Transactions . ee face fromwhich the nature of the process may be inferred, and which willshow the usual sharj), wavy contour or «:ra(lual transition to rockthat is characteristic of replacement ore deposits. Stap^nation of solutions and consequent increa.^^e in the lateralextent of an ore body need not, of course, be determined by animpervious barrier. The constriction of openings which de-crease the rapidity of flow can occur within a single h()mon:ene()usrock mass and will cause the solutions to spread laterally andproduce the wandering forms which are shown in many of theLeadville ore bodi
Transactions . ee face fromwhich the nature of the process may be inferred, and which willshow the usual sharj), wavy contour or «:ra(lual transition to rockthat is characteristic of replacement ore deposits. Stap^nation of solutions and consequent increa.^^e in the lateralextent of an ore body need not, of course, be determined by animpervious barrier. The constriction of openings which de-crease the rapidity of flow can occur within a single h()mon:ene()usrock mass and will cause the solutions to spread laterally andproduce the wandering forms which are shown in many of theLeadville ore bodies. This will also account for the occurrence of not a few b();eadville of the ores occur directly beneath the impc^rvious capping;but some of them occur wholly within the limestone, and in tlies<cases it is presumable that t)ie solutionis never reached the im-pervious barrier. 422 Replacement Ore-Bodies—Irving r* — / /-/- I I -. o. </ i^^^ ra< 1 // / I /^^^ ./v \ /, / /J/ ^7t •^\^ -1,71 , —I I ^ Fig. 18.—Sketch plan of the mineral farm ore-body near Portland,South Dakota, showing how replacement of the country rock by silica hasproceeded outward from the fractures, but has only completely replacedthe intervening rock when the porphyry barrier has dammed up the min-eralizing waters. Effect of Manner of Operation of Solutions on Rock. If the replacement starts from a fissure in a homogeneousrock mass it may be conceived to operate in one of two ways. (1) The solutions may have first penetrated the rock alongminute pores until they have saturated it for some distance fromthe fissure, the limit being set by the balance V)etween the frictionof diffusion and the difficulty of escape elsewh(;re, and the solutionmay then operate from an innumerable number of separatecentres (those farther away being fewer in number). As theseseparate small masses grow, they gradually interfere until coal-escence unites them
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