Further westward we have the very interesting and magnificent transverse section formed by the stream of the Little Schuylkill, at Tamaqua, where the basin has again contracted to the simple synclinal axis, of scarcely one mile in breadth.

Our figure exhibits this section with the accuracy resulting from an original survey, and it is the more memorable from the presence of a particular seam in the Sharp Mountain, which is worked to the breadth or thickness of no less than seventy feet.

At Pottsville, the same region has widened to the extent of about five miles, affording, by the repetition of the coal beds, a vast industrial area; and at the head waters of the Swatara river there is now a breadth of no less than six miles. In the Pinegrove coal district we have at least three miles of breadth. Thus we perceive that in proportion to the space or breadth between the geological margins of the Schuylkill coal-field, so is the frequency of the undulations, the number of anticlinal elevations or axes, and the consequent repetitions of the same series of coal seams.

Westward of the Swatara or Pinegrove coal region, it bifurcates and stretches, with diminished breadth, for many miles towards the Susquehanna. The geological structure of these two forks is illustrated by the above diagram.

Section showing the North and South Forks of the Southern Coal Region, Pa.

Rausch Gap.

Scale three miles to an inch.

Bear Valley Gap.

The figure represents a cross section, in a north and south direction, of a part of the coal region near the western boundary of Schuylkill county, crossing both the forks of that basin. It shows, in the first place, at A, the simple synclinal axis which forms the south-western fork of the region, and its nearly vertical strata on the southern margin of Sharp Mountain. On the same meridian, crossing the north fork at B, is a specimen of more complicated structure; not a simple anticlinal axis, but a trough which exhibits a subordinate anticlinal ridge, or central saddle at B.

The enlarged details of the portion A, are shown in the sketch below, a few miles to the eastward, at Black Spring Gap.

We have been led somewhat astray from our purpose of devoting this section to the consideration of the usual means of tracing the coal seams along their outcrops in the anthracite region of Pennsylvania. We have previously remarked that in the horizontal beds of the bituminous coal-fields of North America, their position was very readily ascertained. We showed also that in the highly inclined anthracite areas, the range of the outcrops was ordinarily distinguishable by parallel depressions along the mountain flanks of the basins.

During our own investigations we have remarked that the true positions of those veins which had their bassets on the slopes of the mountains were, in most cases, rendered obscure by the curvatures of the crops, almost at right angles to the true inclination of the veins. We ascribe this to the atmospheric agency, operating to a given depth below the surface, and to the mechanical influence of surface waters, decomposition, the sliding down of the higher masses, &c. In every instance which has come under our observation, in relation to the outcropping of coal seams on these slopes, we have perceived the manifestation of the like influences, which have deflected the "wash" or decomposed materials of the coal veins from their true courses and thrown them over among the alluvial detritus, generally in a curve, as shown by the next figure, which is merely the representative of numerous corresponding cases.

Coal Crops on the Fourth Mountain.

In every working a good plan of the mine is of great utility; it is above all indispensable when the subterranean works are considerably developed. In fact, it is necessary to maintain the works in the limits of the property, in order to avoid contests with neighbouring owners, and there always exist some points from which it is necessary to keep removed, under risk of the greatest dangers. Finally, when it is suggested to effect a junction with a point, fixed beforehand by a pit or gallery; if there was not a plan constructed with precision, we should run the risk of missing the object, and of making costly works to no purpose.

The drawing of the plans of mines presents great difficulties. The mines being composed of crooked passages, isolated one from the other, how should we determine, singly, the form and the position of each of them, and render them conformable to the plans of the whole? These difficulties are still increasing from the necessity of working in obscure galleries, often low and difficult of access. To construct correctly a map of the works which only communicate with the surface, by sinuous galleries or by shafts, it is absolutely necessary to have recourse to the needle. The dial, or mining compass, is composed of a magnetic needle tinted with blue steel at the north point, and balanced on a cap of agate.

The mining compass is suspended from the middle of two axes or spindles, tourillons, upon the support of the brackets, crochets; the line N and S corresponding with the axis of the crochet. If then, after having strongly held a cord or copper wire, following the axis of the gallery of which the direction is required, the compass be suspended at this cord, the deviation. of the needle from the north and south line, will give the angle of the direction. In order to facilitate the reading of this course, the letters E. and W. are commonly transposed, so that the true point of the course may be read in degrees and minutes, by means of the figure which approaches the nearest to the blue point. This method is also applicable to the compass which is carried in the hand, and appears to be generally in use in Germany and France. The difficulty of reading off, with sufficient exactness with an uncertain light, and in positions often incommodious, the angles marked by the needle which oscillates during a long time, is one of the many obstacles to the perfect accuracy of the observations, and it is admitted that by this method an observation cannot be taken with greater nicety than a quarter of a degree, or fifteen minutes.

In mines of the magnetic oxide of iron, where the action of the needle is deranged by its proximity to the mineral, the compass cannot be employed, and the graphometer is used. A theodolite, for subterranean service, is also adopted in France, with which plans and surveys of mines can be constructed with equal celerity as with the compass, and in a more exact

manner.

MINE SURVEYING.

In England the usual surveys in mines of all descriptions were made with the dial. The most useful treatise on the art and on the practice of this instrument, is that of Mr. Budge, of Cornwall,* an eminent mine surveyor. Although constantly employing the dial in his business, he, from the first, by no means viewed it as the most accurate that can be employed, and remarks, "There doubtless are instruments much better adapted to the work, both for speed and accuracy, than the dial; and it is matter of surprise that they have not been more generally introduced in our mines: of these instruments the theodolite certainly stands unrivalled, for taking both horizontal and vertical angles."

In the second edition of this work, after a lapse of twenty years, the author devotes a section to the subject of "surveying without the magnetic needle." This is a valuable modern discovery; and as the general introduction of iron railways and tram roads in mines drove the surveyor to seek some substitute for the needle, which the attraction of iron rendered useless, he has happily succeeded. The best circumferentors are now made with

* The Practical Miner's Guide, by J. Budge, second edition, 1845.

L

an external graduation and vernier scale, on the theodolite principle, on purpose for the performance of this work.

The author enters into all the necessary details for proceeding with the observations ascertained by this improved instrument, and for protracting and calculating the work thus performed.

IRON ORE OF THE COAL FORMATION.

In the coal formation, iron only exists in the state of carbonate: it is generally concentrated in particular beds of a basin, and upon a much more limited superficies than that of the beds themselves. The usual form of the carbonate of iron of the coal series is that of oval or kidney shaped balls,rognons, having a brown or greyish fracture; it is a mixture, more or less rich, of clay, and carbonate of iron. These balls occur stratified in the argillaceous beds. They appear to be assembled and precipitated during periods of repose, when the waters deposited at the same time the argillaceous particles with which they were charged. These spheroidal concretions or rognons, in consequence of their mixture with clay, are often arranged in concentric lamina, and frequently also present in their centre, a nodule of clay or of pyrites; sometimes even a fossil substance,-nuclei, which appear to have attracted around them the chemical precipitation.

The position of the nodules of lithoide carbonate of iron is in strata parallel to the coal seams. This is the case at the mine of Treuil, in France, and almost every coal-field in England presents a similar deposition. In those of North America, we find a smaller amount of argillaceous carbonate of iron so interstratified with the coal beds, than in Europe, and the instances where iron works are supplied from these sources in America, at the present day, are but rare.

In the coal basins of England, the carbonate of iron is almost always found in the same beds, extending over very large districts. There are two large beds of ferriferous clay in the Dudley basin,* and sixteen or more in the great anthracite district of South Wales.

According to M. Burat, the numerous coal basins of France are far from containing the carbonate of iron in the same abundance as those of England. Few of the argillaceous beds contain these balls, and still more seldom do they contain those which are concentrically formed. At Saint Etienne, for instance, there exist two which furnish in the concession of Treuil, flattened rognons of fair quality, and contribute to supply the furnaces of Janon; but in the other concessions, the balls are not recognized in the equivalent beds, or they are so small as to be neglected. At some other points, the carbonate of iron appears in great abundance, but with very different characters: at the mine of Cros, it penetrates the entire beds of clay, of one or two yards thickness, and gives to that rock a remarkable solidity and density. But these massive beds are much more impure than the beds with disseminated nodules besides a large proportion of clay, they also contain pyrites and

:

*The Dudley coal-field is remarkable as being one of the earliest positions where the argillaceous iron was smelted by means of pit-coal. The experiment was made by the founder of the noble house of Dudley and Ward, who published an account of it in the time of Charles II. He states that in a large stone furnace, twenty-seven feet square, he made seven tons of iron per week, "near which furnace the author discovered many new coal mines, ten yards thick, and iron mines underneath, which coal-works having brought into perfection, the author was by force thrown out of them, and the bellows of his new furnace and invention by riotous persons cut in pieces, to his no small prejudice, and loss of his invention of making iron with pit-coal."

precipitations of dark silex, to such an extent that the working which ought to have been developed, remains almost unproductive.

The coal basin of Aubin, in France, contains the iron ore in the most abundance. It exists, in the first instance, in balls in the beds of clay which accompany the coal, and, as at Saint Etienne, under that form it is the purest mineral. In the other cases, it constitutes a somewhat schistose bed of from three to fifteen feet thickness, which appears to extend under the greater part of the coal area. This bed is remarkable, inasmuch as it presents at several points a series of contractions and enlargements, which constitute the arrangement called en chapelet; like a string or chaplet of beads, -an arrangement very frequent in all the substances which result from chemical precipitations in sedimentary waters, such, for instance, as is often presented by the flints in chalk. This structure is, in other respects, independent of the other accidents, faults or disturbances, which equally affect this bed as those of the coal series. The kidney ores and the bed of stony carbonate of iron are worked at several places in the basin, and supply the high furnaces of Decazeville.

There are but very few basins which do not possess beds analogous to the argillaceous carbonate of iron; but they are in such slight amount, that there are no other workings than those of the two basins of Saint Etienne and Aubin, which we have just mentioned.

On the whole, if we compare the beds of lithoide carbonate of iron with the mass of coal formations, we see that their existence, but little developed, although frequent, must only be considered an accidental circumstance. It is equally worthy of remark, that, in every case where there was a formation of coal in the series subsequent to the true coal, viz., the ferruginous elements have anew resumed the composition and the characteristic aspect of this epoch.

Thus in the coals and the shales with vegetable impressions of the epoch of the lias of Yorkshire, we find the carbonate of iron stratified in balls; whilst, in the same formations, when their appearance is in the normal state, the ferruginous infiltrations appear only in the state of oxides.

These variations of composition in the ferriferous minerals, establish no real difference in the origin to which they may be attributed. They tend merely to demonstrate that the iron, collected at certain intermittent epochs and at isolated points, most commonly in the state of oxides, has undergone through the influences of the carboniferous epoch, a mineralogical transformation. The coal period appears generally to have been a period of tranquil deposits. It is, then, natural to find that in it the ferruginous infiltrations are more concentrated than in the periods of the old red sandstone, of the new red sandstone, and even of the trias, where the products of these infiltrations are blended with the general materials of the deposit.

We have already remarked, that these infiltrations only become valuable according to their concentration: the formations before mentioned, so highly coloured by the per-oxide of iron, contain, perhaps, altogether much more iron than the coal formation; but, in the latter, it is collected together, and often possesses a concentration of 30 and 40 per cent., constituting serviceable beds. In the red or mottled sandstone formations, we find iron everywhere; but the concentrations even amounting to ten per cent. are but rare exceptions. The presence of the iron would then be scarcely remarked, if the glaring colour of these red and variegated formations did not contrast with the gray and dark rocks of the coal deposits which they cover, and with the white and greenish colours of the thick limestones and clays which are above them.*

* Burat.-Géologie applique, p. 108.