The New Zealand Railways Magazine, Volume 1, Issue 9 (February 25, 1927)
The Romance Of Coal
Just as the history of a nation is printed in books of parchment and paper, so we have a history of ages immensely remote and of forms of life that have long since disappeared, written on tablets of stone. Some of its pages are illegible, but the page with which this article deals is, to the instructed reader, fairly decipherable. It is the rocky slab of the carboniferous age. There is reason to believe that it was an age of shallow, inland seas, and marshy lagoons, and that the plants were of rapid growth, loose and succulent of texture. Of the five hundred odd species of plants that flourished, one half were ferns (acrogens). So familiar in this country are their present day representatives that description is unnecessary. Growing as a dense thicket at the waters edge or in bog-land, a reed-like plant (calamite) with jointed and finely ribbed stems, and whorls of leaves springing from each point (Fig. 1) raised itself fifty or more feet high.
A multitude of trunks darkened above by thick foliage and with each branch heavily fruited with spores mark the presence of the lycopods (Fig. 2). Their modern representatives (club mosses) are spread throughout the world as a low-growing plant, but in that age of luxuriance, like the calamites, they attained the height and bulk of an ordinary tree.
Another plant of a peculiar character had a prominent place in the carboniferous jungle. Such was its abundance that whole seams of coal appear to be entirely composed of its remains. It is the sigillaria. Its foliage probably resembled the ferns, but the trunks and roots alone have been preserved to us. (Fig. 3.) This strange tree flourished in swamps or in low ground and was remarkable for its elaborately carved and fluted stems. Each hollow or flute had its line of sculptury running down the centre. Even the roots were roughened with ornament. Other plants of the period which grew in the high tracts of the interior were stately cone-bearing pines which stood as giants of the hillside forests. Mutilated remains, devoid of root and top have been found seventy feet in length, and eighteen feet in circumference. It must not be supposed that these trees represent the entire flora of that time. Dissipation, decay and denuding agencies, have spared us only a small fragment of that rich and luxuriant growth. The experiments of Lindley have shown that different plants have different powers of resisting decay. After immersion in water for rather more than two years the mosses had disappeared and all the higher organised woody trees, such as oak and ash, had gone. Plants such as ferns, club-mosses and the pines, alone remained together.
The sameness and monotony of the flowerless vegetation of the coal measures has often been noted, but there was a beauty, not of shade or hue, but of form and ornamentation. According to their species the lycopods bore sculptured scales or lozenges set in a frame beautified by knobs and furrows—“Diaper work exquisite in design and finish.” (Fig. 4.)
If we take an ordinary piece of coal we see that it can be divided into two portions, one hard, shining and brittle, which produces the greatest amount of flame, and the other dull, black, friable, and resolvable into layers. The first is bituminous (or flame giving), and the second is anthracite. A piece of bituminous coal reduced to such thinness as to be transparent shows when examined microscopically that it consists of a series of minute bodies, more or less rounded and interlacing with one another. There are, too, some larger bodies also page 37 of a rounded shape, which resemble small bags pressed flat. The labours of scientists have demonstrated that the smaller bodies are seeds or spores and the larger bodies the cases that contained them.
Although coal bears only a faint impress of vegetable origin, it is a fact nevertheless that it is compressed and mineralised vegetation. It may well be asked, “How has such a transformation taken place?”
Geology acquaints us with the fact that the coal measures are an association of different kinds of rock. In the main we may say four different kinds. There are sandstones and shales, fireclay (or under-clay) and coal itself.
We must first note that the material, out of which rocks deposited under water have been formed, is carried down in two ways. When it is light and finely divided it can be held in suspension in the waters of the river that bears it along, but when coarse and heavy can be moved forward only by being forced along the bed of the river by the action of the water. The first makes the river muddy, and the second causes the grating sound so marked in times of flood. When the river enters the sea or lake its velocity is checked, but the matter held in suspension does not fall at once. Currents and tides in the sea and any velocity the river has been enabled to retain, spread the sediment over a large area. Should any interruption or pauses occur in the flow of material, each layer when deposited will have time to harden slightly before the next layer falls upon it. This causes a laminated structure. In this way the shales have been formed, for they are nothing but hard clay splitting up readily in thin parallel layers, and are always found deposited over wide areas.
The sandstones are formed of sand—or more correctly, quartz. Being a hard substance and not easily reduced to a finely divided state it cannot be carried as the materials for the shales. It is forced along the bottom of the river and on the checking of the river's velocity sinks rapidly and accumulates as a wedge shaped bank.
The underclay or fireclay, is thickly penetrated with roots and rootlets. It does not divide like the shales, but separates into irregular, lumpy masses, and is clearly the ancient soil.
Then for the coal itself. There is clear evidence that it accumulated in the place where the vegetation grew, and that that place was not far from the water's edge. It is spread in tolerably uniform seams of greater or lesser thickness over areas, embracing sometimes hundreds of square miles. The illustration (Fig. 6) shows how it is interbedded with sandstones and shales. The fireclay represents so many buried land surfaces. How is this succession of buried land surfaces and interbedding of sedimentary rocks to be accounted for? There is but one solution to the problem. The area over which the coal seam extended—the vast landlocked sea in which it was being formed—was slowly sinking. The rank and luxuriant flora of the carboniferous period has been noted in the previous article. For a space plants would spring up and flourish in maritime jungles and forests, and as they died would fall and gather in a layer of dead vegetable matter.page 38
After a time this layer would slowly sink beneath the waters of the sea. So slow and gentle would be the depression that the loose and pasty mass would not, as a rule, be disturbed. In the manner before described, sand, mud and silt would be transported from the neighbouring land to form strata of sandstone and shale. As the weight of sediment increased the vegetable matter would be gradually compressed and pass slowly into coal. A pause in the subsidence would allow the shallow sea to be silted up. Assisted perhaps by a slight elevation of the ocean bed, a new land surface would be formed. Marsh-loving plants would creep outward from the swampy shore to cover the tract with a mantle of growth. There would be a period of rapid development of the plants followed by a renewal of the downward movement, and resultant deposition of sedimentary rocks. This process would continue until the whole of our present coal measures were formed. A thick seam of coal probably marks a prolonged interval of rest when a luxuriant vegetation flourished unchecked, or the sinking was so imperceptible that generations of plants springing up on their predecessors managed to keep themselves above the water. As some of the coal beds are ten to twelve thousand feet in thickness an immeasurable period of time must be assigned for this sinking process. That it occurred in a remote age can be seen by the immense depth these beds lie buried in the earth's crust. Whole systems of strata have been formed above, entombing them beneath thousands of feet of superincumbent rock.
The calm of the summer sea, the music of the tranquil brook tell little of the fearful energies that underlie the calm. It is when Nature is roused that we recognise her terrific power. When the earthquake crumbles cities into dust, when the volcano belches forth its streams of liquid fire, and the tidal wave and hurricane hurl man and his proudest works to destruction — it is then that we learn of her hidden and mighty forces. By eruptive elevation, rocks that were originally level, have been pitched up upon their ends, or bent into arches and hollows or even inverted upon themselves to lie as a gigantic fold and expose their treasures of minerals at the surface of the globe. Not the least of these treasures is our coal.
To sum up the evidence of the origin of coal. Just as the bituminous portion owes its bitumen to resin stored up in the seed vessels of plants allied to the lycopods (and perhaps the ferns), so is the anthracite made up of woody fibre more or less decomposed. Both have been profoundly modified by the action of heat and pressure. In short, the resinous matter has been bitumenised, and the woody fibre mineralised. Sunlight and sunheat of distant ages, bottled up in the tissues of vegetables, and preserved in stony chambers for periods of vast duration—this is what coal is. Surely it has its romance.