The edge series, which forms the lowest, and of course oldest of the above groups, averages from 800 to 900 feet in thickness. It contains about thirty seams of coal above a foot thick, and many more of less size. They occur irregularly, some lying only a few inches apart, others from eighty to ninety feet, the intervening space being occupied by sandstone or shale.
Now as each coal-seam, with its associated under-clay, appears to mark a former land surface, it will follow that there must be as many old land surfaces in this series of strata as there are such coal-seams, and that for every intervening mass of sandstone or shale, the area of vegetation must have been submerged. This conclusion would have been violently resisted by the supporters of the "drift" theory. They would have roundly asserted that such an unsteady surface was a mere supposition to suit a hypothesis, unsupported by fact, and contrary to the analogy of existing nature; and they would not perhaps have hesitated to maintain, that such an oscillating land could be little fitted to nourish so rich and luxuriant a vegetation as that of the Carboniferous period. But it will not be difficult to show that our conclusion, so far from being contrary to analogy, is amply borne out by the processes of existing nature, and that its opponents, and even its original asserters, failed to perceive that what it demands is not a rapidly oscillating crust, but one as steady and uniform as that of many of the least disturbed countries at the present day; and that we do not require to call in the aid of a special elevation and submergence for every coal-seam, but that for the most part the hypothesis of a steady sinking of the area of a coal-field, interrupted perhaps by occasional elevatory movements, along with an active and constant deposition of sediment by the varying currents of a large river, is sufficient, if not thoroughly to explain, at least to throw great light upon the origin of those enormous masses of strata composing our present coal-basins. The oft-recurring variations in the nature of the strata that form our coal-measures, sandstones alternating with shales, these again with coals and fire-clays, together also with the terrestrial origin of the coal-seams, and the occasional presence of true marine organisms, make it evident that, to obtain any modern analogue to such a condition of things, we must examine those localities where large bodies of fresh water, carrying sediment and vegetation from the land, mingle with the sea. Let us then look for a little at the operations now in progress at the mouths of the larger rivers, and mark how far they elucidate the structure and history of a coal-field.
"Egypt is the gift of the Nile." Such was the conclusion arrived at by one of the most diligent observers of ancient Greece—the venerable Herodotus.[67] He sailed up the river marking all the leading features in its scenery, and noting the more apparent evidences of ancient physical changes. His remarks on these subjects form one of the earliest specimens of scientific reasoning that have come down to us, and are remarkable for their correctness and the truly inductive mode of thought which they evince. Modern travellers have amply confirmed the opinions of the father of history, and we now know that but for its central river, Egypt would be a vast dreary expanse of arid sand like the neighbouring deserts of Lybia. The Nile, by annually inundating the country, deposits over it a stratum of rich loam, and thus not only waters the land, but continually renews the soil. The sediment in this way brought down has gradually encroached upon the waters of the Mediterranean, being heaped up at the river mouth into shifting sand-banks, islets, and great tracts of low, swampy ground, slightly raised above the sea-level. Through this series of silting deposits, the river sends a number of branches, often winding in labyrinthine convolutions, and ever changing their course, by wearing away the silt at one place, and throwing it down at another. The area traversed by the mouths of the Nile was called by the Greeks the Delta, from its similarity in form to the Greek letter, and the name has since been given to all such fluviatile deposits, whether they have this general form or not.
[67] Euterpe, 5.—His words are very emphatic. "To one of ordinary intelligence, who has not heard of it before, but sees it, Egypt is manifestly land acquired by the inhabitants, and a gift from the river—δωρον τον ποταμον." The 10th and 12th chapters of the same book deserve especial study for the admirable inductive style in which the historian compares the phenomena observable in Egypt with what were well known as the results of river action in other lands. The passages might be quoted word for word in the most rigid scientific argument of any modern geologist.
The sediment annually deposited by the Nile varies in thickness in different years. The mean thickness of the annual layers at Cairo has been calculated not to exceed that of a sheet of thin pasteboard, so that "a stratum of two or three feet must represent the accumulation of a thousand years."[68] Such thin laminæ must resemble greatly some of the more fissile shales in the Carboniferous system, which were, perhaps, formed by as slow a process, and in their aggregate depth probably took many thousand years to accumulate. But those fluviatile depositions of the Nile vary little in kind, for when cut through they are found regularly stratified down to their base, which rests upon the great underlying sand. They show us how the argillaceous seams of the coal-measures may have originated; but the diversity of character in these Carboniferous rocks indicates a more varied kind of sediment, and probably more rapid and active transporting currents. A closer analogy to such a condition of things meets us on the shores of the New World.
[68] Lyell's Principles, p. 262.
The Mississippi, so magnificent in all its proportions, has raised a delta which covers a tract of about 14,000 square miles, equal to almost half the area of Ireland. The lower parts of this delta are formed of low, shifting banks, traversed by innumerable streams that diverge from the main river, and alternately throw down and remove vast quantities of earthy sediment, intermingled with rafts of drift-wood. These swamps are covered with a rank growth of long grass and reeds, and for about six months of the year are more or less submerged below the waters of the river, while liable at the same time to continual inundation and encroachment from the sea. The higher parts of the delta, though also subject to a similar periodical submergence, nourish a more luxuriant vegetation. Vast tracts of level sandy soil are densely overgrown with pine, which is used extensively for making pitch. Large districts of the swampy ground are covered with willows, poplars, and thickets of the deciduous cypress, an elegant tree that rises more than 100 feet above the soil. When in hot seasons these swamps get dried up, "pits are burnt into the ground many feet deep, or as far down as the fire can descend without meeting with water, and it is then found that scarcely any residuum or earthy matter is left. At the bottom of all these 'cypress swamps' a bed of clay is found, with roots of the tall cypress, just as the underclays of the coal are filled with stigmaria."[69] In this way a thick accumulation of vegetable matter goes on forming for years, until either the river changes its course, and inundating the swamp gradually covers it over with sand and mud, or until, owing to oscillations of the earth's crust, the district is either permanently submerged, so as to be silted over, or elevated to nourish a new and different kind of vegetation.
[69] Lyell's Elements, p. 386.
That such changes have taken place in the past history of the river we have several interesting proofs. Thus, owing to the great earthquakes of 1811, 1812, an area of more than 2000 square miles was permanently submerged.[70] Since then it has gone under the name of the "Sunk Country;" and Sir Charles Lyell, who visited the locality in 1846, that is, thirty-four years afterwards, tells us that he saw innumerable submerged trees, some erect, others prostrate. Now, it is easy to see how such an area may, when the climate suits, become the receptacle of vast accumulations of peat, which, by pressure and chemical action, will ultimately pass into coal. If we suppose the submergence carried on more rapidly at some periods, the plants might have been unable to keep pace with the ever-increasing inroads of sand and mud. In such cases the layer of vegetation would become eventually entombed beneath succeeding deposits of earthy matter. Were the amount of sediment thus thrown down sufficient in the end to counteract the downward motion of the earth's crust, and so raise the bottom of the river or lake to the level of the water, vegetation would spring up afresh and clothe the new raised surface as densely as in former years. This alternation, according as the amount of sinking or the amount of sediment predominated, might go on for thousands of years, until a series of strata many thousand feet thick were accumulated, and tranquilly carried down bed after bed below the level of the waters.
[70] See Sir Charles Lyell's Second Visit to United Stales, chap, xxxiii.