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ORIGIN OF MINERAL VEINS.
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 veinstones 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.
Climate of the north-eastern side of Nicaragua-Excursions around
Santo Domingo – The Artigua-Corruption of ancient Names –
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 “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 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 lap inside, make a round of the apartment, and pass out again, or iris-winged moths, attracted by the light, flit about the ceiling, or long-horned beetles flop down on the table; and 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 De
In moths, numerous fine Sphingidæ and Bombycidæ ; and in beetles, amongst many others, the rare Xestia nitida (Bates) and Hexoplon albipenne (Bates) were first described from these evening captures.
cember is generally a very wet month, the rains extending far into January, so that it is not until February that the roads begin to dry up.
I had much riding about. The mines worked by us, when I first went out, extended from Consuelo, a mile higher up the valley, to Pavon, a mile below Santo Domingo; and even after I had concentrated our operations to those nearer to our reduction works, there were many occasions for me to ride into the woods. I had to look after our woodcutters and charcoal-burners, to see that they did not encroach upon the lands of our neighbours, as they were inclined to do, and involve us in squabbles and lawsuits ; paths were to be opened out, to bring in nispra and cedar timber; our property had to be surveyed, and new mines, found in the woods, visited and explored. Besides this, I spent most of my spare time in the forest, which surrounded us on every side, so that we could not go a mile in any direction without getting into it: longer excursions were frequent. The Nicaraguans, like all Spanish Americans, are very litigious, and every now then I would be summoned, as the representative of the company, to appear at Libertad, Juigalpa, or Acoyapo, to answer some frivolous complaint, generally made with the expectation of extorting money, but entertained and probably remanded from time to time by the often unscrupulous judges, who are so badly paid by the government that they have to depend upon the fees of suitors for their support, and are often open to corruption. These rides and strolls into the woods were very fruitful in natural-history acquisitions and observation. I shall give an account of some of those made in the immediate vicinity of Santo Domingo, and I wish I could transfer to