Page:A Treatise on Geology, volume 1.djvu/197

 CHAP. VI. coal fields of Ashby de la Zouch, Coventry, Dudley, and Colebrook Dale, are very valuable: some smaller fields are known south of Shrewsbury, in the Clee Hills, and at Newent. The Forest of Dean is a rich though small tract, and the disunited patches of coal in Kingswood, and south of the Bath Avon, are valuable. Almost the largest coal field in Great Britain is the great oval elongated tract of South Wales, from Pontypool to St. Bride's Bay, which furnishes fuel to the great iron works of Merthyr, Tredegar, Neath, &c. Murchison and Sedgwick show the culm of Devonshire (Bideford, &c.) to be in a deposit of the same Age as the culm of Swansea and other parts of South Wales, which is known to belong to the true coal formation.

When we recollect, that, in addition to this large expansion of rich coal tracts, in most of which 50 feet of coal (in many beds) exist, the millstone grit and mountain limestone tracts, north of Derbyshire, also yield some coal, it is easy to see that the popular opinion of the extraordinary abundance of coal in Great Britain is perfectly well founded. But does it follow that the supply of British coal is inexhaustible? will it last for one thousand or five hundred years, and during that period meet the hourly enlarging consumption at home, and the augmenting demands from abroad? This question has often been replied to, never answered. Nor have the replies been often dictated by a comprehensive view of the subject. If indeed the only data required were the superficial area of a coat tract, and the sum of the thickness of the several coal beds, nothing could be more easy than to convert this into a term of years, by assuming some fixed or regularly varying rate of annual consumption. But to this it must be objected, that all the coal in a given district cannot be worked, in consequence of natural impediments (thinness, bad quality, disturbed position, &c.), and of the wasteful and unscientific method of establishing coal works. It is not here meant to speak otherwise than with praise of the working of the collieries,