The Naturalist in Nicaragua

f the ground at

ion of

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tion of t

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us quart

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nd reduction

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eins: their connect

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nd. From the main valleys numerous auxiliary ones cut deeply into the ranges, and bifurcate again and again, like the branches of a tree, forming channels for carrying off the great quantity of water that falls in these rainy forests. The branching vall

mposed. Even during the few years I was in Nicaragua there were some modifications of the surface effected; I saw the commencement of new valleys, and

the surface they are often as soft as alluvial clay, and may be cut with a spade. This decomposition of the rocks near the surface prevails in many parts of tropical America, and is principally, if not always, confined to the forest regions. It has b

ds of doleryte lie at low angles, through which the quartz veins cut nearly vertically. Excepting that they are very irregular in thickness, and often branch and send thin offshoots into the enclosing rocks, they resemble coal seams that have been turned up on edge, so as to be vertical instead of horizontal. They run for a great distance. Near Santo Domingo they had been traced for two miles in length, and probably they extend much further. They are what are called fissure-veins, owing their origin to cracks or fractures in the rocks that have been filled up with mineral substances through chemical, thermal, aqueous, or plutonic agencies. In depth, the bottom of fissure-veins has never been

worth from one to four ounces per ton, occurs in irregular patches and bands very small in comparison with the bulk of the ore stuff, which varies in value from two to seven pennyweights per ton. The average value of all the ore treated by the Chontales Mining Company, up to the end of 1871, has been about seven pennyweights per ton, and during that time small patches have been met with worth one hundred ounces of gold

it are seldom met with, even in the richest portions of a lode. The fine gold-dust can, however, easily be detected by washing portions of the lode-stuff in a horn. The quartz and clay is washed away, and the gold-dust sinks to the bottom, and is retained in the horn. This is the usual way in which a lode is tested by the mining age

Y OF CHONTALES GOL

ace, no very rich ore had been met with. Below that, when the works are prosecuted still deeper, there does not appear to be any further progressive deterioration in the value of the ore, and it varies in yield from two to seven pennyweights of gold per ton, upon which yield further depth does not seem to have any effect. The cause of these rich deposits near the surface does not appear to me to be that the lodes originally, before they were exposed by denudation, contained more gold in their upper portions than below, but to be the effect of the decomposition and wearing down of the higher parts, and the concentration of

ales, Nova Scotia, Brazil, and Central America has led me to doubt its correctness, excepting in cases such as we have been considering, where there has been an accumulation of gold in the superficial portions of lodes since their original formation. Gold is distributed in quartz veins in bands, and in patches of richer stone of more or less extent. These richer portions of the lodes, if sunk upon perpendicularly, will be passed through, but so also they would be if followed horizontally, their extent in one direction being as great as it is in the other. The chances of meeting with further

MINE SHOWING METHOD O

OF GOL

d of excavating ore a

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n which the

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led with clay a

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then excavated above the topmost level, and thrown down the shafts to the lowest, where it is received into waggons and conveyed to the reduction works. As both the ore and the enclosing rocks are greatly decomposed and very soft, the whole of the ground has to be securely timbered as the work proceeds. The levels are timbered with "nispera," a wood of great durability and strength, but the excavated portions between them are only temporarily secured w

and and the sulphurets of iron, copper, and silver are caught, and with the sand from the blankets re-treated in arrastres. These arrastres are round troughs, twelve feet in diameter, paved with stones. Four large stones of quartz are dragged round and round in this trough, and grind the coarse sand to fine powder. The gold liberated sinks into the crevices in the stone pavement, a little mercury being put into the trough to form it into amalgam. The arrastres and all the amalgamating apparatus is cleaned up once a month. The amalgam obtained is squeezed through thin dressed skins, and is then of the consistence of stiff putty, and of a silver colour. These balls of amalgam are placed in iron retorts, and the mercury driven off by heat and condensed again in water. The balls of gold so obtained are then melted into bars weighing

ION ACROSS SAN

L

mposed

rface

ocks in sur

d Santo Domingo. Besides the working of the mines proper, some surface deposits, called by the Spaniards "Mantos," are also worked for gold, especially in the neighbourhood of Libertad. The "Mantos" consist of broken quart

of the lodes is always towards the face of the hill, and is, I think, produced by successive small landslips. It is evident that if carried still further than in the case shown in the diagram, the lode would be brought down over the face of the hill, and the result

lutonic rocks that has been thoroughly explored, a similar succession of events, culminating in the production of mineral veins, has been proved to have taken place,* (* "Mineral Veins" page 16.) and it appears that the origin of such veins is the natural result of the plutonic intrusion. There is, also, sometimes a complete gradation from veins of perfectly crystallised granite, through others abounding in quartz at the expense of the other constituents, up to veins filled with pure quartz, as at Porth Just, near Cape Cornwall; and, again, the same vein will in some parts be filled with felspar; in others, contain irregular masses of quartz, apparently the excess of silica beyond what has been absorbed in the trisilicate compound of felspar.* (* Mr. John Phillips in "Memoirs, Geological Survey of Great Britain" volume 2 page 45.) Granitic, porphyritic, and trappean dykes* also sometimes contain gold and other metals; (* Sir R.I. Murchison "Siluria" pages 479, 481, 488 and 500; and R. Daintree "Quarterly Journal of the Geological Society" volume 28 pages 308, 310.) and I think the probability is great that quartz veins have been filled in the same manner-that if dykes and veins of granite have been an igneous injection, so have those of quartz. By an igneous injection, I do not mean that the fused rock owed its fluidity to dry heat. The celebrated researches of Sorby on the microscopical fluid cavities in the quartz of granite and quartz veins, have shown beyond a doubt that the vapour of water was present in comparatively large quantities when the quartz was solidifying. All strata below the surface contain water, and if melted up would still hold it as super-heated 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

rior, 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 metals, all readily dissolved and re-formed by hydro-thermal agencies. Our mineralogical museums are filled with beautiful specimens of crystals of quartz, fluor spar, and various ores deposited one on

re fully into it in another place,* (* "Mineral Veins" by Thomas Belt. John Weale 1861. ) I shall onl

great heat, which, aided by the water contained in the rocks and various chemical reactions, has effected a re-arrangement of the mineral contents o

been completely fused, and the liquid or pasty mass, invading the contorted str

ither 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

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,

d 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

z 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, exceptin