70 ■ REVIEWS 



3. Copper sulphides in limestone. — These ores occur only in fissured 

 regions of limestone marmorized by the contact with monzonite. Inasmuch 

 as it retains the bedded structure of the limestone, and shows every con- 

 ceivable stage of replacement when observed microscopically, the conclu- 

 sion is that it was deposited by a molecular replacement of the limestone. 

 The source of the copper was the intrusive monzonite, while the trans- 

 porting agents were hot solutions or vapors emitted from the intrusives 

 either from the top or from great depths. 



The district is a steady producer of low grade ore, and is the foremost 

 camp of Utah in the production of copper. More of the sulphide ores are 

 being found continually, and progress is constantly being made in the pro- 

 duction of low-grade copper, which seems likely to prolong the mining 

 activity of the region indefinitely. 



The report is clearly written, and betrays systematic and thorough- 

 going work. A. C. T. 



Geology of the Tonopah Mining District, Nevada. By Josiah 

 Edward Spurr. (Professional Paper No. 42, U. S. Geological 

 Survey.) Pp. 295, 24 plates, 78 figures. 



Ore deposits were discovered in the Tonopah district in April, 1900, by 

 James L. Butler. The geologic structure is complex. The rocks are of 

 volcanic origin, probably Miocene-Pliocene, except for a series of water- 

 laid tuffs. The successive flows have been named earlier andesite, later 

 andesite, five recognized rhyolite-dacite series, the Siebert tuffs, and finally a 

 little basalt. The region has been profoundly faulted. It is concluded 

 that the faulting was initiated chiefly by the intrusion of the dacitic 

 rocks. After the intrusion there was a collapse, a sinking of the various 

 vents. ''The still liquid lava, in sinking, dragged down with it adjacent 

 blocks of the intruded rock." 



The veins occur principally in the earlier andesite, and do not extend 

 into the over-lying rocks. Less rich veins are found in the later andesite and 

 one of the rhyolite-dacite series. These veins are formed by replacement 

 in fissured zones. Transverse fissures have determined the position of 

 cross- walls and ore shoots by limiting and concentrating the circulation. 



The ores contain silver sulphides, silver selenide, gold in an undeter- 

 mined amount, chalcopyrite, pyrite, some galena and lead, with a gangue 

 of quartz, adularia and some carbonates. Oxidation has occurred to vary- 

 ing depths, but has not reduced the amount of gold and silver. 



