1226 A TREATISE ON METAMORPHISM. 



Australasian and Nova Scotian gold ores and the Lake Superior iron ores, 

 these relations are perfectly clear. Doubtless in many mines there are 

 minor flexures which have been overlooked, but which may be sufficient 

 to control the movement of the circulation, and thus produce the 

 chimneys of ore. These minor flexures may be parallel with the dip of 

 a deposit, or they may pitch to the right or to the left as one looks 

 down the dip. 



The intersections of fractures furnish one of the most frequent explana- 

 tions of ore shoots. The intersections may be those of faulted fissures, 

 those of fissures and joints, or the intersections of joints. In many instances 

 one set of fractures carries the larger ore deposits, and the intersecting set 

 or sets of fractures are known as side fractures. In some instances deposits 

 may occur in two or more sets of intersecting fractures, and in others one 

 of the intersecting sets may be barren of deposits. 



In all cases where intersecting fractures occur solutions are contrib- 

 uted from two or more sources. The solutions invariably have different 

 compositions, and therefore precipitation is likely to occur at or near the 

 iunctions. In some cases metalliferous material may be furnished by more 

 than one set of fractures, while in other cases it may be contributed by one 

 set of fractures and the precipitating agents by the others. In those 

 instances where the intersecting veins all carry ore it is easy to see why the 

 deposits at the intersections are unusually large and rich. Where the side 

 veins are small or are wholly filled with gangue material their importance 

 in the genesis of ore deposits has been very generally overlooked. In 

 many instances there is little doubt that the metallic material was precip- 

 itated in a main fissure at or near the point where the side veins join, 

 through the influence of the solutions contributed by the latter veins. A 

 very clear case of the influence of barren side veins is that already cited of 

 the Enterprise mine, of Rico, Colo., where the pay shoots are especially rich 

 in the main fissures at the places where barren side veins intersect them. 



Not infrequently, where a side fracture intersects a vein carrying the 

 main deposit, the high values are not exactly at the intersection, but a 

 little way from.it. This is a natural consequence of the laws of circulating 

 waters. Where two streams come together from different sources they are 

 sure to have different heads. If the side fracture which furnishes the pre- 

 cipitating solution has the greater head its water will occupy the mainr 



