22 OUTLINE OF PAPER. 



test well. The theory is reached that an original magma of composition similar to that of the earlier 

 andesite has split up by differentiation, first into more basic andesite (later andesite) and siliceous 

 dacite, and later, by a continuation of the process, into siliceous rhyolite and basalt. 



The structure is so complicated that no general cross sections have been made. Some interesting 

 information on faulting has, however, been obtained, chiefly from mine workings. The faults are 

 reversed or normal, straight or curved, perpendicular or flat. Many varieties of movement are 

 illustrated by them. 



The most important mineral veins occur in the early andesite, and do not extend into the overlying 

 rocks. These veins have been formed, chiefly by replacement, along narrow-sheeted zones, and have 

 all the characteristics of true veins. Transverse fractures have determined the position of cross walls 

 and ore shoots by limiting and concentrating the circulation. The mineralization was probably 

 caused by hot ascending waters immediately after the earlier andesite eruption. The primary 

 ores have a gangue of quartz, adularia, and some sericite and carbonates, and contain silver 

 sulphides such as argentite, polybasite, and stephanite silver selenide, gold in a yet undetermined 

 form, chalcopyrite, pyrite, and some galena and blende. The depth of oxidation is irregular. In the 

 ore of the oxidized zone no important changes in the amount of gold or silver, as compared with the 

 primary ore, has been proved to take place. The ore near the surface is not a truly oxidized ore, 

 however, but is an intimate mixture of original sulphides (and selenides), together with secondary 

 sulphides, chlorides, and oxides. Secondary sulphides include argentite and pyrargyrite. 



The Tonopah ore deposits, when compared with others, find their closest resemblances in the 

 Comstock in Nevada and in the Pachuca and other districts in Mexico, while the Silver City and De 

 Lamar districts in Idaho are also similar in many respects. These deposits all occur in Tertiary lavas, 

 chiefly andesitic. The writer has previously described the Great Basin region as forming part of a 

 great petrographic province, and later it has been shown that this province extends into Mexico, and 

 may reach much farther northeast and southwest. The similarity of the ore deposits in the district 

 mentioned indicatee that there is a metallographic province, which coincides in part at least with the 

 petrographic province. 



A series of veins, of small importance commercially within the Tonopah district, was formed after 

 the eruption of one of the members of the rhyolite-dacite series the Tonopah rhyolite-dacite. These 

 veins may be large, but are usually low grade or barren. They frequently contain a greater proportion 

 of gold than the earlier andesite veins, and have other distinguishing characteristics. In some cases 

 the waters accomplishing this latter mineralization probably attacked and concentrated the ores in the 

 earlier andesite veins. 



A series of veins of still less importance was formed after the eruption of one of the later members 

 of the rhyolite-dacite series a siliceous rhyolite, which makes up some of the hills near Tonopah. 

 One of these, Mount Ararat, a denuded volcanic neck, is traversed by fissure veins, carrying very 

 little values. These veins are restricted to the neck, and the openings they fill were evidently formed 

 by an upward movement of the plug after consolidation. 



Part of the earlier andeeite ia profoundly altered, chiefly to quartz, sericite, and adularia. Other 

 portions are altered chiefly to calcite and chlorite. These alteration phases are transitional into one 

 another, and were evidently caused by the same waters. The maximum effect of these waters was 

 the formation of the mineral veins along their circulation channels. Near the veins they effected the 

 quartz-sericite-adularia alteration, and penetrating farther away they effected the calcite-chlorite 

 alteration. The discussion of these processes is followed by the detailed study of analyses of 

 typical specimens. The conclusion is drawn that the mineralizing waters were charged with an excess 

 of silica, and probably of potash, together with silver, gold, antimony, arsenic, copper, lead, zinc, and 



