The rocks of the district are dolerytes, with bands and protrusions of hard greenstones. The decomposition of the dolerytes is very great, and extends from the tops of the hills to a depth (as proved in the mines), of at least two hundred feet. Next 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 been ascribed, and probably with reason, to the percolation through the rocks of rain-water charged with a little acid from the decomposing vegetation. If this be so, the great depth to which it has reached tells of the immense antiquity of the forests.
Gold-mining at Santo Domingo is confined almost entirely to auriferous quartz lodes, no alluvial deposits having been found that will pay for working. The lodes run east and west, and are nearly perpendicular, sometimes dipping a little to the north, sometimes a little to the south, and near the surface, generally turning over towards the face of the hill through which they cut. The trend of the main ranges, also nearly east and west, is probably due to the direction of the outcrops of the lodes which have resisted the action of the elements better than the soft dolerytes. The quartz veins now form the crests of many of the ranges, but are everywhere cut through by the lateral valleys. The beds 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 reached, and taking into consideration the deep-seated forces required to produce fissures of such great length and regularity, we may safely assume that they run for miles deep into the earth—that their extension vertically is as great as it is horizontally. The possibility that they extend to immense depths is increased when we reflect that mineral veins occur in parallel groups that run with great regularity for hundreds of miles; and further by the fact that, in all the changes of the earth's surface, by which deep-seated rocks have been brought up and exposed by denudation, no instance is known of the bottom of a fissure-vein having been brought by such movements within the reach of man.
The gold-mines of Santo Domingo are in veins or loads of auriferous quartz that run parallel to each other, and are so numerous that across a band more than a mile in width one may be found every fifty yards. All that have been worked vary greatly in thickness; sometimes within a hundred yards a lode will thicken out from one to seventeen feet. Their auriferous contents vary still more than their width. The richest ore, 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 per ton. The gold does not occur pure, but is a natural alloy of gold and silver, containing about three parts of the former to one of the latter. Besides this metallic alloy (to which, for brevity, I shall, in the remarks I have to make, give its common designation of gold), the quartz lodes contain sulphide of silver, peroxide of manganese, peroxide of iron, sulphides of iron and copper, and occasionally ores of lead.
The quartz is generally very friable, full of drusy cavities, and broken up into innumerable small pieces that are often coloured black by the peroxide of manganese. The gold is in minute grains, and generally distributed loosely amongst the quartz. Pieces as large as a pin's head are rare, and specimens of quartz showing the gold in 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 agents, and long practice has made them very expert in valuing the ore by the wash in the "spoon." Although most of the gold occurs loose, amongst the soft portions of the lode, the hard quartz also contains it disseminated in minute grains throughout. These can be obtained in the horn by pounding the quartz to powder and then washing it.
(PLATE 6. MACHINERY OF CHONTALES GOLD-MINING COMPANY.)
One feature in the distribution of gold in the quartz lodes of Santo Domingo led to a most exaggerated opinion of their value when they were first mined by English companies. On the hills, near the outcrops of the lodes, the ore was in some places exceedingly rich. One thousand ounces of gold were obtained from a small patch of ore near the surface of the Consuelo lode, and at Santo Domingo, San Benito, San Antonio, and Javali lodes, very rich ore was also discovered within a few fathoms of the surface. When, however, these deposits were followed downwards, they invariably got poorer, and at one hundred feet from the surface, 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 the gold they contained in the lode below that worn away. We have seen that in the decomposed parts of the lode the gold exists in loose fine grains. During the wet season water percolates freely from the surface down through the lodes, and the gold set free by the decomposition of the ore at the surface must be carried down into it, so that in the course of ages, during the gradual degradation and wearing away of the surface, there has, I believe, been an accumulation of the loose gold in the upper parts of the lodes from parts that originally stood much higher, and have now been worn away by the action of the elements.
This accumulation of loose gold near the surface of auriferous veins, set at liberty from its matrix by the decomposition of the ore, and concentrated by degradation, is probably the reason of the great richness of many of what are called the "caps" of quartz veins; that is, the parts next the existing surface, and has also, perhaps, originated the belief that auriferous lodes deteriorate in value in depth. I at one time, after having studied the auriferous quartz veins of Australia, advocated this theory, which was first insisted upon by Sir R.I. Murchison, but further experience in North Wales, 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 patches of rich ore in depth, after one has been passed through, are about the same as they are in driving horizontally, and the frequency therefore with which the auriferous ores are met with along the surface will, as a rule, be an index of their occurrence in depth, if we be careful in distinguishing deposits belonging to the original condition of the lodes, and those due to subsequent concentration. To do this we must get below the immediate surface, and take as our guide the gold occurring in the solid undecomposed quartz, and not the loose grains contained in the fissures and cavities.
(PLATE 7. SECTION OF MINE SHOWING METHOD OF EXTRACTING THE ORE.
SECTION OF GOLD MINE.
Diagram showing method of excavating ore at Santo Domingo Mines.
A, Levels.
B, Rise, down which the ore is thrown.
D, Stopes.
C, Stopes refilled with clay and barren rock.
Lowest level, Tramway to Stamps.)
The lodes of Santo Domingo are worked by means of levels driven from near the bottoms of the valleys that intersect them. When these levels have entered sufficiently far into the hills, shafts are driven upwards from them to the surface, and other levels driven sixty feet higher than the first. This process is continued until the lode lying above the lowest level has been divided off into horizontal bands, each about sixty feet in depth. The quartz is 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 with common soft wood, and at the end of every fortnight filled up with clay and barren rock. The mining is entirely executed by native workmen, principally Mestizos from the border lands of Honduras and Nicaragua, where they have been engaged in silver-mining. They are paid according to the amount of ground excavated, and are very industrious when poor; but when they accumulate a little money, they take fits of idleness and dissipation until it is spent.
The ore is taken down to the reduction works in waggons that run down by gravitation, and are drawn up by mules. It is then stamped to powder by iron beaters, each of which is lifted by cams, and let fall seventy times per minute. The stamped ore, in the form of fine sand, is carried by a stream of water over inclined copper plates covered with mercury, with which is mixed a little metallic sodium. Nearly the whole of the free gold is caught by the mercury, for which it has a great affinity, and accumulates as amalgam on the copper plates, from which it is cleaned off every twelve hours. The sand and water then pass over inclined tables covered with blankets, the fibres of which intercept particles of gold and mercury that have escaped from the first process, and afterwards into a concentrating box, where the coarsest grains of sand 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 about one hundred ounces each, and in that state sent to England. At Santo Domingo about two thousand tons of ore are treated monthly, and the whole cost of treatment, including all charges for mining, carriage, reduction, amalgamation, and management, is only about eight shillings per ton. The loss of mercury is about twenty pounds for every thousand tons of ore treated; the smallness of the loss in comparison with that of many other gold-extracting establishments being greatly due to the employment of sodium in the amalgamating process. The loss of mercury usually occurring in amalgamation work is principally caused by its mineralisation, and sodium has such an intense affinity for oxygen and sulphur, that it reduces the mercury to its metallic form again, and prevents its being carried off in light mineralised flakes and powder.