C.—3

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so happen to the Carboniferous limestone, is respectively 4,974 ft., 5,325 ft., and 8,181 ft., taking into account a modification which it is advisable to make in.the chart of the Charleroi basin. Nevertheless, the aluminiferous ampelite of the Liege Province, the layer of phtanites flinty slates in Hainault, are marine sediments which it is necessary to deduct, and which bring down the preceding figures to about 4,926 ft., 5,160 ft., and 7,854 ft. In addition, let us notice, that if in the sections of the Liege basin the last small vein of coal disclosed is shown at 48ft. only of ampelite, yet in those of the Charleroi and Mons basins the Leopold seam on the one hand, and the Eenard below the Grand Bouillon or the Grand Eenom on the other, are respectively from 1,242 ft. to 1,962 ft. from the layer of phtanites. It is probable that upon the thick coal-rocks there exist beds, or at least small veins of coal, that may be discovered later on. Yet, deducting from the thickness of the coalformation also that of the strata comprised between the ampelites or phtanites and the recognised lower seam on either side, we arrive at 4,875 ft. for the Liege basin, 3,861 ft. for Charleroi, and 5,892 ft. for Mons. Such would be the minimum depressions since the formation of the oldest-known coalseam that permits of the application to the Belgian coalfield of the theory set forth by M. Briart. The importance of these subsidences is one of the difficulties stated in 1892 by M. A. de Lapparent as against this theory. Although only estimating at 3,927 ft. approximately the thickness of the Mons coal-formation, this geologist does not suppose such a depression to have been possible, because it would have caused the greater part of Europe to disappear under the waters of the sea. The same objection may, moreover, be made to all the theories of the formation on place ; for example, the ancient hypothesis of d'Elie de Beaumont, which supposes the existence of islands covered with rich vegetation sinking into the sea and being covered with sand, next emerging therefrom in order to produce afresh a rapid vegetation, and so on. This difficulty also applies, up to a certain point, to the recent theory of M. Grand-Eury, though this must not be placed in this category of hypotheses, transport there playing a preponderant part. Besides, we shall see further on how this objection can be met. Theory of M. L. Breton. Before examining the theory of M. C. Grand-Eury and that of M. H. Fayol, where the aid of transport only is called in, it is desirable to say a few words on the theory set forth in 1885 by M. L. Breton, a distinguished engineer, and well-known by his publications on the coal-formation of the Straits of Dover. Though subsequent to those of M. Grand-Eury and M. Fayol, the theory of M. L. Breton grants toturfage, taken in the wide sense that we have indicated, an important role. M. Breton's theory is characterized, as is that of M. Eayol, by not requiring successive depressions of the ground, and, in addition, by the Eolian origin of the earthy matters which enter into the constitution of the shaliy intercalations of the coal'seams, and which were the result of dusts, composed of sand and earth, brought by the winds. M. Breton specially had in view the formation of the great Franco-Belgian coal-basin. He supposes that after the Carboniferous limestone, which is a marine formation, had been deposited, there was produced in this a deep elongated basin, surpassing in length the distance from the Straits of Dover to Ehenish Prussia, and in breadth that of the Franco-Belgian coal-basin to-day. At the north and at the south of this immense basin chere rose arid slopes and denuded hills, formed in great part of Silurian and Devonian rocks. No vegetation afforded protection to these slopes, and, in consequence, they were easily turned with ravines by torrents, which brought with them debris, usually in the form of clayey or sandy mud, and fell into the lake following its periphery. At first the bottom of this deep depression contained salt water, in which the marine animals of the ampelite and of the flinty slates were for some time able to exist; but they were soon imbedded in the first sediments arising from the denudation of the hills. The marine salt was enclosed, partly in these rocks in process of formation, and partly in those which followed them ; as for the waters of the lake, in their deeper parts they were brackish, but, owing to the fresh water flowing into the lake down the sides of the declivities, they eventually filled up the vast depression completely, and became suitable for the development of a floating vegetation of Stigmaria. Stigmaria are rhizomes or root-stalks, and are very abundant in the wall of a great number of coal-seams. Many palaeontologists have supposed them to be the roots of Sigillaria or Lepidodendron, but, in spite of the laborious anatomical studies of M. B. Eenault, their functions are still little known. The labours of this scholar, which belong to the Natural History Museum of Paris, show them to be like floating rhizomes, having leaves and roots either together or separately, and in the form of tubulous appendages. M. Breton considers they were aquatic plants, complete and independent, of a very rapid growth, whose branches, bearing thin and radicle leaves, extended in all directions, forming under the surface of the water an inextricable network, occupying almost the whole extent of the lake. Then, in the interval of the heavy intermittent rains which denuded the hills, occurred the terrestrial dusts forced by the winds from the slopes of the valley. The dusts which fell on to the lake were in great measure arrested by the network of Stigmaria; they lodged in the interstices, increased the weight of the floating isle, which slightly sank, and ended by covering it with a veritable bed of mud—a suitable basis for the aerial vegetation, which, as in the theory of formation on place, would, by the intermediary transformation known by the generic name of turfage, form coal properly so called. With respect to the slates intercalated in the coal-seams, M. Breton's explanation is that there was a time when the floating peat-moss was encumbered with vegetation thrown down and in process of decomposition, and consequently the terrestrial dust could no longer pass through it. Wind and early decay also overthrew the greater number of the trunks on the upper part of the floating isle. A fresh supply of dusts, or rather the maceration and the compression of the mass, increased its average density; and, when this became greater than the density of the water, the turf-moss, which until then had only experienced slight downward tendencies, sank completely, and descended slowly to the bottom of the lake, over which it spread itself and partook of its form.