ORE SHOOTS. 1227 



fissure for some distance on either side of it, the solutions furnished by the 

 main fissure being- excluded; but at a greater or less distance from the 

 intersection the solutions from the two sources become mingled and precipi- 

 tation takes place, thus producing rich ore shoots. This relation is very 

 well illustrated at the Silver Age mine, at Idaho Springs, Colo. Here the 

 ore shoots are connected with intersecting fractures. They are in the main 

 vein at places near, but not directly at intersections of the subordinate 

 fissures. Again, at the Last Dollar mine in the Cripple Creek district the 

 main vein is cut by numerous cross veins, which upon the whole have not 

 been very profitable; but at numerous places the very rich values are found 

 in the main veins comparatively close to the cross veins; that is, at distances 

 from them varying from 15 to 60 meters. Where ore shoots are found to 

 be connected structurally with barren side veins a consideration of the 

 minerals of the main vein and of the side veins ought to lead to more exact 

 knowledge concerning the manner of the precipitation of the metal; for 

 presumably the precipitation of the metals was connected with some of the 

 compounds which occur as gangue, and in the side veins these may be 

 different from those of the main vein. 



Side fractures may be at right angles to the main fractures or incline 

 to them. They may extend directly down the dip or pitch to the right or 

 left along the dip. Therefore almost any curious distribution of the rich 

 shoots may occur. All of these relations of fractures with lodes are beau- 

 tifully illustrated in the San Juan district. Here some fractures strike 

 parallel with the lodes, but dip at considerable angles with them. There 

 are also cross fractures which differ from the lodes both in strike and dip." 



Usually side streams bearing either metals or precipitating agents or 

 both do not issue equally all along the faults or joints, but may be largely 

 converged into large channels, and under such circumstances strong springs 

 enter the fissures. Where such springs empty into a room produced by 

 the structural features, discussed on pp. 1224-1225, bonanzas may be 

 formed, such as those of the Comstock lode. 



While the relative influence of the different sets of intersecting frac- 

 tures is very complex, in an individual mine a close study of the number, 

 order, and relations of the fractures and joints, many of which are, perhaps, 



"Ransome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. 

 U. S. Geol. Survey No. 182, 1901, p. 59. 



