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There is no reason to regard the forces as different in kind or mode of action. If the outside waters gain slow access, at depths below, to the lavas for the ordinary action of a volcano in Hawaii, they can at Vesuvius; and the force from the escaping vapors that, in this ordinary action, will make jets of lava of 30 feet to 600 feet, will make jets of cinders of far greater height. Moreover, as the erupting force at Mt. Loa in non-explosive eruptions, is not due to vapors inside the lava-column, since it does its chief fracturing part way down and sometimes far down the mountain instead of about the summit, and causes a quiet condition in the crater instead of violent action, so it is essentially at Vesuvius. In explosive eruptions at Vesuvius, on the contrary, the explosive force is due to vapor-generation inside of the lava cauldron, the projectile action being vastly increased, as at Tarawera in 1886 and Krakatoa in 1883 (page 104).

As the observations at Vesuvius of Scacchi and others have shown (and my own two visits to Vesuvius, one just before an eruption, enable me to appreciate) high-lava mark in the volcano, or that of readiness for a discharge, is attained in the same way essentially as in Kilauea. After a down-plunge following an eruption (as a result of the undermining), leaving the crater hundreds of feet in depth and the upper extremity of the lava-column at a still lower level, work again soon commences, provided the lava-column was not so profoundly cooled off by the aggressive waters and vapor-generating as to be left too deeply buried. For a while the fractures in the bottom of the crater emit only vapors. Later, projectile action begins at one or more points, making conical cinder-deposits by the pericentric action, with now and then an addition to the inside accumulations from small outflows of lava about the bases of the cones or from their vents. The throws of cinders and flows of lava are kept up at irregular intervals, and the level of the floor rises. After the height within has become much increased, small fissures occasionally open through the outside slopes and let out some lava; but the ejections are mostly retained inside except in the later period of progress when some of the highthrown cinders may fall over the outside of the mountain or drift away with the wind. Years pass; and finally the crater's bottom, bearing a large cinder cone, or more than one, reaches that high level in which it becomes actually the summit-plain of Vesuvius, and the fires are visible in the cracks of the plain because the liquid lavas are not far below it.* High-lava mark is thus attained and an eruption may be at hand. Ševere earthquakes are not needed in the work any more than at Kilauea.

* I may refer here to a cut representing Vesuvius in this condition in my Textbook of Geology, made from my sketch in 1834, and to a paper in this Journal for 1835.

How far the ascensive force in the lava-column contributes to the change of level in the floor of Vesuvius nobody knows. The question has hitherto hardly been considered. It probably does its part, for, the liquid lava rises with the rising floor, following it closely.

With the column of liquid lava thus lengthened, making the mountain ready for a discharge, the danger of catastrophe is great for the same reasons as at Kilauea. But the danger is greater than there. It is greater because the forces from vapor-generation and hydrostatic pressure have a weaker mountain to deal with one that has steeper sides and therefore thinner walls to the lava-cauldron, and walls that are partly cinder-made. It is greater because also of the nearness of the lava-column to the sea, the distance being only four miles, while in the case of Kilauea it is over nine miles and in Mt. Loa over twenty; so that at Vesuvius water from two sources, the sea and the land, is close by.

Causes that produce earthquakes may make a rent in the Vesuvian lava-conduit that will let in water for an explosive eruption; but usually it opens the way, as at Mt. Loa, for a comparatively quiet escape of lava, however disquieting the event may be to deluged villages.

The loss by upthrows and outflows tends to produce a sinking or down-plunge of the floor of the crater, and some fall of its walls to the new bottom, as in Kilauea. At the Kilauea eruption of 1886, the outflow drew off the lavas of a lava-lake half a mile in diameter; the crust of lava that covered the borders of the lake, along with portions of the walls, consequently sunk down, and the cavity or crater left by the discharge was half a mile across and between 500 and 600 feet in depth. This is little different from the ordinary event in Vesuvius, except that the loss by the discharge at Kilauea is almost solely by outflow, and no high, weak-sided cone surrounds the vent to suffer from the disaster. It is true that the Kilauea lava-lake in the eruption just referred to occupied only a small part of the great crater. But its diameter was as large as the lava cauldron of Vesuvius has been before any of its modern eruptions; and the movements in the lake were the same that would take place were all Kilauea one great lake.

Explosive eruptions might prove more disastrous to a Vesuvian cone than to one of massive Mt. Loa style; but not because the explosion has the power of blowing off the mountain's summit--which failed to happen at Tarawera in 1885 although the vent was closed and is not a possibility when the vent is an open one-but chiefly because a steep-sided mountain is likely to lose more in height than a broad lava-cone from the same amount of undermining.

We may hence conclude that (1) Vesuvius and Mt. Loa are instructive examples of the effects of the same volcanic forces and methods under different conditions as to rock materials and heat; and (2) systematic study inside of craters between volcanic eruptions is what the science most needs.

In another paper, the results of volcanic action will be further illustrated from observations made, the past year and earlier, in the islands of Maui and Oahu.

ART. XVIII.—On the formation of deposits of Oxides of Manganese; by F. P. DUNNINGTON.

IN explanation of the transfer of compounds of manganese, which has taken place in the formation of geological deposits, the only agency mentioned in works on chemical geology as taking prominent part is that of carbonated water, forming the soluble manganese bi-carbonate. Dr. Bischof remarks, "With regard to the formation and alteration of manganese spar, the same relations obtain as in the case of iron spar." However, Fresenius has shown+ that bi-carbonate of iron and of manganese are obtained under different conditions. There is possibly a hint of the formation of manganese sulphate given by Kersten, but it is not definitely mentioned.

I propose in this article to call attention to the probability that manganese sulphate has taken a very important part in the formation of deposits of manganese ores.

Bunsen's analytic method for the valuation of manganese ores is based upon the fact that an acid solution of a ferrous salt will dissolve the higher oxides of manganese, with the formation of a corresponding amount of a ferric salt. Having observed the promptness with which such solutions are effected, I was led to make some examination of the action of similar solutions upon compact forms of oxide of manganese, and further to ascertain some of the conditions under which solutions of ferrous sulphate, ferric sulphate and manganese sulphate are decomposed. With this in view, the following experiments were made:

1st. There was prepared a solution of ferrous sulphate and sulphuric acid in such proportions as would result from the oxidation of iron pyrites and solution in 25 parts of water. In 500 c.c. of this solution, a lump of crystallized pyrolusite was suspended near the top of the liquid for 48 hours, resulting in

* Chem. and Phys. Geol., vol. i, p. 57, trans. Cav. Soc.

+ Loc. cit., vol. iii, p. 531.

Loc. cit., vol. i, p. 160.

dissolving a thickness of 804 mm. (as calculated from the specific gravity of the mineral, area of the surface exposed and loss of weight); a rate of solution equivalent to 6 inches in one year.

2d. A lump of compact psilomelane was similarly suspended in the acid ferrous solution and a thickness of 506 mm. was dissolved in 48 hours; equivalent to 3.78 inches in one year. In each of the foregoing experiments a corresponding amount (45 and 12 per cent of the total) of the ferrous salt was converted to ferric.

3d. A glass tube 3 cm. in diameter and 20 cm. in length, contracted at one end, was filled with a mixture of coarse powders of psilomelane, 75 grm., disintegrating pyrites (from Marienburg near Bonn), 100 grm. and glass. Through this tube two liters of water were allowed to percolate in the course of 24 hours, and in the liquid was found manganese sulphate, 7 grm., ferric sulphate, 1·45 grm. and sulphuric acid, 4.34 grm.

4th. Through the above filled tube, two liters more of water were similarly passed and this dissolved of manganese sulphate, ·154 grm., ferric sulphate, 07 grm.

5th. Through the above filled tube, two liters more of water were passed, while a continuous current of air was drawn through the tube; this dissolved of manganese sulphate ·292 grm., with about 07 grm. of ferric sulphate.

6th. The glass tube was then refilled, as at first, but using a bright granular pyrite (from Louisa Co., Va.). The current of air was kept up, and two liters of water, dropping at a uniform rate, were passed through the tube in the course of twenty-one days; this dissolved of manganese sulphate 098 grm. and no iron, while a small amount of ferric hydrate was formed and remained in the tube.

7th. A neutral solution of ferrous sulphate digested cold with manganese carbonate, air being excluded, no change takes place; but when heated to 100° C. for two hours, considerable manganese sulphate and ferrous carbonate were formed.

8th. Ferric sulphate solution reacts immediately upon manganese carbonate, forming manganese sulphate, ferric hydrate and carbonic acid.

9th. By reason of the fact, which is familiar, that ferrous sulphate solution is rapidly oxidized when exposed to the air, we find that when it is in contact with manganese carbonate, in the presence of air, the manganese carbonate is decomposed, forming manganese sulphate, etc., as above.

10th. A neutral solution of ferrous sulphate with calcium carbonate, sealed in a tube under carbonic acid, was heated to 100° C. for two hours; mutual decomposition took place, forming ferrous carbonate and calcium sulphate.

11th. Ferric sulphate solution reacts immediately upon calcium carbonate, forming calcium sulphate, ferric hydrate and carbonic acid.

12th. Ferric sulphate solution digested cold or hot with powdered manganese oxide was not altered.

13th. Ferric sulphate solution was digested cold for nine days with sawdust and powdered psilomelane; considerable manganese sulphate was formed.

14th. Manganese sulphate solution is not altered by exposure to air, but when so exposed, and also in contact with calcium carbonate, manganese oxide is gradually formed.

15th. Manganese sulphate solution, digested cold with calcium carbonate in a sealed tube, is not altered; when heated, it is feebly affected, a little manganese carbonate being formed. In view of the foregoing observations and results, it appears possible that many deposits of manganese in calciferous rocks owe their formation to the action of solutions of sulphates, and possibly an illustration of such action is presented in the manganese deposits of Crimora, Augusta Co., Va., which occur under the following conditions.

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In the Shenandoah Valley, the upper portion of formation I (Rogers) is composed of shales which are well decomposed a great depth, interspersed with remaining calciferous ledges, and as they pass westward these alternate with, and are succeeded by, ledges of siliceous limestone. In these decomposed shales, we find pure neutral iron ores, free from manganese, and associated with them there are manganiferous limonites and also psilomelane of high grade, frequently appearing to have grown in similar condition, and sometimes the same mass is composed in part of manganese and in part of limonite. South of the Potomac, no pyrites is visible in these shales, possibly owing to the great depth to which they have been decomposed; but at Harper's Ferry, the iron ore lying in the same geological horizon has been worked down until compact pyrites has been reached. The presence of the pyrites in the latter renders it probable that it did exist extensively in the shales above described.

Assuming that pyrites did thus exist, generally distributed, we might expect a deposition of ores to have occurred in some such manner as the following.

Where pyrites has been deposited and subsequently has been exposed by reason of erosion, the outcrop is gradually converted into limonite by weathering, and the acid solution of ferrous sulphate which sinks into the underlying deposits, must carry with it all manganese associated with the decomposing sulphide, also that in any disintegrating silicates and

AM. JOUR. SCI.-THIRD SERIES, VOL. XXXVI. No. 213.-SEPT., 1888.

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