water, occupies the estuary, and makes its deposit of clay and sand, containing fragments of such drift timber; in seasons of lot water the ocean returns and makes its deposit, perhaps of limestone, and so on alternately. A coal field is supposed by these theorists to be the position of an ancient estuary ; the limestone strata are the marine deposit, the shale and sandstone the river deposit, and the coal seam the im bedded drift timber brought down by the river from distant forests.
The objections to this theory are all that has been said in favor of the peat bog theory. The pureness of the coal, the fine preservation of even the tenderest parts of plants, the position of such well preserved specimens always on the upper surface of a coal seam, the structureless character of the great mass of the coal, and, above all, stumps and trunks of trees still erect, with their roots still fixed in the clay stratum below--all this seems not only unaccountable but impossible on this theory.
In comparing these two theories it will be seen that the first explains completely the phenomena of an individual coal seam, but signally fails to explain the general phenomena of a coal basin, viz: the alternation of coal seam with marine and fresh water strata ; while, on the other hand, the second explains weil this alternation, but fails utterly to explain the phenomena of an individual coal seam. There is, then, real and substantial evidence in favor of each, and equally substantial objections. If this had not been the case one or the other would have been relinquished ere this. But we find, on the contrary, that they have both found strenuous advocates from the time geology commenced to exist as a science until now. In every such case of vitality in rival theories it will be found, I think, that there is a real germ of truth in both—that both are true and both are false ; both true in some sense, and therefore reconcilable'; and both false through narrowness of view, through exclusiveness, through mistaking a partial for a general view. I can best illustrate my meaning by referring you to the familiar but very instructive fable of the shield, which being distinctly seen by two knights of equally good eye sight and of undoubted veracity, was declared by one to be white and by the other to be black. You will recollect that, after several lances were broken and many wounds and bruises endured to decide the knotty point, it was discovered by some one who, strange to say, was more interested in the truth than in the dispute, that one side was white and the other was black. The disputants were both right and both wrong, but wrong only by exclusiveness, by mistaking a partial for a general view. So it is with almost all vexed questions. There is truth on both sides, but both err in excluding the other. We are seeking in the right direction when we attempt to show the partialness of both views. We have risen to a higher view, to a philosophic truth, when we show that these two partial and apparently irreconcilable views may be united into one ; these two surface views may be stereoscopically combined.
There is an old and much quoted adage, that “truth lies in the middle” between extreme opinions. As generally understood nothing
can be more false or hurtful. Through its influence a merely timid or temporizing policy is mistaken for wisdom, the “ fence man" is mistaken for the philosopher. There is another old adage, that “extremes meet ;' i. e., what to the superficial observer seem to be extremes, to the deeper thinker are often really closely allied. But the converse of this proposition, though not erected into an adage, is even more profoundly true, viz: that what seem to be closely allied are very often real extremes. There is often a superficial resemblance between the highest and the lowest, so that by the unthinking multitude the one is often mistaken for the other; pride for nobility of soul, humility for mean-spiritedness, the serenity of self-command for the serenity of insensibility, &c. It is only in this way that the “ fence man” resembles the philosopher, for they are as wide apart as the poles. It is in this way only that truth seems to “ lie in the middle," although we are further from it there than anywhere else. To refer again to the fable of the shield : It would have been a poor solution of the famous dispute to say that the shield was neither pure white nor pure black, but midway between the two extremes; that it was, in fact, some shade of gray or dusky, or, perhaps, pepper and salt. No; I repeat, truth lies not "in the middle,” but the reconciliation of extremes in the harmonious combination of apparent antagonisms.
Now, it seems to me that the phenomena of a coal seam already enumerated prove most conclusively that the coal was formed in situ, as in the peat swamps of the present day. At the same time the frequent alternation of seams with marine and fresh water strata prove also most conclusively that the deposit took place at the mouths of rivers. Here are two incontestible facts. We must put them together; we must combine them if we would make a true and sufficient theory. I believe the more this subject is reflected on the more we shall be convinced that coal was deposited in peat swamps at the mouths of large rivers, and therefore subject to overflows by the river and occasional inundations by the sea. We are to look for analogies in existing nature, not among the bogs of Ireland, but among the river swamps of the Mississippi.
It is well known that such peat swamps, some of them of enormous extent, exist now on the margins and in the delta of the Mississippi and probably many other large rivers, and that pure peat unmixed with mud is constantly forming in these swamps, although they are annually flooded by the river. This seems at first incredible, when we recollect that the river water is loaded with sediment, and that sediment prevents the growth of peat plants, or at least would entirely destroy the purity of the peat. But this apparent anomaly has been entirely explained by Mr. Lyell. According to this high authority, although the peat swamps of the Mississippi are annually flooded by river water they are entirely untouched by river mud. These favored spots are surrounded, particularly on the side next the river, by dense vegetation, which, acting as a sieve, completely strains the water of its mud before it reaches the peat swamp. The water of these swamps is therefore pure, and pure peat has been quietly depositing there for ages.
Let us now suppose that
Fig. 13. there existed during the carboniferous period a large river, perhaps less than the Mississippi, but with enormous swamps and delta, overgrown with rank vegetation far surpassing in luxuriance anything know at the present day. In the midst of such swamps there would evidently occur spots of great extent, the waters of which, for the reasons already mentioned, would never be contaminated with sediment, as at (a) fig. 13. Here for untold ages pure carbonaceous matter would accumulate undisturbed. In the course of time the surrounding portions of the swamps (6) where the mud is detained would rise by deposit of sediment, while the peat swamp (a) would remain as a sunken country, such as exist now in the swamps of the Mississippi. Finally, at uncertain intervals, a more than usually. large freshet, or perhaps some change in the level of the land, would deluge the swamp with mud and bury the peat. Gradually the vegetation would return, and the former condition of things be restored, to pass again through the same changes. We have but one other supposition to make, viz: that the whole river swamp and delta were gradually subsiding during the whole carboniferous period. This is by no means a violent supposition, but one which we have a right in this case to make for two good reasons: 1st. We have the best evidence that many of the large deltas of the present day are thus subsiding. This is proved in the case of the Mississippi delta by cypress stumps in situ below the level of the sea. 2d. The coal strata themselves give indubitable evidence of gradual subsidence during the period of their deposit. The character of these strata and their fossils shows that they were deposited in shallow water, but their enormous thickness (nearly three miles in Nova Scotia) renders this clearly impossible, unless we suppose such subsidence; for, if the bottom was stationary, it must have been three miles below the surface of the water when the lowest stratum was deposited. Now, if such subsidence went on constantly, but slowly, so that, under ordinary circumstances, the delta could be maintained by deposit from the river, but at uncertain intervals, more rapidly than the river could build up, so that the sea would again usurp possession and make its deposit of limestone, and again more slowly, so that the area might again be reclaimed by the river, and become a peat swamp, and so on alternately, we should easily, without any violent hypothesis, account for all the phenomena of a coal basin.
It will be observed that by this hypothesis the area of a coal basin has, indeed, been successively above and below the sea-surface, but not by successive upheaval and depression, as it has been supposed necessary on the peat bog theory, but by the contention, with various
success, of opposing forces, aqueous and igneous, the river constantly building up, and igneous forces beneath as constantly striving to depress; sometimes one force predominating, sometimes the other. Of such contention we have many instances in existing nature. It is evidently going on in the delta of the Mississippi at the present time.
It may not be possible, in the present condition of science, to picture to ourselves all the circumstances connected with this process. Perhaps I have already gone too far in this attempt; but the general facts upon which the theory rest are incontestible. Coal has almost certainly accumulated in situ in extensive peat swamps at the mouths of large rivers, upon ground which was slowly subsiding during the whole period. Under these circumstances it seems not difficult to account for all the phenomena of a coal basin. All we have to do in future is by study of the peat swamp of the Mississippi and the phenomena of delta deposit to discover the details of the process, to fill up the outline of the picture.
There is a fact noticed by Mr. Lyell, which is strongly confirmatory of this theory. In the sandstone of the coal measures it is common to find trunks of trees, but only trunks-no small branches, leaves, or tender parts. Moreover these trunks are observed to be mostly pines, highland trees, while the trunks in the coal seam proper are sigillaria, lepidodendron, calamites—swamp trees. Now, when we recollect that coarse sandstone is the deposit of rapid current, does it not seem evident that the sandstone was deposited by the freshet which overwhelmed the peat swamp, and that the pine trunks are the remains of drift timber brought from the highlands. Here, then, we have ancient drift timber, but how different from a coal seam !
Let us now attempt to estimate approximatively the time necessary to bring about these stupendous results. I believe we should never neglect an opportunity of this kind, because the popular mind has not yet grasped the idea of illimitable time required by geology to the same extent as it has the idea of illimitable space required by astronomy; and, as I believe, this is one of the greatest difficulties with which geology has to contend.
According to Boussingault luxuriant vegetation at the present day takes from the atmosphere about a half ton of carbon per acre annually, or 50 tons per acre in a century. Fifty tons of carbon of the specific gravity of coal, about 1.50, spread evenly over the surface of an acre, would make a layer of less than of an inch. Humboldt makes the estimate a little higher, viz: 1 an inch. We are willing to take the higher estimate. It appears, then, that if all the carbon taken from the air was preserved in the form of coal, our most luxuriant vegetation would make but a 1 inch of coal in a century. But in the coal measures the aggregate thickness of the coal seams in the same basin is sometimes 150 feet or more. In 150 feet there are 1,800 inches, or 3,600 half inches. At the present rate of vegetation, then, it would take 3,600 centuries, or 360,000 years, to accumulate this amount. But it will be objected that the vegetation of the coal period was probably much more luxuriant than the present, and the tendency of this difference would be to shorten the time. True; but it will be recollected that this estimate was made upon the ground