porphyritic ridge and the ore-mass must have originated at one and the same time, and in the same way.

The eruptive origin of the great Lake Superior ore-masses seems also well sustained by the phenomena which they exhibit. They alternate with trappean ridges whose eruptive origin cannot be doubted, and which, themselves, contain so much magnetic oxide disseminated through their mass, as one of their essential ingredients, that they might almost be called ores. These eruptive masses include the largest and purest deposits of ore which are known in the Lake Superior or the Missouri iron regions; but there are other localities in both these districts where the mode of occurrence of the ore is somewhat different, and where the evidences of a direct igneous origin are less marked. This class comprehends those lenticular masses of ore which are usually included within gneissoidal rocks, and whose dip and strike coincide with that of the gneiss itself, but whose dimensions are limited. Such is the character of most of the Swedish deposits, and of many of those of Northern New York. Such beds of ore as these may in some cases be the result of segregating action; but the facts seem rather to indicate that they are made up of the ruins of pre-existing igneous masses, which have been broken and worn down, during the turbulent action which we may suppose to have been pre-eminently manifested during the azoic epoch, and then swept away by currents, and deposited in the depressions of the sedimentary strata in process of formation. In confirmation of this hypothesis in regard to the origin of these lenticular masses of ore in the gneissoidal rocks, it may be noticed that the ores occurring in this form and position are less pure than those of decidedly igneous origin, as if they had become more or less mixed with sand during the process of reconstruction, so that they not unfrequently require to be separated from their earthy impurities by washing before they can be advantageously used. Again, it may be observed in the case of some of the ore-beds of this class, that the bed-rock or foot-wall is considerably rougher or more irregular in its outline than the hanging wall or roof, as if depositions had taken place upon a surface originally rough and uneven, the upper surface of the ore being considerably smoother and more regular than the lower one, and sometimes separated from the rock above by a thin seam of calcareous matter.

There is still another form of deposit which is not unfrequently met with in the Lake Superior region, and which may be seen on a grand scale in the Pilot Knob of Missouri. This consists of a series of quartzose beds of great thickness, and passing gradually into specular iron, which frequently forms bands of nearly pure ore, alternating with bands of quartz more or less mixed with the same substance. Some of the deposits in the Lake Superior region are of this class, and they are very

extensive, and capable of furnishing a vast amount of ore, although most of it is so mixed with silicious matter, as to require separating by washing, before use. Heavy beds of nearly pure ore occur at the Pilot Knob, interstratified with beds of a poorer quality. Deposits of this character are usually very distinctly bedded, and the ore shows a greater tendency to cleave into thin laminæ parallel with the bedding, in proportion to its freedom from silicious matter. These deposits seem to have been of sedimentary origin, having been originally strata of silicious sand, which has since been metamorphosed. The iron ore may have been introduced either by the sublimation of metalliferous vapors from below during the deposition of the silicious particles, or by precipitation from a ferriferous solution, in which the stratified rocks were in process of formation.

The great deposits of ore which have been alluded to above, agreeing as they do in the characteristic features of their mode of occurrence, especially in the magnitude of the scale on which they are developed, are all, beyond doubt, situated in the same geological position; they all belong to the oldest known system of rocks, the azoic. This name was first applied by Murchison. to the ferriferous rocks of Scandinavia, and the geological position of the great iron regions of this country is precisely similar to those of Sweden. There is ample evidence that the lowest known fossiliferous strata, characterized by the same peculiar types of organic life, both in this country and in Europe, rest uniformly upon the iron-bearing strata throughout the Northwest, from New York to Missouri and Arkansas.

We have thus seen that the earliest geological epoch was characterized by the presence of the ores of iron in quantity far exceeding that of any succeeding one; indeed, we may infer that the ruins of the iron ores of this class have furnished the material from which many of the ores of more recent geological age may have been derived. The condition of things in reference to the ores of iron which existed during the azoic period underwent a complete change, and rarely do we find in any fossiliferous rocks any signs of unmistakably eruptive ores. It is certain that we nowhere, out of the azoic system, find masses of ore of such extent and purity as those which have just been alluded to. By far the larger portion of the azoic series on the earth's surface being covered up by the fossiliferous rocks, the ore which that formation contains is equally concealed, and it is only in those regions. where no deposition of newer strata upon the oldest rocks has taken place that the treasures of iron are made accessible. In this respect our country is pre-eminently favored, and there can be no doubt that the immense deposits of iron ore stowed away in the Northwest, are destined at some future time to add to our national wealth more than has been or ever will be contributed by the gold of California. It may seem absurd to speculate on

the exhaustion of the stratified ores of England or of the Eastern United States; yet nothing is more certain, than that the present rate of production in the former country cannot be kept up for any very great length of time, without making the cost of procuring ore so great, that other regions which now seem very remote from a market will be able to compete with the most favored iron producing districts of England.

Practically, the views which have been presented above are of importance, as leading us to expect large and valuable deposits of the ores of iron wherever the azoic rocks are found to exist over any considerable surface. Thus it may safely be predicted that important discoveries of ore will be made, in the now almost unexplored regions of British America, which are covered by rocks of the azoic period. Indeed, large beds of ore have already been found in Canada, which are, in character and position, analogous to those of Northern New York.

ART. VII.—THE MINING OF COAL, &c., &c. By A. T. PONSON. No. 5. [Continued from page 331, Vol. VI.]

36. External figure of the coal deposits.

THE carboniferous strata and the incumbent rocks are not broken by mountains and hills of steep and sharp descent, or by deep and narrow valleys. On the contrary, these formations present to view only plateaus, or vast plains through which some streams flow, or simply groups of rounded hills of a moderate height.

Immediately after the convulsions which rent the crust of the globe, after the folding and rupture of the strata, the coal series probably presented on the surface vast fissures and abrupt precipices; the summits extended to sharp peaks, such as are frequently met with in the midst of formations composed of rocks which are very hard and little affected by atmospheric influences.

But the seams and the enclosing rocks, being usually of a tender and brittle nature, have not been able to preserve their irregularities; and yielding to the erosive action of water, they have been speedily reduced to a level-their masses being made round, the hills washed down, and carried and deposited in the adjacent cavities. The same result is presented where the coal strata is found covered by more recent formations; those stratified in beds sensibly horizontal, and rarely affected by dislocations, have not become uneven in surface only in consequence of the irregularities of the deposition, or in consequence of currents of

water, which, furrowing the surface, have caused the gentle slopes of plains and hills.*

It results from the preceding remarks, that the slopes of the surface are not necessarily in conformity with the inclination of the beds; or, at least, that the irregularities of the actual surface should be the result of the inclinations and faults of the coal strata, is a very rare circumstance. Thus, to indicate or determine the bearing of seams in conformity to the unevenness of the surface, and its external configuration, is a useless speculation; and the opinion of miners, who having discovered an accidental parallelism between the inclination of the slopes of valleys and the seams of coal, have thence inferred a general rule, is erroneous to such a degree, that the exceptions are probably more numerous than the cases to which the rule applies. It often happens, if one regards the bearing of seams in a general manner, that their direction is parallel to the great axis of the basins, and this axis is confounded with that of the valleys.

37. Origin of the water encountered by the miner when pene

trating the earth.

At different depths beneath the surface the miner meets with water in greater or less quantities, according to the disposition of the layers and the structure of the strata. It is now well proved, although a different opinion was at one time entertained, that all comes from the surface, where it has been absorbed by the different strata cropping beneath the vegetable soil. The rains which do not flow off to form streams and rivers, are infiltrated through the beds of the strata, the pores, joints, and fissures of which they traverse, descending until they meet a strata which is devoid of the properties of filtration, where they are arrested and retained. If the plane of a stratification of a bed is such that its lowest part outcrops upon the side of a hill, or if it is entirely above the bottom of a valley, the waters flow off naturally; but if the impermeable bed folds and forms a basin, without an outlet, the water accumulates in a quantity dependent upon the porosity of the rock and the number and capacity of the fissures, thus forming simple internal springs, or vast sheets ready to drain into the shafts or other cavities which present a diminished lateral resistance. If, unfortunately, the layers which have been penetrated by an excavation have an outcrop in the bed of a little pond or

*It should be remembered that the superincumbent strata composed of rock of sufficient hardness to resist the action of the atmosphere, although they do not form actual mountains, yet present, always very elevated summits, having steep sides, and peaks even 150 to 200 feet in height. We find likewise the sur face of the carboniferou sstrata ragged and broken by trap and other volcanic matters, filling certain fissures, and elevated above the surface in consequence of the destruction of the rocks enclosing coal.

river, or rivulet, the copious and constant infiltrations bring into the mine great volumes of water, of which the miner is oftentimes unable to rid himself. All the considerations apply with equal force to the coal series and the stratifications incumbent upon them.

38. State of the waters in the chalk series.

The dead grounds are dry, or carry water according to their position relative the bottom of the valleys, in which the surface is broken, that is to say, according as they are placed above or below the natural water level; the relative situation of the strata and the inflections to which they are often subjected, create oftentimes modifications in the general system that controls the waters.

The permeable beds of the chalk formations are traversed in every way by innumerable fissures, or straight cracks, which are horizontal or inclined, and communicating together and resembling in the mass, vast reservoirs, into which the rains settle. This water, unable to penetrate the dieves usually covering the coal strata where it collects, or to expand itself upon a horizontal ground, rises upward, and attains a level in the bottom of adjacent valleys, where it finds a free course. The upper portion can then be drained, but the lower is divided into as many cavities as there are fractured or permeable strata contained between the two beds of im penetrable clay. This is the cause of the gushing up of artesian springs which the miner encounters in the midst of the layers of the chalk formation, and which rise in the excavations towards the surface of the ground. The springs contained in this strata are called niveaux. To penetrate these water-bearing layers and to restrain the water from entering the excavation, is called passing

a niveau.

The abundance of the springs flowing in the cavities is a cause of the greater or less resistance they encounter in passing from one point to another; so likewise the number and the extent of the fractures containing them. Thus, where these are rare, slightly open, and such that they present serious obstacles to the circulation of the water, a simple windlass is sufficient to draw it off. But where the fractures are numerous, and extend a great distance, the collection of water is great, and its drainage at any one point is perceived in all the adjacent shafts. If, in short, any fissure open into a rivulet, the drainage becomes impracticable.

Fissures are very irregularly scattered in the mass of the chalk

We hear of cases in which the miner, after having long contended against the obstacles presented by the abundance of the springs, has abandoned his labor only to recommence it again at a short distance from the same spot, where the water being less in quantity has not prevented him from triumphing over every obstacle.

It might seem, at first view, that a coal formation completely