Transactions . y, the veins are narrow and lean. There are somany instances in which the same veins can be seen cutting allthree rocks that there can be no doubt as to the correctness ofthese conclusions. The vein-filling consists of quartz whichshows no comb-structure, with pyrite and various copper-min- l\lli ENCE 01 INI RY-ROi K ON MIMI; \l. \ NS. erals, of which chalcocite, enargite and bornite are the moslcommon. The walls on each Bide are much altered, and theless altered rocks al Borne distance from the veins show theferromagnesian Bilicates altered to pyrite. The relative richness
Transactions . y, the veins are narrow and lean. There are somany instances in which the same veins can be seen cutting allthree rocks that there can be no doubt as to the correctness ofthese conclusions. The vein-filling consists of quartz whichshows no comb-structure, with pyrite and various copper-min- l\lli ENCE 01 INI RY-ROi K ON MIMI; \l. \ NS. erals, of which chalcocite, enargite and bornite are the moslcommon. The walls on each Bide are much altered, and theless altered rocks al Borne distance from the veins show theferromagnesian Bilicates altered to pyrite. The relative richness of the veins in the Butte granite is be-lieved to be due to the basic character of the rock, and itsgreater content of the easily replaceable iron silicat Therock is a quartz-monzonite, the composition of which has beencarefully calculated from chemical analyses of the rock, and ofthe biotite and hornblende isolated from it,and from microscopicanalysis o\ the rock as well.* This shows it to contain Pia. 1. y vv 1 Granite * *?*<?*?* Aplite fcfoV^l Quartz Ideal Plan of Conditions in a Copper-Vein at Butte, Mont., Passing from BasicGranite into Aplite Masses. The solid black represents high-grade copper-ore,when the vein is in basic granite. per cent, of hornblende and of biotite. There is a littleaugite also. The aplite contains over 10 per cent, more silicathan the rock just noted, no hornblende, and very little Modoc porphyry also has, when fresh, a very little mica,and is as high in silica as the aplite. Mr. H. V. Winchell hastailed my attention to the condition shown in the diagram,Fig. 4. In the case here given as a general type, the vein isworkable onlv in the Butte granite. A study of thin sections of the rock adjacent to the ore showsthat the hornblende is the first mineral to be altered into ore,and that the bunches of this mineral form the nucleus for amore or less complete replacement of the entire rock. The * Granite Rocks of Butte, Mont. W.
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
