218  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PAET    I. 
The  relations  of  the  alum  rock  to  the  adjacent  basalt  are  well  shown 
on  t  lie  steep  eastern  slope  of  Alum  Canyon.  The  contact  here  extends 
about  N.  60°  E.,  and  is  marked  by  a  deep,  narrow  ravine.  To  the 
north  the  alum  rock  forms  a  series  of  pinnacles  and  in  places  the  origi- 
nal outer  surface  of  the  mass  is  shown,  the  basalt  having  been  the  more 
easily  eroded  of  the  two  rocks.  At  this  point  the  contact  is  vertical 
and  the  alum  rock  contains  numerous  fragments  of  the  basalt  up  to 
several  feet  in  diameter.  The  relations  here  indicate  clearly  that  t lie 
alum  rock  lias  broken  through  the  basalt  in  the  form  of  an  igneous 
Fig.  6.— Sketch  map  of  (ii la  River  alum  deposits,  based  on  claim   map  prepared  by    R.  L.  Powell, 
i  nited  States  deputy  surveyor. 
intrusion,  probably  with  intense  explosive  violence.  The  basalt 
shows  a  foliation  or  platy  si  rucl  lire  parallel  to  the  contact  and  extend- 
ing several  feet  from  it.  Within  this  zone  it  is  deeply  weathered  to  a 
greenish  clay,  though  the  structure  of  the  rock  remains. 
Similar  relations  between  the  alum  rock  and  the  basalt  were  ob- 
served at  several  other  points,  notably  on  the  point  of  the  ridge 
between  the  forks  of  Alum  Creek  and  along  the  western  margin  of 
the  main  mass.  In  every  place  where  the  contact  was  well  exposed 
it  was  either  vertical  or  at  a  high  angle,  and  the  alum  rock  contained 
fragments  of  the  basalt.     One  peculiar  feature  shown  by  the  map  is 
