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extent is often very considerable; but the vertical range is so limited, that the most extensive basins may be in time exhausted, when worked on so extensive a scale as is the case in some of the celebrated iron districts of Great Britain. The deposits of iron in the azoic, however, are many of them developed on a scale of such magnitude, that the term “ mountain masses” may be applied to them without exaggeration, while, from the very nature of their occurrence, they must extend indefinitely downwards, and cannot be exhausted. Thus the great iron mountain of Gellivara, in Sweden, has a length of three or four miles, and a width of not less than a mile and a half. Of course such a mass of ore, without limit in depth, might be worked on the most enlarged scale for any length of time without fear of exhaustion. The same may be said of some of the iron knobs and ridges of Lake Superior and of Missouri. They form veritable mountains of ore, and ages must elapse before their dimensions will have been perceptibly diminished. This is not necessarily the case with all the localities of ore of these districts. Indeed, in Northern New York and in Scandinavia, although there are accumulations of iron which may be measured by hundreds of feet, or even by miles, yet those which are best known and most worked are of much more reasonable dimensions.
The character of the ores thus occurring is mineralogically peculiar. They consist uniformly of the oxides, either the mag. netic or the specular. Hydrous ores, carbonates and the like, are altogether wanting, unless it be upon the borders of the ore deposits, where a secondary metamorphic action between the ferriferous mass and the adjacent rocks may have taken place. The oxides found in this geological position are in general remarkably free from all injurious substances, such as sulphur, arsenic, lead, or zinc, and usually the approach to chemical purity in the ores is in proportion to the extent of the mass, the largest deposits being the purest. The principal foreign ingredient mixed with these ores is silica, which is always present, although frequently in minute quantity. Indeed, the analyses of the Lake Superior and Missouri ores show, in some instances, a surprisingly near approach to a state of absolute purity. It would not be difficult in some localities to procure large quantities of an ore not containing more than two or three tenths of one per cent. of foreign matter, and that exclusively silica. The purity of the ores may be inferred from the high character and value of the iron manufactured from them when they have been skilfully worked, as, for instance, in Sweden. Some samples of iron manufactured from Lake Superior ore have, when tested, exhibited a degree of tenacity unequalled by that from any other part of the world. The ores of L-ke Superior and Missouri are mostly peroxides ; those of Northern New York almost exclusively magnetic; while in Scandinavia the magnetic and specular ores are both of
frequent occurrence. Those of New York are often coarsegrained and highly crystalline, while the peroxides of Lake Superior and Missouri are rarely distinctly crystallized, but are very compact.
The mode of occurrence of these ores in the regions above mentioned is so peculiar, that, from this point of view alone, it is apparent that these deposits should be classed together as distinct from those in the later geological formations. In all the characteristics of true veins, the great masses of ore now under consideration are wholly wanting. Some of the least important of them approach much nearer to segregated veins, and might with propriety be classed with them, were they not developed on so large à scale as to render it difficult to conceive of segregation as a sufficient cause for their production.
In the case of the most prominent masses of ore of these regions there is but one hypothesis which will explain their vast extent and peculiar character. They are simply parts of the rocky crust of the earth, and, like other igneous rocks, have been poured forth froin the interior in the molten or plastic state. No other origin can be assigned to the dome-shaped and conical masses of Lake Superior and Missouri, or to the elongated ridges of the first-named region. The Iron Mountain of Missouri forms a flattened dome-shaped elevation, whose base covers a surface of a little less than a square mile, and which rises to a height of 200 feet above the general level of the adjacent country. The surface of the mountain, where bare of soil, is found to be covered with loose blocks of peroxide of iron, without any admix. ture of rocky pebbles or fragments, which increase in size in ascending to the summit, where large blocks of ore many tons in weight lie scattered about, and piled upon each other. It is a most singular fact, that the ore is nowhere seen in place about the mountain, although the whole mass evidently consists of nothing else. Near its base, an excavation of seventeen feet deep has been made, which exhibits nothing but small, somewhat rounded fragments of ore closely compacted together, without any other substance present except a little red, ferruginous clay, which seems to have been formed by the friction of the masses against each other. This feature in the Iron Mountain is one of peculiar interest, and one which it seems difficult to ex. plain. Evidences of drift action in this region are exceedingly faint. The ore itself is one which seems little likely to undergo decomposition from any exposure to atmospheric changes. The blocks upon the summit, although somewhat moss-grown, have their angles and edges but little rounded. As a key to the origin of the ore, we find in close proximity on the north a long elevation of a reddish porphyry of unmistakably eruptive character, connected with the Iron Mountain by a narrow ridge of a rock composed of iron ore and feldspathic rock, showing that the
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 wbich have just been alluded to. By far the larger portion of the azoic series on the earth's surface being cov. ered 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, &o., &o. 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