222  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    I. 
ascent  of  acid  solutions  and  gases  from  a  deep  source  with  the  result- 
ing solfataric  action  which  often  characterizes  the  final  phases  of  vol- 
canic activity. 
The  andesitic  breccia  generally  contains  fine  disseminated  grains  of 
pyrite,  probably  enough  to  produce  by  their  oxidation  the  necessary 
sulphuric  acid  for  all  the  sulphates  present.     So  far  as  observed,  how- 
ever, the  pyrite  in  the  alum  rock  is  perfectly  fresh  and  unoxidized. 
Further,  the  mineral  halotrichite,  which  seems  to  have  been  the  form 
of  sulphate  iirst  formed,  is  extremely  unstable4  under  oxidizing  con- 
ditions, the  iron  passing  readily  to  the  ivvvlc  condition  and  becoming 
al    mice   insoluble.      Hence   halotrichite  could   not    be   formed  under  j 
conditions  favorable  for  the  oxidation  of  pyrite.      If  the  pyrite  had  I 
been  the  source  of  the  sulphuric  acid  the  mineral  produced  would  have  \ 
been  one  of  the  more  stable  sulphates. 
Another  source  for  the  sulphuric  acid  must  therefore  be  sought, 
and  its  volcanic  origin  is  at  once  suggested.  As  has  been  shown,  the  1 
alum  rock  was  originally  an  andesitic  breccia  which  filled  the  neck  of  a 
volcano.  This  volcanic  neck  cuts  through  and  is  therefore  later- than 
the  great  basalt  ic  lava  sheets  w  hich  occupy  so  much  of  t  his  region.  It  ; 
may  \^v\  easily  bave  been  active  in  late  Tertiary  or  Quaternary  time 
and  have  been,  in  part,  at  least,  the  source  of  the  great  beds  of  tuff 
and  breccia  which  (ill  an  old  valley  to  the  north,  now  in  part  reexca- 
vated  by  Gila  River.  The  brecciated  character  of  the  material  filling 
the  volcanic  conduit  permitted  the  cum  ascent  and  circulation  of 
gases  and  .solutions  from  great  depths,  and  if  these  contained  i'vvv, 
sulphuric  acid  they  would  be  ent  i red \  competent  to  produce  tin4  altera* 
tion  observed  in  the  breccia  and  the  sulphate's  which  it  contains. 
The  alteration  is  confined  almost  entirely  to  the  breccia  within  (he 
volcanic  conduit,  generally  extending  only  a.  short  distance'  into  the 
surrounding  basalt.  The  alteration  of  the  basalt  is  most  extensive 
where  it  was  most  fractured  by  the  intrusion,  as  in  the  vicinity  of  the 
numerous  small  int  rusions  of  breccia  southeast  of  t  he  main  mass.  As 
the  basalt  consisted  largely  of  ferromagnesian  minerals,  it  yielded 
under  the  action  of  the  acid  solutions  chiefly  hydrate'd  magnesian  sili- 
cates and  gypsum,  the  latter  being  rather  abundant  as  selenite  crys- 
tals in  the  residua!  greenish  clay. 
The  determination  of  the  agent  producing  the  chemical  changes  in 
the  breccia  has  more  than  a  theoretical  scientific  interest.  If  these 
changes  were  due  to  the  acid  produced  by  the  oxidation  of  pyrite, 
they  would  not  be  expected  to  extend  beyond  the  relatively  shallow 
zone  of  oxidation.  If,  on  the  other  hand,  they  are  due  to  ascending 
acid  solutions,  they  may  be  expected  to  extend  to  much  greater 
depths. 
It.  is  probable  that  the  fumarolic  activity  to  which  the  chemical 
changes  appear  to  be  due  has  entirely  ceased  in  this  particular  vol- 
canic neck.     No  thermal  springs  and  no  emanations  of  acid  gases 
