VARIATION OB 1 POROSITY AND STRUCTURE. 1201 



]IISTRIH[-TIi>X AND SIZE OF OPENINGS. 



From the foregoing general statement it is clear that one of the essen- 

 tial conditions for the production of large ore deposits is that openings should 

 be of adequate size. As will be seen, these openings may be original or 

 they may be largely secondary. A close analysis of the manner in which 

 the waters work shows that it is not of advantage in ore deposition to have 

 equal porosity in all directions. Were the rocks equally porous in every 

 direction there would be no reason for the concentration of the water in 

 trunk channels, and therefore the conditions which have been discussed as 

 essential for the mingling- of solutions of different kinds would not exist. 

 Oftentimes the essential condition is to have considerable masses of rocks 

 nonporous or moderately porous adjacent to or associated with other areas 

 which are very porous. Oftentimes where the porous belts are too numer- 

 ous there is a segregation of a small amount of ore in them all, and not an 

 adequate segregation in any one. 



The most frequent combination of porous and relatively nonporous 

 rocks favorable to the production of ore deposits is to have certain belts 

 with openings of adequate size associated with more extensive areas in which 

 the openings are much smaller, although they may be equally continuous. 

 As illustrations of such combinations we may have layers of sandstone or 

 conglomerate between igneous rocks or shale. The large openings may be 

 furnished by fault fissures. 



For many reasons trunk channels may vary greatly in size and con- 

 tinuity. For instance, fault fissures, in consequence of the irregularity of 

 the fractures and the displacements, may range in width from great rooms 

 to mere seams. The effect of the dip upon fissures is of great importance. 

 Where the dips are vertical or nearly so, the veins may be wide; whereas, 

 where the dips are flat, the veins may be relatively narrow. This is a nat- 

 ural consequence of the effect of gravity. The more inclined the veins the 

 more forcibly does gravity pull down the overhanging wall, and thus tend 

 to close the openings. This association of wide veins with steep dips, and 

 narrow veins with low dips, is beautifully illustrated at the Smuggler Union 

 mine, of southwestern Colorado. 

 mon xlvii — 04 76 



