LIkansume*.ND]         CRIPPLE    GREEK    DISTRICT,    COLORADO.  89 
genetic  relationship  of  the  rocks.  The  "  phonolite,"  "  nepheline- 
syenite,"  "  trachytic  phonolite,"  "  syenite-porphyry,"  and  "  andesites  " 
of  Cross  are  all  very  closely  related  and  have  been  found  to  be  in  most 
cases  connected  by  intermediate  types.  None  of  the  massive  rocks 
can  properly  be  called  andesite,  and  although  it  can  not  be  affirmed 
that  andesitic  fragments  are  entirely  absent  from  the  usually  much 
altered  volcanic  breccia,  the  term  "  andesitic  breccia  "  does  not  seem 
applicable  to  this  formation  as  a  whole.  It  would  be  more  accurate 
to  describe  it  as  a  phonolitic  breccia,  although  in  places  it  consists 
chiefly  of  particles  of  the  older  rocks  through  which  the  Tertiary 
erupt ives  broke. 
While  it  is  undoubtedly  true  that  some  of  the  breccia  in  the  north- 
eastern part  of  the  volcanic  area  rests  upon  a  very  uneven  surface  of 
granite,  gneiss,  and  schist,  the  results  of  held  work  during  the  last 
season,  favored  by  deep  workings  not  in  existence  when  the  district 
was  originally  surveyed,  have  emphasized  the  fact  that  the  breccia 
lying  southwest  of  a  general  northwest-southeast  line  drawn  through 
Big  Bull  Mountain  and  Gold  Hill  occupies  a  chasm  of  profound 
depth  in  the  fundamental  rocks  of  the  region.  From  the  Conundrum 
mine  on  the  western  slope  of  Gold  Hill  to  Stratton's  Independence 
mine  on  the  south  slope  of  Battle  Mountain  the  contact  plunges 
steeply  down,  with  dips  ranging  in  general  from  70°  to  vertical.  In 
some  instances  the  granite  walls  of  this  chasm  actually  overhang  the 
breccia.  It  is  probable  that  this  entire  southwest  contact  represents 
a  part  of  the  wall  of  the  great  pit  formed  by  the  volcanic  explosions 
that  produced  the  breccia.  It  is  further  probable  that  an  arm  or 
branch  of  this  volcanic  abyss,  now  filled  with  breccia  and  intrusive 
rocks,  extends  northeastward  past  Bull  Cliff  and  the  town  of  Altman. 
ECONOMIC   GEOLOGY. 
EARLIER    WORK. 
To  the  excellent  work  of  Mr.  R.  A.  F.  Penrose,  jr.,  apply  statements 
'  similar  to  those  made  in  the  discussion  of  the  purely  geological  branch 
I  of  the  subject.     Few  shafts  had  then  attained  a  depth  of  400  feet,  and 
1  most  of  the  exposures  were  masked  by  surface  oxidation.     It  would 
be  surprising,  in  view  of  the  facilities  created  by  the  later  develop- 
ment of  hundreds  of  mines,  if  a  subsequent  investigation  should  not 
bring  out  some  slight  modifications  of  earlier  results. 
EXTENT    OF    PRODUCTIVE    TERRITORY. 
There  is  nothing  in  the  history  of  the  district  since  ISO!  warranting 
any  extension  of  the  bounds  of  the  productive  territory  as  then 
known.  Now,  as  then,  a  circle  of  3  miles  radius  described  from  the 
summit  ol  Gold  Hill  would  include  all  deposits  of  known  or  prospec- 
