those we have mentioned can, of course, have no existence, and we have there to seek for other phenomena which may indicate the presence of coal. Happily these are so abundant, that no coal region in the world, probably, presents more ready facilities for the ascertainment of what lies at so insignificant a depth beneath the surface.

To return to the more disturbed region which is occupied by anthracite in Pennsylvania. The disposition of the outcrops, to which we have alluded, materially influences the physical features of these coal districts, and modifies the contour of the surface by a numerous succession of terraces, steps, or benches, on the inner slopes of the mountains, facing the centres of the basins. Those who have ascended, from either side, the long parallel mountains which border the southern coal region of Schuylkill county, to the height of 1350 feet above the Susquehanna river, and more than 1650 feet above tide-water level, need not to be reminded of these characteristic details.

Between the external margins of the principal coal basins of Pennsylvania, subordinate axes of elevation are of frequent occurrence. Even the undulations of the surface between these limits are all attributable to these minor axes, and correspond, in great measure, with the local inclination of the upheaved stratification beneath. These undulations of groups of coal seams, so important to the proprietor, yet whose existence, until of late years, was scarcely suspected, are daily becoming more familiar to us, as the progress of development and practical investigation gradually advances.

The long narrow troughs, of which there are so many in central Pennsylvania, owe their contour to parallel synclinal axes, which present highly inclined or vertical coal beds; and occasionally even exhibit the strata of one of their sides tilted or leaning over so much that their inclination becomes almost parallel to, or conformable with, those of the opposite side of the basin.

In proportion as the anthracite basins become wider, their interior is the more disturbed or broken by undulations, consisting of one, two or three subordinate axes, each maintaining itself for a space as a parallel inferior ridge, and thus interrupting the general troughlike arrangement of the stratification. It is to be expected that the carboniferous beds in the vicinity of the centres of these synclinal axes are liable to be too much crushed to permit an advantageous working of their contents.

The southern anthracite region, in particular, furnishes numerous instances of the modified arrangement of which we speak, and we might introduce several illustrations from our own observation, which would exemplify the extent of the forces to which the anthracite country has been subjected, in the area between the Lehigh and the Susquehanna.

Beginning near the eastern extremity, at Nesquehoning, we see the ordinary basin-form arrangement modified by an upheaving or saddle in its centre, it being here scarcely one mile in width.

Next westward is a section in the meridian of the Mauch Chunk summit mines, where the basin has now expanded to almost double. the breadth that it occupied at Nesquehoning. The structure of the interior is now considerably complicated, and the enlarged breadth allows of a triplication at least of the coal series. So confused is its aspect at this point that we are by no means certain that our section embraces all the details.

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.

A

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.

ON THE MAPS OR PLANS OF MINES.

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.

To

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. 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 trans

posed, 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 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

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