Transactions . first ore-bodies studied by Emmonsin Leadville he was naturally confronted by what seemed to bean extremely difficult problem, and it was only when he had seenthe ore-bodies in the incompletely replaced rock that the originof the ores by replacement of limestone became clear.^ When more than one susceptible layer is present, , whensusceptible strata alternate with resistant strata the resultant orebody will finger out into the surrounding sediments or even retainthe unsusceptible layers throughout the entire mass. Such orebodies have, in cross-section, saw-tooth forms or a s
Transactions . first ore-bodies studied by Emmonsin Leadville he was naturally confronted by what seemed to bean extremely difficult problem, and it was only when he had seenthe ore-bodies in the incompletely replaced rock that the originof the ores by replacement of limestone became clear.^ When more than one susceptible layer is present, , whensusceptible strata alternate with resistant strata the resultant orebody will finger out into the surrounding sediments or even retainthe unsusceptible layers throughout the entire mass. Such orebodies have, in cross-section, saw-tooth forms or a sort of gridironstructure. An excellent illustration is given in Fig. 15 and alsoin Plate II-G. Masses of susceptible rock such as limestonemay also be enclosed in irregular porphyry intrusions and maythen have any form, so that the replacement will be confined tothe susceptible rock, as in the Fryer Hill ore bodies just describedand the resulting ore masses will have the same form as the re-placed mass of Fio. 15.—Cross-section of ore-shoot in alternating layers of shale andrlolomite, Portland, South Dakota. The shales are only slightly replacedby silica, but the ore in the limestone layers runs out to greater distancesfrom the Oral communication. Replacement Ore-Bodies—Irving 419 Effect of Impervious Barriers and Constriction of Cavities on Form. Waters which circulate rapidly through natural conduitsproduce less replacement than those which move more slowlyor come to rest, probably because more time is afforded for re-action with the surrounding rock than when circulation is thus happens that if impervious barriera lie along the pathsof fissures or rocks in which the fissures become lost or constricted,the solutions will be dammed up, spread out beneath or uponthe impervious rocks and the mass of rock replaced at the point ofstagnation will be very much greater than elsewhere. Whenwaters are ascending (which is generally the case with primaryore
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