216  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
Professor  Blake  devoted  parts  of  only  two  days  to  the  examination 
of  these  deposits,  and  the  actual  time  spent  on  them  by  the  writer  was 
less  than  three  days,  so  that  the  geologic  work  thus  far  done  must  be 
regarded  as  purely  of  a  reconnaissance  character  and  the  conclusions 
as  preliminary  and  tentative. 
TOPOGRAPHIC    RELATIONS. 
The  alum  deposits  occupy  a  nearly  circular  depression  below  the 
general  level  of  the  volcanic  plateau.  Gila  River  cuts  through  the 
northern  edge  of  this  depression,  and  Alum  Creek  intersects  its  eastern 
portion  from  south  to  north.  The  elevation  of  the  river  is  about 
4,000  feet  above  sea  level;  the  rim  of  the  basin  is  between  2,000  and 
3,000  feet  higher.  The  inner  slopes  of  the  basin  rim  are  very  steep 
and  generally  surmounted  by  basaltic  cliffs.  Except  where  the  river 
enters  and  leaves  the  basin  and  at  one  point  where  Alum  Greek  heads 
against  the  upper  branches  of  Copperas  Creek,  the  rim  is  practically 
continuous.  The  latter  point  has  been  selected  for  a  trail  which  by 
way  of  Copperas  Creek  and  the  Sapio  a  fiords  the  most  direct  approach 
to  the  basin. 
Within  this  basin  is  a  group  of  hills,  the  highest  of  which,  Alunogen 
Ridge,  rises  from  1,000  to  1,200  feet  above  the  river.  These  hills  are 
separated  by  Alum  Creek  and  its  branches  into  three  groups.  The 
largest  group,  with  Alunogen  Ridge  along  its  southern  border,  lies 
west  of  Alum  Creek,  between  the  west  fork  and  the  Gila.  On  the 
west  it  is  separated  by  a  deep  depression  from  the  western  rim  of  the 
basin.  East  of  Alum  Creek  is  a  smaller  group  of  hills,  culminating  in 
a  sharp  peak  something  over  800  feet  above  the  river  and  separated 
from  the  cast  rim  of  the  basin  by  a  shallow  depression.  Between  the 
two  main  forks  of  Alum  Creek  is  a  third  and  smaller  group,  forming 
the  point  of  a  spur  from  the  southern  rim  of  the  basin.  This  group 
consists  of  a  series  of  pinnacles  and  isolated  buttes  with  nearly  vert  ical 
sides.  North  of  the  rim  a  fourth  group  of  hills  rises  abruptly  from  the 
slope  of  the  basin. 
These  hills  within  the  basin  are  entirely  distinct  in  form  from  any 
other  topographic  features  of  the  region.  They  are  characterized  by 
great  numbers  of  pinnacles  and  isolated  buttes  rising  abruptly  from 
steep  talus  slopes.  The  cliffs  are  weathered  into  innumerable  intri- 
cate and  fantastic  forms  and  in  many  places  are  covered  with  heavy 
masses  of  incrustations.  Even  more  striking  than  their  peculiar  form 
is  their  varied  and  brilliant  coloring,  which  forms  a  strong  contrast 
with  the  somber  greenish  gray  and  black  of  the  surrounding  basaltic 
basin  rim.  The  prevailing  colors  are  white  and  red,  with  all  shades 
of  pink,  yellow,  and  green. 
