XVIII GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Pngo. 

 Older eruptives — Continued. 



Diorite ;3?3 



Quartz-mica-diorite — Horubleude-diorite 333 



Augite-bearing diorite 334 



Porphyrite 334 



Principal group 335 



Sacramento Porphyrite 341 



Silverheels Porphyrite 342 



Miscellaneous porphyrites 343 



Younger eripti ves 345 



Rhyolite 345 



Chalk Mountain Nevadite 345 



Black Hill Rhyolite 349 



McNulty gulch Rhyolite 350 



Empire gulch Rhyolite 351 



Other rhyolites 352 



Rhyolitic tufa — Dike in Teu-Mile amphitheater — Breccia 352 



Qnartziferous trachyte 352 



Audesite 353 



Pyroxene-bearing hornbleude-andesite '. 353 



Hypersthene-andesite — Tufaceoua andesites 354 



R6sum6 354 



Rock structures observed — Individual rock types 355 



Mutual relations of rock types — Rock constituents — Their decomposition 356 



Negative observations — Chemical composition 357 



Notes upon the Henry Mountain rocks 35!l 



Hornblendic rocks 359 



Augitic rocks 361 



R€8um6 362 



MINING INDUSTRY. 



Chapter I. 



Ore DEPOSITS 3*57 



Classification of ore deposits in general 367 



Leadville deposits 375 



Manner of occurrence 375 



Composition 376 



Distribution 377 



Secondary alteration — Mode of formation 378 



Age of deposits — Origin of the metallic contents 379 



Chapter II. 



Iron Hill GROUP OF JUNES .' 380 



Iron Hill 380 



General description 380 



Geological structure 381 



Later intrusive sheets 382 



White Porphyry 383 



Blue Limestone — Silurian — Cambrian — Iron fault .384 



California fault 385 



Dome fault — Emmet fault — Dome Hill 386 



Ore deposits 388 



