by a force which permanently increases its length without at the same time compressing it.

Another effect of the lateral motion of the atoms of a soft heavy body, when acted upon by a percussive force with a hammer of small dimensions in comparison with the mass of metal, -for example, if a large shaft of iron be hammered with an ordinary sledge, is a tendency to expand the surface so as to make it separate from the middle portions. The interior of the mass by its own inertia becomes as it were an anvil, between which and the hammer the exterior portions are stretched longitudinally and transversely. I here exhibit to the Association a piece of iron originally from a square bar four feet long, which has been so hammered as to produce a perforation of the whole length entirely through the axis. The bar could be seen through, as if it were the tube of a telescope.

This fact appears to me to be of great importance in a practical point of view, and may be connected with many of the lamentable accidents which have occurred in the breaking of the axles of locomotive engines. These, in all cases, ought to be formed by rolling, and not with the hammer.

The whole subject of the molecular constitution of matter offers a rich field for investigation, and isolated facts, which are familiar to almost every one when attentively studied, will be made to yield results alike interesting to abstract science and practical art.

ART. VI.-On the Occurrence of the Ores of Iron in the Azoic System; by J. D. WHITNEY.*

THE object of the present communication is to call attention to the geological position and mode of occurrence of one of the most interesting and important classes of the ores of iron, namely, those which are associated with rocks of the Azoic System.

The term Azoic, first employed by Murchison and De Verneuil in their description of the geology of the Scandinavian Peninsula, has been adopted by Mr. Foster and myself in our Reports on the Geology of the Lake Superior Land District, and has been shown by us to be applied with propriety to a series of rocks which covers an immense space in the Northwest. We have called attention to the fact, that this system of rocks, wherever it has been demonstrated to exist, has been found characterized by the presence of deposits of ores of iron, developed on a scale of magnitude beyond anything which occurs in any of the succeeding geological groups or systems of rocks.

*Proceedings of the American Association for the Advancement of Science, Ninth Meeting, held at Providence, R. I., August, 1855, p. 209.

In illustration of these views, we have briefly described some of the great ferriferous districts of the world, and particularly those of Lake Superior, Scandinavia, Missouri, and Northern New York, all of which exhibit a most marked analogy with each other, both in regard to the mode of occurrence and the geological position of the ores. The two last-named regions, however, not having been thoroughly examined by us in person, we were obliged to content ourselves with information obtained from others, in making a comparison of their most striking features.

Strongly impressed with the interest attaching to this subject, I availed myself of the first opportunity, after the publication of our Report, to visit the iron regions of Missouri and Northern New York, from the last-named of which I have just returned, after a careful examination of the most important localities where ore is now mined in that district. While it is intended to take another opportunity for giving a minute and detailed account of this region, I may be permitted to recapitulate here the prin cipal points maintained by Mr. Foster and myself, to the gen eral correctness of which my more recent explorations have furnished me with additional evidence.

We maintain therefore,

1. That deposits of the ores of iron exist in various parts of the world, which in extent and magnitude are so extraordinary as to form a class by themselves. The iron regions mentioned above offer the most striking examples of the deposits now re

ferred to.

2. That the ores thus occurring have the same general character, both mineralogically and in their mode of occurrence, or their relations of position to the adjacent rocks.

3. That these deposits all belong to one geological position, and are characteristic of it.

The extent of the workable deposits of the ores of the useful metals is usually quite limited. Most of the veins which are wrought in mines throughout the world are but a few feet in. width, often not more than a few inches. This is true of the ores occurring in veins. In sedimentary metalliferous deposits, such as those of the ores of iron in the carboniferous, the horizontal 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 oxyds, either the magnetic 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 oxyds 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 unequaled by that from any other part of the world. The ores of Lake Superior and Missouri are mostly peroxyds; 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 peroxyds 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 charac

teristics 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 a 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 from 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 peroxyd of iron, without any admixture 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 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 explain. 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 oremass 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 SECOND SERIES, Vol. XXII, NO. 64.—JULY, 1856.

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oxyd disseminated through their mass, as one of their essential ingredients, that they might almost be called ores. These erup

tive 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 preexisting igneous masses, which have been broken and worn down, during the turbulent action which we may suppose to have been preeminently 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