Page:Popular Science Monthly Volume 72.djvu/541

 waters. It is even possible in some cases to determine by the character of the minerals the origin and nature of the causative solutions.

As to the relative importance of the work of ascending and descending waters there is also divergence of opinion. There are few who still doubt the agency of descending waters in the formation of the oxidized ores, such as carbonates, silicates and oxides of copper, lead and zinc and silver chloride, or in the superficial or shallow alteration of the sulphides, arsenides or antimonides. The iron ores of the Lake Superior region, for example, are generally believed to owe their concentration to descending solutions, in this respect differing from many of the Scandinavian iron ores, according to recent descriptions.

It is not, however, the oxidized or "dry" ores alone that are now believed to owe their formation in large part to the action of descending waters; but the base ores consisting of chemical combinations of the metals with sulphur, arsenic, antimony, tellurium and some rarer elements. It is only within the past decade that it has been considered possible that the sulphide minerals are produced by reaction between sulphate or carbonate solutions and undecomposed sulphides or other minerals found in veins. Laboratory experiments have, however, shown that the operation is not only possible, but easily accomplished and duplicated under normal conditions as to temperature and pressure. This is a fact of great importance and wide significance, for it aids in the explanation of many formerly puzzling phenomena of mines and mining geology.

It has long been noticed by the students of ore deposits that by far the greater number of mines become exhausted at comparatively shallow depths; that veins, instead of continuing downward uniform in size and composition, like dikes of diabase and porphyry, become smaller and of lower value with depth, and often disappear altogether. It is noticed also that the shape of many ore deposits and the distribution and paragenesis of the minerals which they contain can often be better explained on the theory of descending than of ascending mineralizers. Moreover, it is apparent that there are changes constantly in progress in those portions of sulphide ore bodies lying nearest the surface of the ground. These changes consist in the oxidation of the sulphides and their solution as sulphates. These sulphate solutions percolate downward into the veins or rocks below along the most open channels; and thus, by degrees, the upper zone of the vein is robbed of most or all of its sulphide minerals, and only a gossan or iron cap remains.

The process of oxidizing and leaching out of the sulphides in the superficial zone of ore deposits tends, first of all, to disguise the nature