. Transactions. lways able to distin-guish the piedra azul from the ore itself, but on request the minersin-variably decided without hesitation. The piedra azul is shown by themicroscope to be a phase of the mineralization which led to the commonest variety is a very fine-grained aggregate of magnetite and 26 THE GEOLOGY OF IRON-ORE DEPOSITS augite, Fig. 29. One would at first take it to be a narrow dike, but inanother piece a fine-grained mixture of garnet and pyroxene made up therock, allying it with the contact effects. The best explanation is there-fore that while the mineralizing
. Transactions. lways able to distin-guish the piedra azul from the ore itself, but on request the minersin-variably decided without hesitation. The piedra azul is shown by themicroscope to be a phase of the mineralization which led to the commonest variety is a very fine-grained aggregate of magnetite and 26 THE GEOLOGY OF IRON-ORE DEPOSITS augite, Fig. 29. One would at first take it to be a narrow dike, but inanother piece a fine-grained mixture of garnet and pyroxene made up therock, allying it with the contact effects. The best explanation is there-fore that while the mineralizing agents in one place produced ore by re-placement of the diorite with magnetite, they may have afforded denseand closely crystalline bodies of magnetite and augite or both these andgarnet, close alongside. One other striking feature of the larger mines of the Spanish-Americancompany should be emphasized before passing to the smaller openingswhich are based on characteristic contact deposits between intrusives and. Fig. 28. Fig. 29. Fig. 28.—Calcite in Magnetite prom one of the Mines on Lola 29.—Piedra Azul or Malo Mineral, a Fine-Grained Intergrowth ofMagnetite and Augite. limestones. One cannot study the great open cuts in Lola Hill withoutbeing impressed with the evidences of movement and crushing. Thediorite is crushed along planes which run in close parallelism with the is squeezed and sheared around the buttress-like masses of magnetitewhich have presented a resisting front to the movement, but whichprobably antedate some though not all of it. The alteration of thediorite is excessive and it has apparently been subject to some influence,especially in the vicinity of the ore which has almost obliterated the horn-blende and feldspar in favor of chloritic greenstone. In this crushedzone, traversing Lola Hill like a backbone, the great slab-like masses ofore are distributed along a northwest and southeast line. Dr. Spencer sketched several cross-sections of
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
