Alexander AGASSIZ - The naturalist in Nicaragua

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 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