The coal-fields of the British Islands, as given by Dr. Ure,—from whom some of the foregoing facts are obtained, and who is generally very correct, —are more extensive than the areas we have adopted.
We give, however, below, an interesting table from his works, in which it will be observed there is considerable difference in the areas as given by us in Chapter V.
The area of the South Wales coal-field is less than our computation; but that of Ireland is far greater. The Irish coal-fields are not considered valuable or productive to a great extent, and mining operations are very limited, owing to the impurity and unproductiveness of the seams. We have given 250 square miles of available coal area out of more than 2000 square miles of coal formation; and this amount is in excess of that given by most of the late English authorities.
Our estimate of the coal resources of Great Britain is also in excess of the estimates of her modern engineers, though. we give only 6195 square miles of productive coal area, while Dr. Ure gives 8800. The addition, however, of 200,000 acres to the South Wales coal-field and the sub traction of 1,690,000 acres from the coal-fields of Ireland will reduce Dr. Ure's estimate to about the proportion we have adopted.
Estimates of the total amount of coal contained in a given area are by no means reliable : there are so many circumstances affecting the seams that no calculation, without a practical computation of the amount of coal in each seam, can be even an approximation to the truth. But few of the coal-seams underlie the whole area of any coal-field ; some of them are lost or become valueless in the deep basins, or in opposite directions, while dikes, faults, and other interruptions seriously depreciate the amount of available coal, and erosion or denudation affects it still more.
Much discussion has taken place recently in regard to the probable dura tion of the British coal-fields under the present rate of production, which is not likely to diminish, since it has steadily increased for the last 500 years or more. But even at a maximum of 100,000,000 tons per year, it is estimated, by good authority, that the supply of coal in the British Islands will be exhausted within 300 years. Mr. Hull, Sir William Arm strong, and Prof. H. D. Rogers, now of the University of Glasgow, Scot land, place the limit at 212 years ; while Mr. T. Y. Hall, an eminent mining engineer of the north of England, and Mr. Greenwell, a geologist of the same district, estimate the duration of the Great Northern coal-field at 256 years, under a production of 20,000,000 tons per annum, which is less than the present production.
No coal-producing country is so thoroughly developed as England, therefore we must adopt the later estimates as the most correct, though largely at variance with the eminent authorities from which our tables, in Chapter V. were compiled. But, it will be observed, in those tables we simply give the total coal areas, total thickness, and total assumed amount of coal, from the data thus furnished, without deductions for dikes, faults, waste, pillars, &c.; and, moreover, we calculated the pro duction of all seams, from one foot up, instead of three feet, which the gentlemen above quoted take as a mini mum workable thickness. We think, and there can be no doubt of it, when coal becomes scarce and high priced in England, her engineers will find a mode to work many a small or abandoned seam which at pre sent will not pay.