superheated steam; and M. Angelot has suggested that fused rock under great pressure may dissolve large quantities of the vapour of water, just as liquids dissolve gases. The presence of the vapour of water would cause the liquefaction of quartz at a much lower temperature than would be possible by heat alone, unaided by water.* I know that this opinion is contrary to that usually held by geologists, the theory generally accepted being that mineral veins have been produced by deposits from hot springs; but during twenty years I have been engaged in auriferous quartz mining in various parts of the world, and nowhere have I met with lodes, the phenomena of which would be explained on this hypothesis. The veinstone is pure quartz containing water in microscopical cavities, as in the quartz crystals of granite, but not combined as in the hydrous siliceous scoriæ deposited from hot springs. The lodes are not ribboned, but consist of quartz, jointed across from side to side, exactly like trappean dykes. There is often a banded arrangement produced by the repeated re-opening and filling of the same fissure; but never, in quartz veins, a regular filling up from the sides towards the centre, as in veins produced by deposits from springs. Quartz veins extend sometimes for miles, and it is necessary to suppose on the hydro-thermal theory that they kept open sufficiently long for the gradual deposition of the veinstones, without the soft and shattered rocks at their sides falling in, nor yet fragments from above; although there are many lodes, fully twenty feet in width, filled entirely with quartz and mineral ores, without any included fragments of fallen rocks, * H. C. Sorby, Jour. Geol. Soc., vol. xiv.

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and nowhere showing any trace of regular deposition on the sides. The gold also found in auriferous lodes is never pure, but forms various alloys of gold, silver, copper, lead, iron, and bismuth; and no way is known of producing these alloys except by fusion.

Since

It is true that mineral veins contain many minerals that could not exist together undecomposed with even a moderate degree of heat; but it is only here contended that the original filling of the lodes was an igneous injection, not that the present arrangement and composition of all the minerals is due to the same action. the lodes were first filled, they have been subjected to every variety of hydro-thermal and aqueous influence; for the cooling of the heated rocks must have been a slow process, and undoubtedly the veins have often been the channels both for the passage of hot water and steam from the interior, and of cold water charged with carbonic acid and carbonate of lime from the surface, and many changes must have taken place. Auriferous quartz veins have resisted these influences better than others, because neither the veinstone nor the metal is easily altered, and such veins therefore form better guides for the study of the origin of mineral lodes than fissures filled with cale spar and ores of the baser metal, all readily dissolved and reformed by hydro-thermal agencies. Our mineralogical museums are filled with beautiful specimens of crystals of quartz, fluor spar, and various ores deposited one on the other; and the student who confines his attention to these is naturally led to believe that he sees before him the process by which mineral veins have been filled. But the miner, working far underground, knows that such crystals are only found in cavities and fissures, and that

the normal arrangement of the minerals is very different. The deposition of various spars one on the other in cavities is a secondary operation ever now going on, and has nothing necessarily to do with the original filling of the lodes; indeed, their arrangement is so different that it helps to prove they have been differently formed.

It would take a volume to discuss this question in all its bearings, and as I have already entered more fully into it in another place,* I shall only now givé a brief résumé of the conclusions I have arrived at respecting the origin of mineral veins.

1. Sedimentary strata have been carried down, by movements of the earth's crust, far below the surface, covered with other strata, and subjected to great heat, which, aided by the water contained in the rocks and various chemical reactions, has effected a re-arrangement of the mineral contents of the strata, so that by molecular movements the metamorphic crystalline rocks, including interstratified granites and greenstones, have been formed.

2. Carried to greater depths and subjected to more intense heat, the strata have been completely fused, and the liquid or pasty mass, including the contorted strata above it, has formed perfectly crystalline intrusive granites and greenstones.

3. As the heated rocks cooled from their highest parts downwards, cracks or fissures have been formed in them by contraction, and these have been filled from the still fluid mass below. At the beginning these injections have been the same as the first massive intrusive rocks,

* "Mineral Veins," by Thomas Belt. John Weale, 1861.

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either granite or greenstone; but as the rocks gradually cooled, the fissures reached greater and greater depths; and the lighter constituents having been drawn off and exhausted, only the heavier molten silica, mingled with metallic and aqueous vapours, has been left, and with these the last-formed and deepest fissures have been filled. These injections never reached to the surface,probably never beyond the area of heated rocks; so that there have been no overflows from them, and they have only been exposed by subsequent great upheaval and denudation.

4. Probably the molten matter was injected into the fissures of rocks already greatly heated, and the cooling of these rocks has been prolonged over thousands of years, during which the lodes have been subjected to every degree of heat, from that of fusion to their present normal temperature. During the slow upheaval and denudation of the lodes, they have been subjected to various chemical, hydro-thermal, and aqueous agencies, by which many of their contents have been re-arranged and re-formed, new minerals have been brought in by percolation of water from the surrounding rocks, and possibly some of the original contents have been carried out by mineral springs rising through the lines of fissures which are not completely sealed by the igneous injection, as the contraction of the molten matter in cooling has left cracks and crevices through which water readily passes.

5. Some of the fissures may have been re-opened since they were raised beyond the reach of molten matter, and the new rent may have been filled by hydro-thermal or aqueous agencies, and may contain alloy with vein

stones of calcite derived from neighbouring beds of limestone, and some minerals derived from a previous igneous injection; and crevices and cavities, called raghs by the miners, have been filled more or less completely with crystals of fluor spar, quartz, and various ores of metals from true aqueous solutions, or by the action of superheated steam.

6. By these means the signs of the original filling of many mineral lodes, especially those of the baser metals, have been obscured or obliterated; but in auriferous quartz lodes both the metal and the veinstone have generally resisted all these secondary agencies, and are presented to us much the same as they were first deposited, excepting that the associated minerals have been altered, and in some cases new ones introduced, by the passage of hot springs from below or percolation of water from the surface.