Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/70

56 COAL [COAL-FIELDS. 3. Forge coal series, 29 seams. These are chiefly used for smithy purposes and iron works, but the lower mem bers approximate to dry steam coals. 4. Dry or lean coals, 20 to 25 seams, forming the bot tom series. They are of small value, being chiefly used for brick or lime burning. The amount of compression to which, the strata have been subjected in these coal-fields, has caused them to be sharply contorted into zig-zag folds. In the neighbour hood of Mons a single seam may be passed through six times in a pit of 350 yards vertical depth, and the strata, which if flat would be 9 miles broad, are squeezed into a space 7 miles across and about 8200 feet deep to the bottom of the basin. At Charleroi the compression is still greater, a breadth of 8| miles of flat strata being nar rowed to rather less than half that quantity by contortion into 22 zig-zag folds. The thickness of the overlying Tertiary and Cretaceous strata in the neighbourhood of Moris is from 500 to 900 feet; towards the French frontier the thickness is between 200 and 400 feet, and at Valenciennes about 250 feet. At Aniche these overlying measures, or terrains morts, are 400 feet thick, below which the coal measures are found to contain 23 feet of coal in 12 seams. At Anzin, near Denain, there are 18 seams, together 39 feet, which, is about the maximum development in the north of France. This coal-field, which was unknown before 1734, has reached a very high state of production in spite of great difficulties interposed by the water bearing strata covering the coal measures. It extends for about 45 miles, dimi nishing in extent and value to the westward. The struc ture is very similar to that of the Belgian, one of the most remarkable features being the inclined fault called the cran de retour, which brings the lower or dry coal series of the north side against the higher coking coals of the south side, as shown in the section, Plate II. fig. 4. At Hardinghen, near Boulogne, a small patch of disturbed coal strata was formerly worked. These are now supposed to be of the age of the Carboniferous limestone. The coal-fields of central and southern France are mostly small in area and irregular in structure, with at times remarkable single accumulations of coal of enormous thick ness, which do not, however, extend for any distance. The most important basin is that of Saint Etienne and Rive de Gier, south of Lyons, on the right bank of the Rhone. It is of triangular form, about 28 miles long, with a base of 8 miles. The thickness of the three principal seams at the latter place is about 33 feet, but at Saint Etienne there are from 15 to 18 seams, making together about 112 feet in a total depth of measures of about 2500 fe.et. The basin of the Saone et Loire, near Chalons and Autun, is about 25 miles long in a S.W. and N.E. line. At Creusot, on the north crop, the coals, which are in places extremely thick (the main seam averaging 40 feet, but occa sionally swelling out to 130 feet), dip at a high angle below a covering of New Red Sandstone strata, and appear in a modi fied form,bothas regards thickness and position, on the south side at Blanzy. An attempt has been made to prove the continuity of the series in the bottom of the basin by a deep boring, which was, however, abandoned at a depth of over 3000 feet without passing through the overlying strata. At Moutchanin a remarkable seam or mass of coal was found extending for about 650 yards, with a thickness varying from 60 to 200 feet at the surface, which, however, df- minished to one half 60 yards down, and wedged out at 1 40 yards deep. Another coal field of considerable im portance is that of Alais and Grand Combe near Nimes, which is partly covered by Liassic strata, and has a total maximum thickness of 80 feet of coal. In addition to these must be mentioned the anthracitic series of the Alps, which extend along the flanks of that chain from Savoy and the Tarentaise into Styria and Carinthia. They are of small economic importance. The Secondary and Tertiary coals of France are of com paratively small importance. Lignite is worked, among other places, near Dax in the Pyrenees, and at Trets and Fuveau near Marseilles. The coal-fields of Prussia, situated on the extension of Germ; the Franco-Belgian axis, are the two small basins of the Inde and Worm, east of Adelnau, near Stolberg and Esch- vveiler, which are included in single sharply sloped folds of the mountain limestone, and the great Westphalian basin east of the Rhine, in the valley of the Ruhr. The latter, which is one of the most important in Europe, extends for about 30 miles east and west from Essen to Dortmund. The breadth is unknown; the beds are exposed for about 15 miles at the broadest part, but the actual boundaries to the north and north-east are hidden by Creta- taceous rocks. The greatest depth from the surface to the bottom of the basin is probably about 5000 feet. It is divided lengthways by transverse axes of elevation into four principal basins, besides several smaller ones. The total thickness of measures already proved is from 6000 to 8000 feet, with about 130 seams of coal, together about 300 feet thick. These are divided into three series by two bands of barren measures. The thickness of the individual coal seams varies from 8 inches to 7 feet. Seventy -six are considered to be workable, having a combined thickness of 205 feet, and 54 are unworkable, containing 42 feet of coal. The proportion of workable coal to the whole thickness of strata is as 1 to 33. The order of succession as regards quality is similar to that observed in Belgium, the most highly valued gas and coking coals being at the top of the series, and the dry semi-anthracitic seams at the bottom. On the south side of the axis of the Rhenish De vonian strata, which is the high ground known as the Eifel and Hunsruck, carboniferous strata reappear in what is known as the Pfalz-Saarbriicken basin, occupying a rect angular area between Bingen, Donnersberg, Saarbriicken, and Mettlach, about 60 miles long and 20 miles broad, the productive coal measures being restricted to a triangular space of about 175 square miles in the S.W. corner. The Carboniferous limestone is absent, but the thickness of the coal measures is very great, the upper or Ottweiler series measuring from 6500 to 11,700 feet, with about 20 feet of coal in different parts of the district, and the lower or Saarbriicken series from 9000 to 5200 feet, with 82 workable and 142 unworkable coal seams, making a total of about 350 to 400 feet of coal. The greatest thickness of the upper strata is found in those localities where the lower are thinnest, but the total thickness is computed to be about 20,000 feet in the thickest known section. The coals of the lower division are divided into groups by certain well- marked horizons, usually prominent seams, which have this peculiarity that the best coking and gas coals are found ir?, the bottom of the series, and the drier ones at the top, thus reversing the order observed in the basins on the northern slope. The amount of hygroscopic water in the coal is also found to diminish downwards. In the district between the Ems and the Weser, are situated the small coalfields of Ibbenbiiren, on the easterly extension of the Westphalian basin, and the Piesberg, near Osnabriick, which are of true Carboniferous age. Besides these, there is a curious development of coal in the Weal- den strata which extend in a narrow discontinuous band E. andW. for about 150 miles. The coals are or have been worked at Tecklenburg and Borgloh in the Teutoburger Wald, at Biickeburg in Schaumburg, and in the Osterwald south of Hanover. The coal seams are small and of infe

rior quality, but are interesting as showing how nearly the