intercalated slate, and by the continuous barres which divide the beds into several courses. Finally, the coals themselves often present a great number of interruptions, and of veins which render its structure striped, lamellar, and following the direction of the stratification.

The stratification of coal ought not, however, to be considered as absolute, and to be compared to that of the calcareous or argillaceous beds of the sedimentary formations, nor even to that of the sandstones and shales which alternate with them. Certain beds present massive undulated forms, yet without these undulations having been occasioned by the course or strike of the formation. This shows that the origin of the coal permits, at one and the same time, thin layers, continuous and of the greatest regularity, and thick beds, so limited and irregular, that they may be assimilated to masses. The ulterior details, respecting the forms and courses of the principal coal deposits, will decide perfectly our ideas in this respect. The thin and regular beds, although very much disturbed, of the basins of the north of France. and Belgium, and the thick and limited beds of Montchanin represent the two extremes of position.

The number of coal beds, in the same formation, as well as their thickness and continuity, appear to be subject to very great variations. Nevertheless, there is a certain connection between these different conditions. The thin and regular beds are commonly continuous and multiplied; the thick and unequal masses are, on the contrary, limited in their extent, and there are rarely more than two or three superposed in the formation which encloses them. Thus, in the basin of Mons, in Belgium, more than one hundred distinct seams of coal are counted, whose ordinary thickness varies from eight inches to near five feet. In the collieries of the department of the North, in France, there are few centres of exploitation which do not count six, eight, twelve, or more beds of coal; but their maximum thickness does not exceed three feet, and the greater part of those which are worked have only about twenty inches.

Although coal beds have frequently been traced along a distance of many miles, yet we ought not, even in the case of very great regularity, to suppose that certain beds of coal are absolutely coextensive with the whole formation. For example, there is an interruption between Valenciennes and the Belgian frontier, to such an extent that the beds of Anzin are not those of Mons, and these, again, have no relation of continuity with the beds of Liege or Charleroy. The same remarks can be applied to other coal basins. Thus, in the basin of the Loire, the beds of Rive-de-Gier, are not the same as those of Saint-Etienne. We may, then, in a basin of some extent, consider the coal as forming, within the beds of sandstones and shales, special districts; often isolated, the one from the other, by sterile portions, and of which the coal beds, differing in number and power, have no relation of continuity.

Although, then, even if we have discovered the sandstones and the shales of the coal formation, it does not follow that we have also found the coal, though we were on the prolongation, in direction or inclination, of known beds. To form a probable hypothesis on this subject, it would be necessary first to study the peculiar conditions of the formation on which to operate, and calculate, from the known portions, the chances that we may have.

The strata of the basin of the Saône-et-Loire appear to form basins subordinate to the principal basin, which is filled up with coal, sandstones and schists. These subordinate basins are bounded like the basin which contains them, and have, besides, nearly similar proportions between the axes.

Further, the coal appears there to diminish in length in proportion as it acquires thickness. In the valley of Creuzot, the great bed which is worked has twelve metres, or forty feet, of mean thickness. In the enlarged portions it has 40 metres, or 130 feet from wall to roof. In direction it is not prolonged above 2000 yards; and, with regard to its limits, its divided extremities, there impoverished, present all the symptoms of a total suppression.

The bed of Montchanin, greatly inclined, whose thickness attains even to 70 metres, or nearly 230 feet, measured from roof to floor, representing, consequently, the thickest known coal bed, is equally one of the most limited in extension. In fact, in the upper part of the exploitation, this direction is about 650 yards, at the end of which the bed terminates abruptly, and is confused or entangled in the rocks of the roof and floor. At a lower stage, about thirty yards below the first, the length is reduced to 450 yards, and it is probable that at the depth of about 150 yards the bed will terminate.

The basin of the Loire contains, in the region of Rive-de-Gier, but three beds, of which the average united thickness does not exceed 32 feet; but, in the district of Saint-Etienne, the sum of the regular beds amounts to 114 feet, in fifteen to eighteen beds. At Brassac they amount to from 27 to 40 feet; 45 feet at Comentry and Doyet; and 48 to 65 feet in the basin of Aubin. It is remarkable that, in all these basins, the coal beds, of 15 to 30 feet, are occasionally reduced, by contractions, to 6 or 10 feet, and at times are swelled out to the thickness of 60 to 90 feet, an ordinary and normal fact.

In the department of the North, on the contrary, 30 feet of total thickness are divided into 14 beds, worked at Fresne and Vieux-Condé. The 12 beds of Aniche only form 22 feet; four successive beds at Douchy have only 11 feet aggregate; at Denin 7 feet only; and 38 feet are occupied by not less than eighteen beds at Anzin. There are also still more veins, which are unworked, and whose thickness is below one foot each. But these beds are regular and prolonged, and are not disturbed by those enlargements and entanglements so frequent in the beds of the southern basins.

This difference of power and continuity in the coal beds agrees also with some very important differences indicated by geological observation.

The southern basins of France appear to have been deposited, during the coal period, in isolated lakes of fresh water; encircled, and entirely commanded by the neighboring summits, from whence the materials have often been drifted with violence: forming breccias and conglomerates. In studying this debris, especially in the lower parts of the deposit, we can recognize the transition rocks of the surrounding countries. The northern basin of Belgium and France, containing at its base the carboniferous limestone, is, on the contrary, only composed of sandstones and fine schists. It appears, from the character of the fossils, to have been formed in marine waters, and thus represents, with the coal basins of England, the pelagic accumulations of an epoch, of which the basins of the south are but the lacustrine terminations. It is, then, natural to discover, in these northern deposits, a regular and continuous disposition which comports not with the deposits of the south.

The southern basins, deposited in isolated lakes of fresh water, form the principal riches of France. The aggregate thickness of the coal is, besides, nearly as great as in the basins of the north. To sum up all, we can lay down no absolute rule for the number and power of the beds of coal, any more than for their continuity. The indices, which result from the direction of the stratification, have, nevertheless, a real value, even in the coun

tries where the continuity presents the most frequent exceptions; because they always conduct to the possibility of finding, if not the prolongation of the beds, at least to formations analogous to such as have been already discovered.

ACCIDENTS, FAULTS, AND IRREGULARITIES OF COAL BEDS.

The beds of coal are rarely in the position where they have been produced, for that position would approach sensibly to the horizontal; a condition compelled, if not by the mode of production of the coal itself, at least by that of the beds of sandstone and slates between which it is stratified. Most frequently, the uniformity of the formation is disturbed, not only by inclinations, more or less great, but also by the folds which change these inclinations, and distort the beds to such an extent that a vertical shaft might cut them several times. There often, also, prevails one or several systems of faults [failles] which change the levels and isolate, one from the other, the divers parts of a bed.

This accidentation, subsequent to the production of the beds, and resulting from dynamic perturbations, commonly influenced by determinable conditions of direction, must be distinguished from the contemporaneous accidents inherent even to the production of the coal; such as the undulations of the roof and floor, which swell or contract a bed, and the intercalations of layers or of amygdaloid rocks, which interrupt the regular order of the stratification. Nevertheless, there is an evident connection between these two causes of irregularities; because the dynamic perturbations appear to have sometimes operated upon the coal beds before they were solidified; or, at least, when they were in such a state, that they were capable of being compressed, squeezed, and even completely suppressed, by a compression between the rocks of the walls, and, consequently, enlarged at other points.

The distorted structure, often smooth and polished, of the shales which accompany the coal, thus troubled; the state of the coal itself, which is there not only more crushed than at other places, but sometimes twisted or contorted and in a manner kneaded, seem to confirm the existence of these almost contemporaneous perturbations.

We may, besides, by attentive observations, frequently distinguish the dynamic and violent perturbations of such as result even from the circumstances of the deposit. The regular seams [nerfs] of shale and their beds or bands [barres] of clay, which are almost always interposed in beds of coal, following the direction of the stratification, can furnish many indices in this respect. Thus, in a natural expansion, not only the seams (nerfs) and bands which exist, do not experience perturbations, but they add other parallels to the increasing thickness of the coal. A natural contraction is often produced by the dilatation of the "barres," and, at other times, the barres submit gradually, like the coal itself, to the influence of diminution. In the dynamic accidents, on the contrary, the "nerfs" and barres are broken suddenly, and their fragments, blended with the coal, announce, in advance, to the miner the accident which comes to modify the "allure" of the bed.

The accidents to which coal beds are subjected, are those of the inclination, the folds, the faults, the displacements and the disturbances.

Inclination is the most general casualty: it is rarely, in fact, that the seams are presented in a horizontal position. These inclinations are evidently the result of perturbations, of upheavings or of sinkings of the earth, subsequently to the deposit of the formation.

The direction of all the beds is commonly the same in a coal basin, but their inclination varies. Thus, it is remarked that, upon the opposite borders of a basin, the slopes were most frequently directed towards each other; and it has been proved that sometimes there was a junction of the two slopes in the middle of the basin by a plane or curved portion which has been called the bottom of the boat-[" fond de batteau,"] because, in fact, the section of the two slopes, thus united, bears resemblance to the section of a boat. This disposition, which has been too generalized for the ensemble of the formation, it having been very frequently deranged by accidents of another nature, and as regards the coal beds, when the continuity is not always established between the beds of which the slopes tend theo. retically towards each other, is nevertheless, with these exceptions, a very

common occurrence.

This fact indicates that coal basins have generally been compressed by lateral upheavings or pressure.

The change of inclination often involves the existence of curves of adjustment, which are only the folds [plis] of the beds. In the greater part of the circumscribed basins, these folds are of great radius: but, in the beds of the great northern basin of France, the folds are sometimes so sudden and decided that they change the inclination of from 10° or 15°, to that of 75° or 80° at any given point.

The section of the basin of Mons, in Belgium, is good example of the plication or doubling back of the coal beds of the northern basin, even to such an extent as to permit the vertical shafts to pass two or three times through the same beds. Most frequently there is an enlargement of the thickness at the angle or bend [" crochon,"] of a ply, and the thickness of a bed of one yard is increased to one and a half or two yards. Those beds whose inclination is below twenty degrees, permit the establishment of working galleries according to the method which bears the name of flats'plats," and they call uprights-" droits"—those which possess a high inclination. The same beds, therefore, that occur in our section of Mons, assume alternately the disposition of flats and uprights.

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These folds have, at the same time, both a direction and an inclination, and form a sort of sloping gutter which is called by the Belgian and French miners “ennoyage."

The plications are evidently the effect of dynamic causes, which have produced these inclinations; they result from upheavings which have undulated the surface of the basin, and from lateral pressure, which has forced the groups thus undulated, to occupy a greatly contracted space.

Contractions and enlargements are frequent accidents in coal beds; they both exist, generally, in the same vein, and, in some basins, the miners are in the habit of saying, when the roof and floor widen suddenly, “the bed enlarges, we are going to lose it." A gradual and prolonged contraction, a division of the bed, whose planes of stratification become entangled in the rocks of the roof and floor; in fine, the alteration of the coal which becomes more and more mixed with argillaceous slate, constitute an impoverishment which is the ordinary precursor of a total suppression.

When the two walls of the vein, approaching each other, come to unite, and for a while to suppress the coal bed, the accident takes the name of fault, [crain or coufflée.]

Faults are more frequent accidents in the thick beds than in those which do not exceed a yard. By following the carbonaceous thread which almost always exists as a trace left by the coal itself, or, in default of that, by fol

lowing the rocks of the roof and floor, of which nature furnishes indices sufficient to preserve the plane of stratification, the bed will be recovered, after an interruption of greater or less extent. In the mines of Rive-deGier, the lines of white slate serve as a guide, in default of carbon, by which to pass over or go by the faults. In the mines of the environs of Nantes, where the suppression is equally complete, it is necessary to consult attentively the rocks of the walls, in order to pass by, without deviation, the very considerable spaces which often separate the prolongation of the same bed. Miners are sometimes very much embarrassed, when they have pursued the trace of a fault during a long space; for, not finding again any indication of resumption, they do not know whether the interruption should be attributed to the presence of a fault or to the definitive cessation of the coal. No rule can be laid down in this case; the observation of the structure of the whole can only furnish the data which we have already indicated above. Faults are very common accidents. They are fractures which affect the entire character of the formation, and cause greater or less disturbances. These faults have a determinate direction, and frequently a basin is affected by a system of faults parallel with each other. At other times, there are several systems which follow different directions, but are each composed of faults, connected with each other, by a parallelism of direction.

The intensity of the faults is very variable; sometimes they scarcely interrupt the formation, and appear as fissures which have changed the level of the two ruptured parts, but not enough to constitute the total interruption of the coal, which is always easy to follow, when the offset is not greater than the thickness of the coal. The bed of Lucy, in the basin of Saône-etLoire, frequently presents such faults.

The section of the coal bed of Monceau furnishes an example of interruptions of continuity caused by movements subsequent to the formation of the coal deposit, and of a confusion [brouillage] which totally interrupts the coal.

The brouillages are nothing but the intervals comprised between the planes of fracture; in these intervals all the beds are broken and reduced to angular blocks mingled together.

When these faults form part of those which have determined the outline of the surface of the ground, the offsets or upthrows are, in some measure, proportionate to the inequalities which it presents. Thus, there are offsets and upthrows of several hundred feet in the basins of England, of Wales, and of Rive-de-Gier, whose surface is highly disturbed.

In the basins of the Saône-et-Loire, they are rarely more than from thirty to ninety feet, and are, notwithstanding, in agreement with the undulations of the surface.

Endeavours have been made to establish, from the most detailed and available information, agreements between the arrangement of the beds of coal and the superficial accidents of the soil. Thus, in a great number of basins, the direction of their beds coincide with that of the great axis of the coal formation, and this great axis is itself directed in the same course as the existing valleys, in such a manner that the direction of the beds is confounded with that of the dividing lines of the waters and the principal valleys. In some other basins, the planes of stratification of the formation have not only the same direction, but also the same inclination as that of the surface declivities.

The basin of Bert offers a striking example of this agreement. The inclinations of its coal beds change as often as five times whilst conforming