CHAPTER VII. 



ORIGIN OF MINERAL VEINS. 



ORIGIX OF THE MIXERALJZIXG AXD ALTERING WATERS. 



ANTITHESIS BETWEEN WATERS AND ASSOCIATED ROCK. 



In view of the composition of the waters which produced the veins and the 

 chief alteration of the early andesite, it has been argued that they were rich in 

 silica and potash and noticeably poor in the other common rock-forming elements. 

 They seem to have directly followed the earlier andesite eruption. In considering 

 the alteration of the later andesite in the vicinity of Mount Oddie, it has been 

 concluded that the waters which wrought the change were rich in magnesia, lime, 

 and iron, and low in silica and the alkalies; in this case the data seem to point 

 to the explanation that the waters followed the eruption of the Oddie rhyolite. 

 Both are concluded to have been hot-spring waters, which were active after volcanic 

 eruptions for a i - elatively short time, geologically speaking, and which differed in 

 composition as much as the rocks. If these conclusions are true, it is right to 

 notice an apparent antithesis in each case between the composition of the erupted 

 rock and that of the accompanying and succeeding hot solutions. The eruption 

 of the earlier andesite, a rock of intermediate composition, containing perhaps 

 about 60 per cent of silica, and about five times as much soda, lime, iron, and 

 magnesia as it does potash, was followed by the advent of waters which were 

 rich in the elements characteristic of extremely acid rocks (alaskites) namely, 

 silica and potash with the proportion of silica probably largely in excess of that 

 in these rocks and probably approximating that in feldspathic quartz veins of 

 granitic origin, as the composition of the Tonopah veins indicates. The eruption 

 of the Oddie rhyolite, a rock made up almost entirely of silica and potash, with 

 alumina, and only trifling quantities of magnesia, lime, and iron, was followed by 

 the advent of waters rich in these three last-named elements (which are charac- 

 teristic of basic rocks) and poor in the elements represented in the rhyolite itself. 



Testing this latter conclusion, we may recall the calcitic veins of Ararat 

 Mountain, which are certainly directly due to hot solutions that ascended immedi- 

 ately after the eruption of the neck or plug of Oddie rhyolite (p. 101). It has been 

 shown that these waters give evidence of having contained chiefly lime, iron, 



253 



