In 1773 there were only 13 collieries on the Tyne, and these had increased to upwards of 30 in 1800. The number of collieries in 1828 had increased to 41 on the Tyne, and 18 on the Wear, in all 59, producing 5,887,552 tons of coal. The out-put of coal in Northumberland and Durham in 1854 was no less than 15,420,615 tons, and now in these two counties there are 283 collieries. Mining began on the Tyne and continued on the Wear, where the industry has been largely developed. There are in all about 57 different seams in the Great Northern coal-field, varying in thickness from 1 inch to 5 feet 5 inches and 6 feet, and these seams comprise an aggregate of nearly 76 feet of coal. Taking the area of this field to be 750 square miles—a most probable estimate—we may classify the contents as household coal, steam coal, or those employed in steam-engine boilers, and coking coal, employed for making coke and gas. Of household coal there is only 96 square miles out of the total 750, all the remainder being steam or coking and gas coal. The greater part even of this 96 square miles has been worked out on the Tyne, and the supply is rapidly decreasing also on the Wear, where the largest bulk of the household coal lies. The collieries of the Tees possess but six square miles out of the 96, as far as we at present know. Turning, however, to that part of the coal-field regarded as precarious, and consisting of first, second, and third-rate household coal, we have for future use 300 square miles. London was formerly supplied from the pits east of Tyne Bridge, where is the famous Wallsend Colliery, which gave the name to the best coal. That mine is now drowned out, and, like the great Roman Wall, at the termination of which it was sunk, and from which it derived its name, is now an antiquity. There is now no Wallsend coal, and the principal part of the present so-called coal comes from the Wear, but the seam which supplied that famous pit is continued into Durham, and that seam, or its equivalent, sends a million or two of tons every year into London. The supply, however, in this district is rapidly decreasing. Careful calculations have been made as to the probable duration of this coal, of which the following is a summary. The workable quantity of coal remaining in the ten principal seams of this coal-field is estimated at 1,876,848,756 Newcastle chaldrons (each 35 cwt.). Deducting losses and underground and surface waste, the total merchantable round or good-sized coal will be 1,251,232,507 Newcastle chaldrons. Proceeding on this estimate, formed by Mr. Grunwith in 1846, we may arrive at the probable duration of the supplies: taking the future annual average of coal raised from these seams to be 10,000,000 of tons—and this is under the present rate—the whole will be exhausted in 331 years. A still later estimate was made by Mr. T.G. Hall in 1854, and he reckoned the quantity of coal left for future use at 5,121,888,956 tons; dividing this by 14,000,000 of tons as the annual consumption, the result would be 365 years; and should the annual demand arrive at 20,000,000 of tons, the future supply of this famous coal-field would continue for 256 years. The total available coal (1871) in the British coal-fields, at depths not exceeding 4000 feet, and in seams not less than 1 foot thick, is 90,207,285,398 tons, and taking into account seams which may yet become available, lying under the Permian, New Red Sandstone, and other superincumbent strata, this estimate is increased to 146,480,000,000 of tons. This quantity, at the present annual rate of production throughout the country—namely, 123,500,000 tons—would last 1186 years. Other estimates of various kinds relative to our coal supply have been put forth: some have asserted that, owing to increasing population and increasing consumption in manufactures, it will be exhausted in 100 years, and between this extreme and that of 1186 years there are many other conjectures and estimates.
In the United States there are about 120,000 square miles underlaid by known workable coal-beds, besides what yet remains to be discovered; while on the cliffs of Nova Scotia the coal-seams can be seen one over the other for many hundred feet, and showing how the coal was originally formed. With this immense stock of fuel in the cellars of the earth, it seems evident that we need not trouble our minds or be anxious as to the duration of our coal supply. Besides, the conversion of vegetable matter into coal seems to be going on even now. In the United States there are peat-bogs of considerable extent, in which a substance exactly resembling cannel coal has been found; and in some of the Irish peat-beds, as also in the North of Scotland, a similar substance has been discovered, of a very inflammable nature, resembling coal.
Yes! what could have produced this singular-looking, black, inflammable rock? How many times was this question asked before Science could return an answer? This she can now do with confidence. Coal was once growing vegetable matter. On the surface of the shale, immediately above the coal, you will find innumerable impressions of leaves and branches, as perfect as artist ever drew. But how could this vegetable matter ever accumulate in such masses as to make beds of coal of such vast extent, some not less than 30 feet thick? It would take 10 or 12 feet of green vegetable matter to make 1 foot of solid coal. Let us transport ourselves to the carboniferous times, and see the condition of the earth, and this may assist us to answer the question. Stand on this rocky eminence and behold that sea of verdure, whose gigantic waves roll in the greenest of billows to the verge of the horizon—that is a carboniferous forest. Mark that steamy cloud floating over it, an indication of the great evaporation constantly proceeding. The scent of the morning air is like that of a greenhouse; and well it may be, for the land of the globe is a mighty hothouse—the crust of the earth is still thin, and its internal heat makes a tropical climate everywhere, unchecked by winter's cold, thus forcing plants to a most luxurious growth.
Descend, and let us wander through this forest and examine it more closely. What strange trees are here! No oaks, no elms, or ash, or chestnut—no trees that we ever saw before. It looks as if the plants of a boggy meadow had shot up in a single night to a height of 60 or 70 feet, and we were walking among the stalks—a gigantic meadow of ferns, reeds, grasses, and club-mosses. A million columns rise, so thick at the top that they make twilight at mid-day, and their trunks are so close together we can scarcely edge our way between them, whilst the ground is carpeted with trailing plants completely interwoven. What strange trees they are! Beneath us lies an accumulation of vegetable matter more than 200 feet in thickness—the result of the growth and decay of plants in this swamp for centuries. All things are here favourable for the growth of vegetation—the great heat of the ground causes water to rise rapidly in vapour, and this again descends in showers, supplying the plants with moisture continuously. The air contains a large proportion of carbonic acid gas, poison to animals but food to plants, which, by means of its aid, build up their woody structure. Winds at times level these gigantic plants, for their hold on the earth is feeble, and thus the mass goes on increasing.
We are now on the edge of a lake abounding with fish, whose bony scales glitter in the water as they pursue their prey. Lying along the shore are shells cast up by the waves, and there are also seen the tracks of some large animals. How like the impression of a man's hand some of these tracks are! The hind-feet are evidently much larger than the fore-feet. There is the frog-like animal which made them, and what a size! It must be six feet long, and its head looks like that of a crocodile, for its jaws are furnished with formidable rows of long, strong, sharp, conical teeth.
The continued growth and decomposition of the vegetation during long ages must have produced beds like the peat-deposits of America and Great Britain. In the Dismal Swamp of Virginia there is said to be a mass of vegetable matter 40 feet in thickness, and on the banks of the Shannon in Ireland is a peat-bog 3 miles broad and 50 feet deep. When conditions were so much more favourable for these deposits, beds 400 feet in thickness may easily have been produced. This accumulated mass of vegetable matter must be buried, however, before we can have a coal-bed. How was this accomplished? The very weight of it may have caused the crust of the earth to sink, forming a basin into which rivers, sweeping down from the surrounding higher country, and carrying down mud in their waters, the weight of which, deposited upon the vegetable matter, pressed and squeezed it into half its original compass. Sand carried down subsequently in a similar manner, and deposited upon the mud, pressed it into shale, and the vegetable matter, still more reduced in volume by this additional pressure, is prepared for its final conversion into shale. In time the basin becomes shallow from the decomposition of sediment on its bottom, and then we have another marsh with its myriad plants; another accumulation of vegetable matter takes place, which by similar processes is also buried. Where thirty or forty seams of coal have been found one below another, we have evidence of land and water thus changing places many times.
When vegetable matter is excluded from air and under great pressure, it decomposes slowly, parting with carbonic acid gas; and is first changed into lignite or brown coal, and then into bituminous coal, or the soft coal that burns with smoke and flame. I have been in a coal-mine where the carbonic acid gas, pouring from a crevice in the coal, put out a lighted candle. The high temperature to which the coal has been subjected when buried at great depths has also probably assisted in producing this change; and where that temperature has been very high, the coal by the influence of the heat having parted with its inflammable gases, we have the hard or anthracite coal, which burns with little or no flame and without smoke. It is indeed coal made into coke under tremendous pressure, and this is the kind of coal which Americans use exclusively in their dwelling-houses and monster hotels.
It was at first supposed that the plants of the carboniferous times were bamboos, palms, and gigantic cactuses, such as are now found in tropical regions, but a more careful examination of them shows that, with the exception of the tree-fern now found in the tropics, they differ from all existing trees. A large proportion of the plants of the coal-measures were ferns, some authorities say one-half. From their great abundance we may infer the great heat and moisture of the atmosphere at the time when they grew, as similar ferns at the present day are only found in the greatest abundance on small tropical islands where the temperature is high. Coal often contains impressions of fern leaves and palm-like ferns—no less than 934 kinds are drawn and described by geologists. Many animals and insects are found in the coal, such as large toad-like reptiles with beautiful teeth, small lizards, water lizards, great fish with tremendous jaws, many insects of the grasshopper tribe, but none of these are of the same species as those found now living on this globe.
Wood, peat, brown coal, jet, and true coal, are chemically alike, differing only in their amount of oxygen, due to the difference of compression to which they were subjected. The sun gave his heat and light to the forests now turned into coal, and when we burn it ages afterwards, we revive some of the heat and light so long untouched. Stephenson once remarked to Sir Robert Peel, as they stood watching a passing train: "There goes the sunshine of former ages!"
COST OF WORKING.