1228 A TREATISE ON METAMORPHISM. 



almost imperceptible, may furnish rules which will enable one to more 

 intelligently search for ore. 



Between the two cases of trunk channels produced by flexures 

 (described on pp. 1225-1226) and by cross fractures (described on pp. 

 1226-1228) there are complete gradations. 



Late orogenic movements explain certain ore shoots. After openings 

 have received a first contribution of ore, and are, perhaps, fully cemented 

 by ore and gangue materials, orogenic movements frequently recur, which 

 again fracture the ground and produce openings. Some parts of a deposit 

 may escape fracture, while other parts may be broken. The fracturing of 

 the broken parts may be simple or complex. The latter may produce 

 zones of parallel fractures, zones of intersecting fractures, brecciated zones, 

 or even zones in which the material is finely mashed. Between the parts 

 of a deposit where fracturing is absent and those where it is most complex 

 there may be all gradations. The fractures may be confined to a narrow 

 belt of a deposit or to one side of it, and to varying limits laterally or 

 vertically. Entirely new sets of openings may be produced in the wall 

 rock. All of the above statements in reference to a main deposit apply 

 equally well to intersecting deposits. Therefore an ore deposit which has 

 received a first contribution and is subjected again to orogenic movements 

 is in such a condition that it may again receive a contribution of ore 

 material under the same complex laws as at first. The late fractures may 

 be filled with new contributions of metals from the original source; they 

 may be fed by the solution of the deposits before formed; or they may 

 derive metal both from the original source and from the earlier deposit. 



The new ore may be distributed very irregularly through the older 

 ore, may be superimposed upon the old material where there are openings, 

 may be deposited as relatively small secondary veins, or any combination 

 of these may take place. The original irregularities in the tenors of the 

 veins, combined with the irregular deposition of the secondary material, 

 may give extraordinary and apparently inexplicable variations in richness. 

 Excellent examples showing how secondary fracture produces very rich 

 subordinate veins are furnished by the San Juan district of Colorado. At 

 the Smuggler Union, deep in the mine, in a secondary fracture adjacent 

 to the main vein, are found extraordinarily rich deposits of free gold in 

 quartz, having an exceedingly irregular and "pockety" distribution of 

 .values. It seems probable that in this fracture the gold has been precipi- 



