Page images

Ch. VI.]



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.

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. Since 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 calc 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 give 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. Carrieù 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.

Ch. VI.]



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 vaghs 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.

[ocr errors][merged small]

Climate of the north-eastern side of Nicaragua-Escursions around

Santo Domingo-The Artigua-Corruption of ancient Names
Butterflies, Spiders, and Wasps-Humming-birds, Beetles, and
Ants-Plants and Trees—Timber—Monkey attacked by Eagle
White-faced Monkey — Anecdotes of a tame one- Curassows
and other game Birds—Trogons, Woodpeckers, Motmots, and

THE climate of Santo Domingo and of the whole north-eastern side of Nicaragua is a very damp one. The rains set in in May, and continue with occasional intermission until the following January, when the dry season of a little more than three months begins. Even during the short-lived dry season there are occasional rains, so that although the roads dry up, vegetation never does, the ground in the woods is ever moist, and the brooks perennial. In the shady forest, mosquitoes and sand-flies are rather troublesome; but the large cleared space about the houses of the mining company is almost free from them, and in the beautiful light evenings one can sit under the verandahs undisturbed, watching the play of the moonbeams on the waving silky leaves of the bananas, or the twinkling north star just peeping over the range in front, or

or “Charlie's Wain" in the upper half of its endless circlings. In the opposite direction we can see the beautiful constellations of the southern hemisphere, whilst on the darkest nights innumerable fire-flies flash

« EelmineJätka »