Page images
PDF
EPUB

various spars one on the other in cavities is a secondary operation even 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 by other deposits, 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, invading 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, either granite or greenstone; but as the rocks gradually cooled, the fissures reached greater

* “Mineral Veins,” by Thomas Belt. John Weale, 1861.

[ocr errors][merged small][ocr errors][ocr errors]

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 exposed 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, along with veinstones of calcite derived from neighbouring beds of limestone, some minerals due to a previous igneous injection. Crevices and cavities, called vughs 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 super-heated 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][merged small]

· Climate of the north-eastern side of Nicaragua—Excursions around

Santo Domingo—The Artígua—Corruption of ancient namesButterflies, spiders, and wasps - Humming-birds, beetles, and ants -Plants and trees—Timber—Monkey attacked by eagle-Whitefaced monkey-Anecdotes of a tame one-Curassows and other game birds—Trogons, woodpeckers, mot-mots, and toucans.

The climate of Santo Domingo and of the whole northeastern 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 summer 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 silky, leaves of the bananas, the twinkling north star just peeping over the range in front, with “Charlie's Wain” in the upper half of its endless circlings, whilst in the opposite direction the eye rests on the beautiful constellations of the southern hemisphere. On the darkest nights innumerable fire-flies flash their intermittent lights as they pass amongst the low bushes or herbage, making another twinkling firmament on earth. On other evenings, sitting inside with lighted candles and wide opened doors, great bats flap inside, make a round of the apartment, and pass out again, whilst iris-winged moths, attracted by the light, flit about the ceiling, or long-horned beetles flop down on the table. In this way I made my first acquaintance with many entomological rarities.*

The heaviest rains fall in July and August, and at these times the brooks are greatly swollen. The one in front of my house sometimes carried away the little wooden bridge that crossed it, and for an hour or two became impassable, but subsided again almost as soon as the heavy rain ceased falling, for the watershed above does not extend far. Every year our operations were impeded by runs in the mines, or by small landslips stopping up our tramways and levels, or floods carrying away our dam or breaking our watercourses; but after August we considered our troubles on this score at an end for the season. Occasionally the rains lasted three or four days without intermission, but generally they would come on in the afternoon, and there would be a downpour, such as is only seen in the tropics, for an hour or two, then some clear weather, until another great bank of clouds rolled up from the north-east and sent down another deluge. In September, October, and November there are breaks of fine weather, sometimes lasting for a fortnight; but December is generally a very wet month, the rains ex

* In moths, numerous fine Sphingidæ and Bombycidæ ; and in beetles, amongst many others, the rare Xestia nitida (Bates) and Heroplon albipenne (Bates) were first described from these evening captures.

« EelmineJätka »