GILA   RIVER    ALUM    DEPOSITS. 
219 
the  large  body  of  basalt  entirely  surrounded  by  the  alum  rock,  near 
the  center  of  the  main  mass.  Although  its  relations  are  obscure,  this 
appears  to  be  merely  a  large  inclusion,  broken  off  from  some  under- 
lying basalt  sheet  and  floated  up  to  its  present  position  by  the  intruded 
mass. 
ALUM    ROCK. 
The  most  striking  peculiarity  of  the  alum  rock  is  its  extreme 
alteration.  This  is  evident  to  the  unaided  eye  and  even  more  so 
when  the  rock  is  examined  under  the  microscope.  Careful  search 
failed  to  reveal  at  any  point  within  the  Alum  Creek  basin  a  trace  of 
the  unaltered  breccia  for  determination  of  its  original  character. 
Where  exposed  to  leaching,  as  in  the  upper  parts  of  pinnacles  and 
cliffs,  the  rock  is  invariably  porous,  giving  a  hollow  sound  under  the 
hammer.  It  is  evident  that  a  considerable  part  of  its  substance  has 
been  removed  in  solution. 
The  least  altered  phase  of  the  alum  rock  is  seen  in  tunnel  No.  2, 
which  is  driven  about  180  feet  into  the  face  of  the  cliff  on  the  east 
side  of  Alum  Canyon.  The  rock  is  fine  grained  an  I  was  originally 
composed  of  minute  crystals  of  plagioclase  feldspar  embedded  in  a 
glassy  groundmass.  Both  feldspar  crystals  and  matrix  are  much 
altered,  only  the  outlines  of  the  former  remaining.  There  are  also 
abundant  very  fine,  dustlike  particles  of  pyrite.  Near  the  face  of  the 
cliff  the  rock  is  white  and  chalky,  but  a  few  feet  in  from  the  face  it 
has  a  white  chalky  base  more  or  less  mottled  with  bluish-gray  patches 
in  which  the  alteration  is  evidently  less  complete.  A  rock  originally 
very  similar  to  that  in  tunnel  No.  2  forms  the  point  of  the  spur 
between  the  forks  of  Alum  Creek. 
The  composition  of  the  various  phases  of  the  alum  rock  is  shown  in 
the  accompanying  analyses. 
Analyses  of  alum  rock  from  Gila  River,  New  Mexico. 
[W.  T.  SchaUer,  analyst.] 
A. 
B. 
c. 
D. 
Soluble  in  water: 
AI2O3+  Fe203 
0.51 
1.83 
55.76 
20.64 
4.16 
18.07 
81.19 
0.22 
1.64 
50.45 
18.61 
3.79 
23.85 
76.16 
S03 
Insoluble  in  water: 
Si02 
57.25 
32.27 
Trace. 
12.07 
88.00 
83.51 
AI2O3  (+Ti02)...                                             
5.55 
Fe203  (total  iron) 
7.38 
91.69 
A.  Alum  rock,  white,  mottled  bluish  gray,  from  tunnel  No.  2,  50  feet  from  mouth  of  tunnel. 
B.  Alum  rock,  disintegrated,  from  dump  of  tunnel  No.  2. 
C  Alum  rock,  white,  chalky,  from  point  of  ridge  between  forks  of  Alum  Creek. 
D.  Alum  rock,  pinkish  white,  porous,  from  summit  of  Alunogen  Ridge. 
Of  the  four  samples  analyzed  A  and  B  represent  practically  the 
same  material,  the  only  difference  being  that  the  material  on  the 
