COAL-SEAMS.
There are 57 seams of coal in the Great Northern field, from one to six feet in thickness, averaging 75 feet of coal; of these, ten are workable seams, from 30 inches to six feet thick, with an average working thickness of 40 feet; which leaves 35 feet of coal in seams not now considered un workable, ranging from one to two and two and a half feet in thickness; a fact which it might be well to remember in any consideration of the duration of the British coal-fields.
The working seams are locally named, and some confusion exists in identifying them in different portions of the field. Those generally accepted in the Newcastle District are the High Main, Five-Quarters, Main Coal, Bensham, Hutton, Beaumont, Stone-Coal, Low Five-Quarters, Yard, or Three-Quarters, and the Brockwell.
We give a section of the High Main and Low Main in their most favorable condition. But accompanying will be found the variations of these seams at different localities, which will apply generally to all the other seams, since they change in about the same proportion.
But to this may be added 25 feet of coal existing in seams from one foot to two feet, or from 12 to 28 inches, in thickness, which are not now con sidered workable, but which will be considered valuable long before the British coal-fields are exhausted.
Assuming only half the entire area of the Great Northern coal-field to be underlaid by those small seams, or but half the area productive, they would still yield 20,000,000 tons per annum for five hundred years.
It may be a question to many of our readers, whether seams as thin as 12 inches can be worked at all, and much less to profit. We will here
present some facts from the actual workings in the East Somerset coal field, where the seams are generally thin. Near Bath, in this coal-field, are seven seams whose aggregate thickness is 12 feet,—three of them from 12 to 16 inches, and four from 24 to 28 inches. They are worked ex tensively on the "long wall" system, at the following items of expense.
It will be observed by the above table, which is from an interesting paper by G. C. Greenwell, in Vol. IV., North of England Institute of Mining Engineers, that the average cost of mining coal in the thin veins of Somerset is less than one dollar per ton, and not much, if any thing, over the cost of mining coal in the great anthracite veins of Pennsylvania.
We may also compare it with the cost of mining coal in the Newcastle district, as given by the same author, from the average production of the Five-Quarters, Low Main, and Hutton seams.
These charges do not appear to include machinery or cost outside, but the simple mining charges, or inside work.
The quantity of coal mined in East Somerset during 1855 was about 400,000 tons, or 150 tons to the hand for under-ground work. In the Newcastle district the amount of coal mined per head in 1854 was about 494 tons for under-ground work.
The amount of coal actually realized from an area containing by calcu lation 122,082 tons, was 108,703 tons, leaving only 13,379 tons in the mine as waste or fine coal, dirt, and pillars,—or only 10 per cent. of the whole.