84 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
they lie like the spokes of a wheel. Where they come together near 
the center from which they radiate, they join and fork in the manner 
of linked veins. There is evidence to show that these veins, although 
having the characteristics of true fissure veins, have formed along 
zones of fracturing or sheeting in the lode porphyry, and have replaced 
the porphyry in these zones to a more or less perfect extent. There- 
fore, in passing along the veins, at some places they are found to 
consist of pure quartz, in other places chiefly of highly silicified 
porphyry. 
Probably contemporaneous with the mineralization has been the 
extreme alteration of the original hornblende-andesite. This altera- 
tion, together with the subsequent weathering, has produced a great 
variety of appearance in the originally nearly uniform rock. Pyrite 
and iron carbonate have been derived from the decomposition of 
the dark minerals (mica, hornblende, and pyroxene) of the original 
andesite. The decomposition of the feldspars and other minerals has 
furnished an abundance of secondary quartz. In some of the most 
decomposed and altered phases, therefore, the appearance is that 
of a highly siliceous, nearly fresh rock, apparently a rhyolite. This 
is one of the common phases in the vicinity of the Mizpah. Other 
phases are soft and light colored; others comparatively firm and dark 
colored, with abundant pyrite. 
The mass of Oddie Mountain is made up of a true rhyolite, later in 
point of age than the lode porphyry and containing, so far as yet 
known, no veins belonging to this period. It is also somewhat decom- 
posed, but not nearly to so great an extent as the lode porphyry. 
The later andesite occurs on Mizpah Hill in the same localities as 
the earlier andesite, and between certain phases of the two rocks it is 
often difficult to distinguish. Originally they both had nearly the 
same composition, and they have often been altered in nearly the same 
way. The large feldspar crystals of the later andesite, however, are 
generalty of greater size and more thickly set together than in the 
earlier andesite or lode porphyry. The mica or biotite crystals of the 
later andesite are also frequently intact and can be recognized in the 
hand specimen, while the micas of the older andesite have generally 
completely disappeared. The later andesite is also apt to be more 
highly colored in its present state than the earlier rock. It has often 
assumed various shades of purple and green, which led the writer to 
give it the field name of "purple porphyry." This later andesite is 
found in dike form, cutting the earlier eruptives, and also occurs in 
the form of extensive flows. It was possibl} 7 accompanied by breccias. 
About this time, and perhaps succeeding the later andesite, came the 
formation of the great series of stratified white volcanic tuffs which I 
is exposed around Tonapah. At one point a thickness of several 
hundred feet is shown. The perfect stratification indicates that this I 
formation was laid down in a lake which must have been of consider- 
