18 
ORE DEPOSITS OF TONAPAH, NEVADA. 
[BULL. 219. 
rocks; indeed, the} 7 must also occur sometimes in the early andesite 
itself. The mere fact of a vein lying in the early andesite, therefore, 
is not always evidence that it belongs to the early andesite period. 
In some such cases it may be extremely difficult, if not impossible, to 
tell to which period it belongs. 
The later andesite is in many cases thoroughly decomposed and 
altered, and there has been extensive formation of pyrite, calcite, 
etc., in it, especially in the regions where the richest veins have been 
found. The nature of this alteration is such as to indicate copious 
percolating waters as the agents, and to suggest that the waters may 
have been heated. It is quite possible 
that some of the small sulphide-bearing 
veins, which are frequently found in the 
later andesite, may have been formed 
contemporaneously with this alteration. 
These veins are nonpersistent, and the 
values contained are small. Physically 
they resemble the rhyolitic veins, and as 
they have been noted chiefly in the gen- 
eral vicinity of rhyolite, which is intru- 
sive into the later andesite, they have 
been classed with the undoubtedly rhyo- 
litic veins. It is possible, however, that 
a moderate mineralization, similar in 
quantity to that which succeeded the 
rhyolite eruptions, followed that of the 
later andesite. 
/ 
VEIN GROUPS OF THE EARLY 
ANDESITE MINERALIZATION. 
Fig. 1.— Horizontal plan of portion of 
Mizpah vein, as developed on the 250- 
foot level, Mizpah mine, east of the 
Brougher shaft. 
The only productive veins thus far dis- 
covered in the Tonopah district proper 
are those of the early andesite period. 
On account of the later volcanic rocks 
which cover the early andesite in most 
of the district, these productive veins 
outcrop only in a very small area, outside of which little is known as 
yet. It is probable, however, as indicated by the great amount of 
alteration in the early andesite, that the vein formation, has been 
extensive, and that the veins at present known are only a small por- 
tion of those that will eventually be developed. 
The veins already discovered all belong to the type of "linked 
veins." Their physical characteristic is that they branch and reunite 
in both a horizontal and a vertical direction (fig. 1). There is gen- 
erally in each group a main or master vein from which the smaller 
veins branch. These smaller veins again may subdivide and so finally 
