GILA   RIVER   ALUM    DEPOSITS.  217 
GEOLOGIC    RELATIONS. 
This  portion  of  New  Mexico  is  occupied  wholly  by  igneous  rocks, 
the  nearest  sedimentary  beds  being  Carboniferous  limestone  in  the 
vicinity  of  Santa  Rita,  20  miles  to  the  southeast.  The  canyon  of  Gila 
River  exhibits  a  great  series  of  lava  flows  alternating  with  volcanic 
tuffs  and  breccias.  At  Lyons  Hot  Springs  the  lower  200  or  300  feet 
consists  of  generally  vertical  cliffs  of  light-gray  rhyolite.  Above  this 
is  a  great  thickness  of  andesitic  breccia,  containing  large  fragments  of 
basalt.  This  resembles  a  very  coarse  conglomerate  and  may  be  in 
part  water-laid.  Between  Lyons  Hot  Springs  and  Alum  Creek  the 
rhyolite  and  the  overlying  breccia  disappear  and  are  replaced  by  a 
series  of  basalt  sheets  which  extend  from  the  level  of  the  river  to  the 
summit  of  the  plateau,  3,000  feet  or  more  above. 
The  rocks  which  are  of  chief  importance  in  the  present  connection, 
and  which  occupy  the  greater  part  of  the  topographic  depression 
described  above,  are  andesitic  breccias.  Although  this  rock  presents 
considerable  variation  in  appearance,  its  different  phases  grade  into 
each  other  and  there  is  nowhere  any  difficulty  in  distinguishing  it  from 
the  surrounding  basalt.  Its  breccia  character  is  generally  evident  to 
the  unaided  eye,  the  rocks  being  made  up  of  angular  fragments  embed- 
ded in  a  fine-grained  groundmass.  The  angular  inclusions  are  in  the 
main  similar  to  the  matrix  in  composition  and  structure,  though  some- 
what finer  grained.  Here  and  there  they  are  composed  of  an  entirely 
distinct  type  of  rock.  This  is  most  common  near  the  contact  with 
the  basalt,  where  fragments  of  that  rock  are  in  places  abundant.  Con- 
siderable portions  of  the  rock,  however,  show  no  breccia  character. 
This  andesitic  volcanic  breccia  will  be  referred  to  as  the  alum  rock. 
DISTRIBUTION  AND   RELATIONS  TO  BASALT. 
The  distribution  of  the  alum  rock  is  shown  on  the  accompanying 
sketch  map  (fig.  6).  Owing  to  lack  of  time,  contacts  were  traced  in 
detail  at  only  a  few  points,  and  they  will  doubtless  be  materially 
modified  by  further  study.  In  a  general  way,  however,  the  map 
shows  the  more  important  facts  of  distribution. 
It  will  be  seen  that  the  main  mass  of  the  alum  rock  occupies  a  nearly 
circular  area,  lying  for  the  most  part  south  of  Gila  River  and  west  of 
Alum  Creek.  About  the  margins  of  this  main  mass  are  smaller  areas 
occupied  by  the  alum  rock,  the  largest  being  north  of  the  river  and 
several  small  ones  lying  in  the  upper  basin  of  Alum  Creek.  It  is  pos- 
sible that  the'  mass  north  of  Gila  River  is  continuous  with  the  main 
mass  to  the  south,  though  more  probably  it  is  entirely  distinct. 
Doubtless  there  are  other  small  masses  in  addition  to  those  shown  on 
the  map. 
