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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 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.
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
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.
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 again.
The ore is taken down to the reduction works in waggons that run down by gravitation, and are drawn up again 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 then 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 passes 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 retreated 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. These 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 charge 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 goldextracting establishments being greatly due to the em
A, Lode; B, Decomposed doleryte ; C, Surface soil;
D, Quartz rocks in surface soil.
ployment of sodium in the amalgamating processes. 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.
The band of auriferous quartz veins worked at Santo Domingo continues westward for eight miles, as far as the savannahs near Libertad, and has been largely mined in the neighbourhood of that town, and between that point and 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 quartz, covering the faces of the hills in the neighbourhood of some of the lodes. In some places they form a broken but regular stratum over the whole side of a hill, and I was much puzzled at first to account for their origin.
I have already mentioned that the lodes near their summit incline over towards the face of the hill through which they cut. In some cases, as in the San Antonio mine, the lode is in parts bent completely round, as shown in the annexed section. This bending over of the lodes is always towards the face of the hill, and is, I think, produced by successive small land slips. 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 has, I think, been achieved in some places, and a regular “Manto" produced. I have already stated that small landslips are of frequent occurrence on the sides of the hills. We had several times the entrance to our minés closed by them in the wet season.