Page:Popular Science Monthly Volume 72.djvu/544

 instead of chemically there will be little opportunity for the deposition of secondary concentrations of ores within the rocks. If, for example, the principal effect of the rains and snows is to erode and wash away the exposed portions of veins with all their contained ores, there will be a scattering and wasting instead of an assembling and storing. In other words, secondary enrichment by descending waters depends first of all upon the ratio of oxidation to erosion. Where erosion is more rapid than oxidation the unoxidized sulphides will be found in the rocks and veins at the surface of the ground, and in the sands rolling down the beds of torrential streams as in Alaska. While if oxidation precedes erosion the uppermost zone of a sulphide ore deposit will be oxidized and leached of its base minerals, as is the case here in Butte, and to varying extent over the larger portion of the temperate zones of the earth. Assuming that the conditions are such as to permit the entrance of surface waters, and that the ground-water level is at some depth, which depth naturally varies from year to year and age to age because of many common geological phenomena, the factors upon which depend the extent of secondary enrichment are: (1) Quantity of water, (2) time, (3) temperature, (4) the physical structure and solubility of the rock containing the primary ore, and of the ore itself.

It is manifest that a large supply of mineralizing solution will accomplish greater results than a small supply, provided it follows the course of the ore. For the metals in solution can hardly escape precipitation by reaction with the primary sulphides present, sooner or later, at some depth; and the oxidizing and dissolving effects will certainly increase with the amount of active oxygen-bearing moisture available. In regions of very little rainfall there may be partial oxidation to the depth of several hundred feet; and yet there may still remain particles of the primary sulphides upon the very surface of the rocks. Chemical activity is great; but the thirsty rocks quickly absorb that part of the water of rains and melting snows which is not evaporated, and the work of oxidation is not so complete as in regions more plentifully supplied with rain. On the other hand, there may be such heavy and constant downpourings of rain, even in tropical regions, that erosion is again the most active agent.

The second of our factors is time; a commodity of which the geologist is accustomed to make most liberal and even extravagant use in his arguments and theories. In this he is frequently justified; and the most astonishing results may be produced by the long continued but slow operation of natural forces in any given direction. Events of the past few years have, however, reminded us forcibly that catastrophic phenomena must not be forgotten in comprehensive reviews of the earth's history.