ALTERATION OF THE LATER ANDESITE. 249 



magnetite, etc., and is nearly always closely associated with pyrite. Usually the 

 two occur intercrystallized, yet so clearly separated as to show contemporaneous 

 crystallization; sometimes, however, a rim of siderite around pyrite indicates later 

 crystallization for the former, if not its derivation from the pyrite; while quite 

 as often rims of pyrite around siderite indicate a reversal of this order of crys- 

 tallization, and sometimes the phenomena clearly indicate that the pyrite has 

 formed at the expense of the siderite (PI. XXIII). This is in harmony with 

 the conclusions arrived at that the rock has been altered by solutions at once 

 highly carbonated and sulphureted. 



The siderite occurs usually as a cloudy, opaque or semitranslucent substance, of 

 a characteristic white color by incident light. It has indeed usually the appearance 

 of the mysterious substance called leucoxene by petrographers, and observed as the 

 decomposition product of ihnenite. In many examples of this mineral in the Tonopah 

 andesites, however, rhombic cleavage has been observed, and characteristic rhombic 

 crystal outlines. The nature of the mineral has also been determined by chemical 

 tests (p. 241). 



Concerning similar siderite in the iron-bearing rocks of the Mesabi range in 

 Minnesota, the writer has made the following statement." 



" It is to be noted that siderite * * * surrounds magnetite as a decomposi- 

 tion product, and is cloudy and without crystal form. It thus comes under the 

 group of decomposition products from magnetite called leucoxene. Rosenbusch 

 describes it as an alteration product of ilmenite, titaniferous magnetite, and rutile. 

 Concerning its nature he says: 6 'Its chemical composition is not the same in all 

 cases where it has been investigated, and has been considered the equivalent of a 

 variety of minerals (titanite, anatase, and siderite) by different observers.' In every 

 case where this mineral is present in these rocks, chemical tests show it to be 

 siderite, and no signs of titanium can be found either in it or in the magnetite 

 whence it is derived. The existence of this leucoxenic decomposition product 

 surrounding magnetite has sometimes been held as sufficient evidence that the 

 magnetite was titaniferous, but it is clear that it is not necessarily the case." 



In the altered "propylitic" andesite of the Comstock lode, which in alteration 

 resembles very nearly the later andesite of Tonopah, Dr. G. F. Becker suspected 

 the presence of siderite. He remarks:'" 



"* * * It seems certain that the black border of many hornblendes has been 

 attacked and has given place to a transparent mineral, which is more or less diffused 

 in and obscured by the groundmass. The natural supposition is that it is ferrous 

 carbonate." 



aQeol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 84. 



b Microscopical Physiography of the Rock-Making Minerals, by H. Rosenbusch. Translated by Joseph P. Iddings. 

 Second, revised edition, p. 165. 



Mon. U. S. Geol. Survey, vol. 3, p. 215. 



