spurr.] APPLICATION OF GEOLOGICAL PRINCIPLES. 25 
the fault planes, which are later than the original veins and have dis- 
placed them. This action, however, is only exceptionally the case, 
and as a whole is insignificant. The main ore shoots of the oxidized 
portion of the Mizpah mine are regarded as primary, since evidence 
has been found that these coincide with the original ones and have 
been oxidized nearly in place; therefore it is improbable that the 
transfer of material during oxidation, bringing about local concen- 
tration or impoverishment, has been very great. 
Ore shoots. — In the oxidized portion of the Mizpah vein the richer 
ores lie in roughly defined, broad shoots, which pitch east on the vein. 
At least three of these shoots, parallel to one another and with poorer 
ore between, have been recognized. In the unoxidized ores of the 
Montana Tonopah there is evidence of the existence of similar shoots 
or bunches, but enough exploration work has not been done to enable 
the writer to describe them more closely. 
APPLICATION OF GEOLOGICAL PRINCIPLES. 
From the short description of the occurrence of the veins which lias 
been given, it is seen that it is of the first and highest importance to 
distinguish and determine, so far as possible, the position of the early 
andesite. As has been noted, this early andesite is likely to be con- 
founded with the later andesite and with some forms of altered and 
silicified rhyolite, especially that gray, glassy rhyolite which occupies 
a considerable area in the northern corner of the district. As is seen 
on the accompanying map, the actual area where the early andesite 
outcrops is very limited, and in other portions of the district it is 
covered by later rocks or is cut through and displaced by them. 
These later cappings may consist of the biotite-andesite, the various 
rhyolites and dacites, the volcanic breccias, or the stratified tuff for- 
mation; or they may consist of two or more of these, superimposed 
one on the other. Those later rocks which are intrusive rather than 
in the form of flows (including some of the siliceous rhyolites, some 
of the dacites, and probably even some of the later andesites) can not 
be expected, however, to give way to the early andesite in depth. A 
shaft sunk in a dike or volcanic neck is very apt to continue in this 
same rock indefinitely downward (see fig. 4). 
Of the different rocks which cap the early andesite and the included 
veins, the oldest, the later andesite, is regarded as the most favor- 
able for prospecting; and, in the light of present knowledge, a belt of 
this rock lying east and west of Mizpah Hill is probably especially 
favorable. On the other hand, although the veins indicate a general 
east- west trend of mineralization, it is by no means proved that the 
outcrops on Mizpah Hill occupy the center of the mineralized belt and 
show the greatest mineralization. It may eventually be proved that 
the strongest portion of this belt lies north or south of the locality 
known at present, which would make the whole width of the belt con- 
