100 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull. 225. 
rocks;  indeed,  they  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 vicinit}^  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  era  lotions,  followed  that  of  the 
later  andesite. 
/ 
VEIN  GROUPS  OF  THE  EARLY  ANDES- 
ITE MINERALIZATION. 
Fig. 2  .—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.  2).  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 
