SMITH'S SUPERPOSITIONS OF ENGLISH STRATA. 153 The second the coal measures forming a part of it. which comprehends the most ancient of the secondary limestones, is divided into two principal layers. The third is a newer sandstone called the spotted or speckled sandstone, or sandstone with clay. And the last is a limestone enclosing many shells, and called in consequence the shell limestone (muschelkalk). Mr. William Smith was the first geologist who showed, in his admirable researches and map, that England is regularly distributed into strata, that the order of superposition is never inverted, and that exactly similar fossils are found in all the portions of the same stratum, and at great distances asunder. The following is the order given by Mr. Smith. On the primitive territory which forms the western portion of England, there exists a red or brown sandstone, which appears to be a greywacke, above which lies the encrinal or mountain limestone, which some have described among the intermediate class. We find after this, progressively as we advance towards the east, a succession of great beds which dip towards that point of the horizon. These beds are: 1. The coal formation or coal measures; 2. Yellowish magnesian limestone ; 3. Marl, and red sandstone, gypsum, rock-salt; 4. Argillo-bituminous limestone (Lias), containing many am monites. 5. Blue marl with belemnites, gryphites; 6. Oolitic limestone; 7. Compact limestone with schistose clay; 8. White and sandy limestone-a thin layer; 9. Schistose clay of a deep blue colour; calcareous and bitu 'minous; 10. Ferruginous sand, containing calcareous masses, fuller's earth, clay; 11. Limestone with remains of madrepores and pisolites; 12. Bluish marl; 13. Green sand, often chloritic sandstone with calcareous Schists. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Sectional view of the series of strata from Snowdon to London. 1. Old red sandstone; 2. Mountain limestone; 3. Coal measures; 4. Magnesian limestone; 5. Red marl; 6. Lias; 7. Oolite; 8. Limestone; 9. Clay; 10. Sand; 11. Lime marl; 12. Green sand; 13. Chalk; 14. Sand; 15. Clay; 16. London clay. From the narrow bounds of the figure, strata 4, 5, &c. are placed in an inclined, instead of a nearly horizontal position. An analogous suite of strata has been observed in France, and other parts of the continent; which, if not entirely the same, are at least equivalent formations; that is, parallel and congruous. Before entering into the details of the several strata, we may take a general view of their relations. The red sandstone passes by an uninterrupted continuity into the greywacke; it is the same substance in every respect, only the bed alternating with the clay-slate is regarded as belonging to the intermediate class. The coal-sandstone is linked also with other rocks, which extend even to the primi OF THE COAL MEASURES IN GENERAL. 155 tive formations, of the eruptive kind, namely, granite and porphyry, as at Thuringerwald. The connexion of the intermediate limestone, with the coal measures, is in like manner certain. On quitting the district of Newcastle, which presents the coal formation in its utmost purity, and advancing towards Derbyshire, we observe first of all some beds of encrinitic or mountain limestone, placed between the sandstone strata, which increase in number, and eventually become the predominant rock. It appears, therefore, that in the succession of mineral strata there is such a concatenation, that on departing from one line, it is difficult to point out any broad plane of demarcation, either above or below; and we are led by the most striking relations to consider certain masses as forming but a single suite, which viewed separately, and at a great distance, are perfectly distinct. The coal measures form most regular beds of stratification, with peculiar inflections worthy of attention. A very large portion of this formation is deposited in valleys, at the foot, and along the flanks of primitive mountains; it is moulded on their bottoms and sides, and partakes of all their inequalities; thence arises the contorted shape of certain strata of coal; thence their direction parallel to that of the sides of the basin which encloses them, and determines their sinuosities; thence their great inclination against the precipitous flanks of a valley, their gentler slope towards the middle, where they become horizontal, and their renewed ascent and inclination on the other side of the valley. Geological observation proves in general that these strata are now in the same situation, as at the period of their deposition on the primitive rocks; and consequently that neither eruptions nor revolutions have set them up in the inclined posture which they exhibit in very many places. When they are greatly inclined, they become gradually thinner transversely towards the upper edge of the basin, and thicker towards its bottom, an effect analogous to what would happen to materials deposited on the sloping surfaces of planes lying more or less oblique to the horizon. To this statement, there are no doubt many exceptions; for it is well ascertained that several beds have been raised up subsequently to their deposition and consolidation. We may mention the beds of Anzin, near Valenciennes, so singularly folded backwards and forwards. To form an idea of their derangement let us conceive a stratum dipping towards the south at an angle of 75°; that at a certain depth, two hundred yards for example, it turns suddenly back, rising towards the north at an angle of 15° with the horizon; after proceeding in this direction for nearly 600 yards, it is folded back again, so as to dip southwards anew at an angle of 75°. It thus presents nearly the figure N. Let us now imagine a great number of strata of coal, sandstone, and shale (slate-clay), having all this shape, incased one within another, and thus forming an enormous bale, half a league broad, and several leagues long, and we shall have a pretty exact idea of the system of beds in which lie the valuable coal-mines of Anzin. In these convolutions the strata of coal are frequently broken, their tabular masses are intermixed with those of the rock, but SURPRISING CONTORTIONS OF COAL STRATA. 157 'sometimes also the foldings are rounded off, with-out any appearance of rupture. A bed nearly two feet thick was observed by M. D'Aubuisson, folded into a very regular curvature, forming an arc of about 100°, with a radius of 4 yards; all the tables of the coal followed this curvature, preserving a perfect parallelism with the contour, and the same thing was manifest in the mineral strata of the roof and the floor.* The collieries of the environs of Mons, and of several other places, present foldings of the same kind. What may be the cause of such extensive, extraordinary, and we might even say grotesque convolutions? We may remark, in the first place, that the stratum of slaty clay, or sandstone which forms the roof of a bed of coal in one of its portions, serves as a floor a little further on; and that consequently, had the foldings been of original formation, the stratum of sandstone deposited immediately after that of coal, should have been placed there partly above and partly below, an absolutely impossible event, especially when the deposition is mechanical like sandstone. As no change of position in the coal measures taken in their totality could exhibit a case, where the bed of sandstone would have been deposited entirely above the stratum of coal, we must necessarily admit that the form of the beds is owing to a mechanical cause which has acted posteriorly to their formation, but before their complete consolidation, since the beds present in several places curvatures perfectly rounded off without any fracture. Contortions analogous to the above are displayed • Traité de Geogrosie, Tom. II. p. 285. |