ALTERATION OF THE LATER ANDE8ITE. 251 



later andesite, which occurred at a later period than that of the earlier andesite; 

 otherwise some trace or transition of the later andesite alteration would be found 

 on the earlier andesite side of the fault line. 



RELATION OF ALTERATION TO VEIN FORMATION. 



EXUDATION VEIXLETS IS LATER ANDESITE. 



In the later andesite occur many veinlets of calcite, some of gypsum, and even 

 of quartz. They are almost always very small and nonpersistent, tilling cracks, 

 and are evidently mainly the product of lateral secretion or exudation from the 

 rock. The quartz generally has a chalcedonic or jasper}- look, as compared with 

 the quartz of the earlier andesite veins, although in some cases the resemblance 

 of the two varieties of quartz to one another may be close. 



METALLIFEROUS VEINS IN LATER ANDESITE. 



Some larger veinlets, probably of a different origin, are composed of quartz 

 or quartz and calcite, and contain pyrite. An assay" of such a bluish veinlet in 

 later andesite, from the east base of Mount Oddie, and near the contact of the 

 Oddie rhyolite showed only traces of gold and silver. It was noted that these 

 veinlets were especially characteristic of a zone in the later andesite near the 

 contact of the Oddie rhyolite. 



Near the contact of the glassy Tonopah rhyolite-dacite at many points, as for 

 example, near the Belle of Tonopah shaft, there are numerous small veins of this 

 kind in the intruded later andesite. These veins gave variable but generally small 

 assays for gold and silver, the gold predominating. In the Mizpah Extension, 

 large veins of pyritiferou.s quartz were encountered in the later andesite, but 

 this was at or near the contact with Tonopah rhyolite-dacite, which is, it will be 

 remembered, of more recent date than the later andesite. 



The pyrite in the altered later andesite is sometimes very abundant, and may 

 be segregated so as to be of striking appearance, and to suggest an ore; but assays 

 show in all cases that the mineral is barren of gold and silver. 



CONCLUSION. 



It thus appears probable that the more important quartz veinlets which 

 appear in the later andesite in places were largely formed under the influence of 

 solutions following the contacts of later intrusive rocks the rhyolites and rhyolite- 

 dacites. This being the case, it is likely that a large part of the rock alteration 

 just described may have been due to the same causes. The entirely altered 

 specimens 3 and 4, described and analyzed, were both near the intrusive contact 

 of the Oddie rhyolite, and in general the more altered portions appear to be in 



<" By R. H. Officer & Co., Salt Lake City. 



