sea leaves the cause thus assigned to operate with full power; and if Schulten's hypothesis be confirmed, of which there is now but little doubt, it will, in all probability, serve to explain similar phenomena observed in other close waters, as the Caspian, Lake Balkai, and the Lake of Geneva, to which Saussure has assigned similar causes."*

[An important paper on the fluctuations in the surfaces of the Lakes not referred to by Major Lachlan is published by Charles Whittlesey in Foster and Whitney's Report on the Lake Superior Land District, Part 2, 1851, p. 319. It reviews the facts, adds a large number of observations and sustains the conclusion that the rise is not periodical.-EDs.]

ART. V.-Remarks on the changes which take place in the Structure and Composition of Mineral Veins near the surface, with particular reference to the East Tennessee Copper Mines; by J. D. WHITNEY.

In the number of the American Journal of Science for March last, ([2] xix, 181,) M. Tuomey has given "a brief notice of some facts connected with the Ducktown, Tennessee, Copper Mines." As this notice seems to me to convey a wrong impression with regard to the mode of occurrence of the ore in the veins of the region alluded to, I take the liberty of stating what I conceive to be the correct interpretation of the phenomena displayed on so large a scale in the Polk County, or Ducktown Mines.

Mr. Tuomey remarks as follows: "in every published account of the mines that I have seen, the impression is left, that the ore (b) is derived from the underlying portion of the bed (c) by decomposition." No such impression could, by any possibility, be derived from the descriptions of the East Tennessee Mining region which I have published. To show this, I quote from the report to which Mr. Tuomey refers, at the commencement of his notice, as follows: "On penetrating beneath the surface, the section represented in the annexed figure is obtained. (See section accompanying Mr. Tuomey's article.) Beneath the gossan, is found a bed or mass of black cupriferous ore, of variable thickness and width. This, as well as the gossan, is the result of the decomposition of an ore consisting originally of a mixture of the sulphurets of iron and copper, which was associated with a quartzose gangue or vein-stone. The place of the bed of copper ore marks the limit of the decomposition of the vein; beneath it the

See McCulloch's Geographical Dictionary, Article Baltic.

+ Report on the Mining tract of the East Tennessee and Cherokee Copper Mining Co.; by J. D. Whitney. New York, 1853.

ore exists in its original condition." It seems hardly possible that any one should ever have supposed the black ore to have resuited from the decomposition of any other part of the vein than that which lies above it, as below it everything remains in its original, unaltered condition.

The decomposition of metalliferous lodes in their superficial portions is a matter often noticed and generally expected by the miner, and there is nothing anomalous in this respect in the East Tennessee Copper region. The commonly observed facts are these the predominating metalliferous ores which are wrought in mines, especially of silver, copper and lead, are sulphurets, sulphur being the usual mineraliser, although arsenic and antimony are not unfrequently, found in connection with sulphur in combination with these metals. These ores are sometimes scattered irregularly through the gangue in fine particles and sometimes arranged in nearly parallel bands or plates, which are separated from each other by belts of barren veinstone. This is the normal condition of the veins at a considerable depth and some of them retain their original appearance and remain chemically and mechanically unchanged up to the very surface. In most metalliferous lodes, however, it is found that the ores have undergone decomposition down to a certain depth, which rarely exceeds 300 feet and generally falls between 50 and 100 feet. This decomposition is perhaps more common and more strongly marked in cupriferous lodes than in those of the other metals, although some argentiferous veins in South America exhibit it on a grand scale. The predominating ores of copper are the variegated ore and copper pyrites, both of which are combinations of sulphur with copper and iron and their presence in the veins beneath is indicated on the surface by an outcrop of what the Cornish miners call gossan, a term which has been generally adopted wherever the English language is spoken. This is a hydrated peroxyd of iron, usually much mixed with silicious and earthy matter and having a somewhat open and porous structure. Associated with this ferruginous mass the oxydised combinations of copper are often found occurring, at no great distance from the surface; among these, the carbonate and silicates are the most common, the phosphate and arseniate less so. The oxyds themselves and the native metal are among the products of decomposition. Sometimes these oxydised ores are very abundant in the upper part of a cupriferous lode, and are wrought with large profits, owing to their richness and the softness of the ground and the consequent facility in mining. In other localities, nearly all the copper has disappeared from the upper portion of the vein and only traces of these ores are found with the gossan. On sinking down into such decomposed veins a gradual change is found to take place in their character the oxydised ores are replaced by the sulphurets; the

ferruginous aspect of the lode disappears; the gangue becomes more solid, and the walls are better defined.

These changes in the upper portion of the sulphuret-bearing lodes are usually conceived to be the result of the action of air and water introduced from the surface and penetrating gradually downwards. Through their joint influence the sulphuret of copper and iron is gradually decomposed and while the latter metal remains behind in the form of an impure hydrous oxyd, or gossan, the copper is also converted into an oxyd and may remain in that state, or combine with the sulphuric acid furnished by the oxydation of the sulphur of the original ore, or with any other acid which may chance to be present, thus giving rise to the numerous beautiful ores, most of which contain water, which are so common in the higher portion of cupriferous veins. The nature of the combinations resulting from any such decomposition and their relative quantity must, of course, depend on the quantity and quality of the ore originally in the lode, the proportion and kind of vein-stone and probably still more on chemical and perhaps electric agencies, the precise mode of action of which is as yet but imperfectly understood.

In the Polk County mines, these changes are displayed on a grand scale. The metalliferous veins which belong to the segregated class, are very wide and the decomposition has been very complete, so that the outcrop of gossan is very marked and in some places occupies a width of 100 feet on the surface, consisting of large angular blocks of ferruginous rock piled up along the line of the vein. On sinking into this mass of ferruginous matter it is found to be tolerably soft, but at the same time so compact that excavations in it need but little timbering. If the shaft is commenced on the summit of a hill, it will be necessary to penetrate a hundred feet, perhaps, before any change in the nature of the vein is perceived. In the valleys the distance required for this purpose is much less. The depth at which the gossan terminates is nearly coincident with the water-level, or the point where, in sinking, water is found in considerable quantity. Here a layer or bed of copper ore is met with of very irregular dimensions, in some places occupying large bunches or pockets of many cubic yards in content, and in others forming only a thin stratum. This deposit of ore is quite as variable in composition as it is in dimensions. Its color is usually quite dark, and when rich in copper, almost black. It is evidently a mechanical mixture of black oxyd of copper with sulphurets of iron and copper, sulphate of copper, oxyd of iron, silicious matter, and some manganese. The per-centage yield of copper is usually low; but the purest portions contain from twenty to thirty per cent. of metal. This deposit of black ore is the object of exploration in the mines, and the only source, thus far, from which copper has been obtained in any quantity worthy of notice.

Beneath the black ore is the undecomposed portion of the vein, showing, in two or three points, where I was able to see it at the time of my visit (1853), a hard quartzose gangue with particles of copper pyrites scattered through it, and associated with a considerably larger quantity of iron pyrites. There seems no reason to suppose that the ore which originally existed in the upper part of the vein, from whose decomposition the black ore was derived, was any different in nature from that found below, although there may have been bunches of it considerably richer in copper. The deposit of black ore is insignificant in dimensions, compared with the mass of gossan which overlies it, and when we consider that a large portion of the copper which was once disseminated through perhaps a hundred feet of overlying vein-stone is now concentrated into the thickness of perhaps two or three feet, on an average, it will be seen that it is not necessary to suppose that the whole of that portion of the vein which is above the bed of black ore, "doubtless once consisted of yellow sulpharet of copper," as Mr. Tuomey supposes to have been the case. Certainly there is no reason to believe that the black ore is a sulphuret of copper altered by heat. Apart from the consideration that it is not such a product, or mixture of products, as would be produced by any igneous action on copper pyrites, we can conceive of no way in which the effect of increased temperature could be limited to the upper portion of the vein, so that that only should undergo decomposition. That the changes in question are exclusively the result of a humid process, can, as it seems to me, be hardly doubted. The concentration of the black ore in one stratum seems to have been due to the percolation of the surface water which was constantly carrying it downwards to the point where it was stopped by the solid portion of the vein.

That the subject of the decomposition of veins is one which is thoroughly understood should by no means be inferred from the preceding remarks: there is, on the contrary, much in these phenomena which has not, as yet, been satisfactorily explained. We know, indeed, that the changes of the sulphurets with oxydised combinations do occur, for we see them taking place under our own eyes, through the joint action of air and water holding carbonic acid in solution; but why in some mining districts the metalliferous veins should be thus effected, while in others no change whatever has occurred, is less easily understood. Burat has called attention to this circumstance, and cited some instances in which the sulphurets remain entirely unoxydised up to the very surface. Thus the cupriferous veins of Mouzaia in Algiers project out from the surface like walls, being more permanent than the adjacent rock and the first blow of the hammer reveals the pyritiferous ore in its natural state. The same thing may be observed in this country. Throughout the Northern states the pyritiferous lodes remain apparently in their unaltered condition;

or, at most, have undergone but little change and exhibit hardly any indications of gossan. The enclosing rocks are not at all softened or stained with ferruginous matter. As examples of this he would instance the great veins of Shelburne, and Eaton, in New Hampshire; those of Hampshire County in Massachusetts; and the St. Lawrence County mines in New York. In none of these has any marked change taken place near the surface. In one part of the Southampton (Mass.) lode a few oxydised ores were found when the mine was first opened, but they were but small in quantity compared with the mass of the unaltered ore. This state of things is a great drawback on the opening of the New England mines since the expense of sinking and driving in the hard granite and quartzose rocks is enormous. North Carolina, South Carolina and Georgia, on the other hand, the gneiss and slates are often found over a great extent of territory completely decomposed and softened, so that they may be excavated with the pick and shovel, down to a depth of fifty or a hundred feet. I have known a shaft sunk in North Carolina in the rock to the depth of sixty feet in one week.

In

In the veins of that State, the principal, indeed, almost the only, one near the surface is an auriferous gossan resulting from the decomposition of iron pyrites, with which a little copper pyrites occurs intermixed. Of this latter ore, the quality in several instances seems to increase with the depth of the workings. If the veinstone is wholly quartzose the extent of the decomposition is much less than when it contains feldspathic or slaty portions. Thus in the McCullock mine, in Guildford County, N. C., there is a body of soft ferruginous ores containing good amount of gold which extends downwards more than 100 feet and parallel and coëxtensive with this auriferons mass, which may be mixed with a shovel, there is a heavy bed of quartz with iron and copper pyrites scattered through it, in which no traces of decomposition can be perceived.

With regard to the East Tennessee veins the practical question of the most importance is: what kind of ore and how much of it is likely to be found in sinking into the undecomposed veins beneath the level of the black ore. This, we believe, can only be determined by actual trial. If in cleaning out the deposit of ore, which lies upon the hard veinstone beneath, there should be bunches of cupriferous ore found, the best of them should be opened by sinking on them, and there can be no satisfactory reason given, based either on analogy or on the appearances of the bodies themselves, why considerable quantities of the yellow ore of copper should not be found within a reasonable depth. Still it is not impossible, that, as these views do not exhibit the characteristics of true fissure veins, they may be found to have been richest near the surface and not to be capable of being worked with profit in the hard rock.

SECOND SERIES, Vol. XX, No. 58.-July, 1855.

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