ORES DEPOSITED BY AQUEOUS SOLUTIONS. 1063 



Generalizing in reference to districts, it may be said that, excluding sedi- 

 mentary deposits, probably all of the great mining camps of the world show 

 the phenomena which indicate deposition by aqueous solutions. Certainly 

 this is true for America. The ore deposits which have been shown to be 

 formed by gaseous solutions, or by magmatic segregation, are at compara- 

 tively small and little known camps. 



Objection has been raised to the above general statements as to the 

 importance of aqueous solutions, on the ground that in many ore-bearing 

 districts there is not now a vigorous aqueous underground circulation." 

 This may be true of deposits either in humid or in arid regions. Many 

 important humid districts could be mentioned in which the circulation is at 

 present very feeble. For instance, in the deep copper deposits of the Lake 

 Superior region there is little water. The same is true of the San Juan 

 districts of Colorado, of the deep mines of Przibram, and of many other 

 places in the world. Another instance in which deep workings have shown 

 very little circulating water is that of the Newhouse tunnel, at Idaho 

 Springs, Colo. This tunnel, more than 4,000 meters long, in a granite- 

 gneiss-schist complex of rocks, shows but an insignificant amount of water. 

 A small amount percolates in at different places, but the quantity is so small 

 that it is ignored in the drifting arrangements. For arid regions with small 

 circulation the copper mines of New Mexico and Arizona furnish admirable 

 illustrations. Indeed it not infrequently happens that in such regions, after 

 one reaches a moderate depth, the levels are dry, or even dusty. 



But from the above facts it is a mistake to infer that when the orgs 

 were deposited there was not a vigorous circulation. As has been so fully 

 explained in various places, the very process of cementation lessens the size 

 and number of the openings, and when the process nears completion the 

 openings all become subcapillary and circulation practically ceases. We 

 have already seen that such are the facts in reference to the Lake Superior 

 conglomerates, the San Juan tuffs, and to the rocks of various other regions. 

 The original openings, and the deformation cracks and crevices have been 

 so thoroughly cemented, and since that time so few new fractures have been 

 formed, that but little water can find access from the surface. 



« Kemp, J. F. , The role of the igneous rocks in the formation of veins: Trans. Am. Inst. Min. Eng., 

 vol. 31, 1902, pp. 184-198. Rickard, T. A., Water in veins, a theory: Eng. and Min. Jour., vol. 75, 

 1903, pp. 402-403. 



