54 o POPULAR SCIENCE MONTHLY 



ically 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 ac- 

 complish greater results than a small supply, provided it follows the 

 course of the ore. For the metals in solution can hardly escape pre- 

 cipitation 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 oxida- 

 tion 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 region?, 

 that erosion is again the most active agent. 



The second of our factors is time; a commodity of which the geolo- 

 gist 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. 



