C.—s

55

(8.) That the main mass of argentiferous lead-ores is found in calcareo-magnesian rocks. (9.) That the siliceous rocks, porphyries, and crystalline rocks contain proportionately more gold than copper. On this subject Mr. Curtis states that the ore-deposits in the Eureka Mining District do not differ from the Leadville (Colorado) District, except that their metal-hearing solutions came from below, and that their connection with the eruptive rocks is not plainly shown. The Shape of Deposits. —The deposits sometimes spread out into an immense chamber that measures more than 50ft. in each direction, and which is completely filled with ore, with the exception of an occasional limestone pillar. From the sides of these chambers, which scarcely ever present smooth walls, there are branches, and auxiliary pipes lead up or down or in a horizontal direction to the other bodies. The ore-bodies do not seem to follow any particular direction either as regards dip or strike, and at the first sight they appear to be disturbed throughout the ore-bearing formation without any regularity. This is not wholly the case, and, although no well-defined law can be found governing their occurrence, this is connected with that of certain phenomena in the country rock, such as fissures, caves, and broken limestone. Deposition of the Ore in Chambers. —The ore in the upper part of the larger chambers is mostly in a loose state, sometimes in layers, and is usually covered by beds of sand, gravel, and boulders of variable thickness. It is difficult to believe that this mass owes its structure to any other cause than rearrangement by subterranean water-currents, though it is not likely that the original position of the material was remote from that which it now occupies. There is, of course, every reason to suppose that the waters either from the surface or from below have flowed through these rocks in notable quantities ever since they were intersected by fissures ; but the floods which have left the traces just described in the upper portions of the chambers must be comparatively recent, since the stratified ore has been rearranged since its oxidation. In the lower part of the chambers, on the other hand, the ore is more compact, and usually appears as if it occupied its original position. Connection of Ore-bodies with Quartzite. —In many of the mines in the Eureka Mining District the ore is usually found in the limestone at or near its contact with the quartzite, except close to the surface, where it is generally at some distance from that formation. Although a complete connection has not been established between all these ore-bodies and the quartzite, or between all the ore-bodies themselves, yet their location and the relation that they bear to the secondary or contact fissure between the quartzite and the limestone indicate that the fissure has often served as an ore-channel. The same thing occurs in the North Island with relation to what are called buck reefs, and the lodes in which gold has been found. On the Tokatea Eange, at Coromandel, there is a connection of all the gold-bearing leaders with the main Buck Eeef that goes through the range. Again, in the Waiotahi Mines, at the Thames, the large or main reef does not appear to carry a great deal of gold ; nevertheless there is a connection between this and the lodes and leaders that have been worked and found to have been rich in precious metals. In this mine there are fissures in every direction, the most of which contain rich auriferous deposits. This tends to showthat the fissure in which the large body of quartz is now found was a channel through which the subterranean waters flowed, carrying the mineral solution, and from which they found their way into the joints, crevices, and fissures of the rock in the vicinity. Whether the solution of these minerals came from below or from the surface, or percolated through the adjoining rock into these fissures, is a problematical question which has not been solved. It is nevertheless a question of considerable importance to the miner to ascertain the source of the ore, and how the different minerals were deposited, in order to carry on prospecting mining operations on a scientific and intelligible basis. Far more attention has been directed to this subject of late years in America than in other gold- and silver-producing countries. This is not to be wondered at, seeing the magnitude on which mining operations are carried on, and the enormous annual yields of gold and silver compared with that of any other country. Mr. Curtis attributes solfataric action to have acted a prominent part in decomposition and in the formation of minerals. With regard to the source of the ore in the Eureka Mining District, Mr. Curtis states that there is a strong probability, if it is not absolutely certain, that the eruption of rhyolite preceded the deposition of the ore. Extensive eruptions of this rock took place at no great distance from the mines, and, as has been described, a dike of it follows one of the chief fissures of the mineral zone. The decomposition of this dike, and of other rocks accompanying it, especially the quartz-porphyry, is such as is characteristic of volcanic regions, and its occurrence must almost inevitably be ascribed either to the rhyolite eruption or to the still more recent outburst of basalt. There is no basalt, however, in or near the mines, and therefore nothing to indicate a connection between its ejection and the deposition of the ore. The solfataric action traceable in the mines is therefore most naturally referred to the rhyolite eruption. It is, of course, no objection to this hypothesis that the rhyolite is itself decomposed, since the decomposition of lavas within a few days after their ejection, by the gases and solutions of the same eruption has frequently been observed, while the period of the rhyolite eruptions near Eureka may have covered centuries. The character of the decomposition of the rhyolite is familiar, and consists largely in the extraction of the heavy bases and alkalies, leaving siliceous clay as a residue. Sulphuretted hydrogen is almost invariably an accompaniment of volcanic action, and the alkalies in solution were in part converted into sulphides. As most of the sulphides of metals are soluttle in solutions of the alkaline sulphides, a vehicle was thus formed for the transportation of any metallic sulphides which might be present. Those sulphides, or compounds which w 7ould yield them, might have formed constituents of the rhyolite, but, as a matter of fact, there is no rhyolite in the vicinity of the ore which contains sufficient gold, silver, or lead to admit of its being regarded as the source of these minerals in the ore. The rhyolite, however, is not the only eruptive rock met with in the mines—quartzporphyry also occurs. This rock, however, contains considerable quantities relatively of gold