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Configuration of the ground at Santo Domingo-Excavation of valleys
-Geology of the district--Decomposition of the rocks—Gold-mining-Auriferous quartz veins—Mode of occurrence of the goldLodes richer next the surface than at lower depths-Excavation and reduction of the ore—Extraction of the gold—“Mantos
rigin of mineral veins — Their connection with intrusions of Plutonic rocks.
THERE is scarcely any level land around Santo Domingo, but in every direction a succession of hills and valleys. The hills are not isolated; they run in irregular ranges, having mostly an east and west direction, but with many modifications in their trend. 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 valleys, all leading into main ones, and these into the rivers, have been excavated by sub-aërial agency, and almost entirely by the action of running water. It is the system that best effects the drainage of the country, and has been caused by that drainage.
The wearing out of valleys near Santo Domingo proceeds more rapidly than in regions where less rain falls, and where the rocks are not so soft and decomposed. 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 widening and lengthening of others, caused not only by the gradual denudation of the surface, but by landslips, some of which occur every wet season.
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 probability 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 deepseated 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 lodes 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 designa
tion 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 lodestuff 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.
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 Jayali lodes, very rich ore was also discovered within a few fathoms of the surface. When, however, these deposits