#8  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
area  in  the  center  of  the  district,  about  5  miles  long  in  a  northwest] 
southeast  direction  and  about  3  miles  wide.  According  to  Cross  these 
breccias  and  tuffs  rest  in  part  upon  an  earlier  flow  of  andesite,  but 
mainly  upon  an  unevenly  eroded  surface  of  the  granites  and  schists, 
although  along  the  southwest  edge  of  the  area  the  contact  was  found 
to  be  so  steep  as  "  to  support  the  idea  that  the  central  vent  or  vents  of 
the  volcano  were  adjacent  to  this  line."  The  breccia  is  much  indu- 
rated and  altered,  but  Avas  thought  by  Cross  to  consist  mainly  of  andel 
sitic  fragments,  although  it  was  recognized  that  fragments  of  phono- 
lite  are  locally  abundant.  The  most  characteristic  massive  rock  of 
the  Cripple  Creek  volcano  is  phonolite,  which  was  erupted  at  several 
periods  and  more  abundantly  than  any  other  type,  It  occurs  as 
dikes  and  masses,  not  only  in  the  breccia,  but  in  the  surrounding 
granitic  rocks. 
The  general  succession  of  igneous  rocks,  according  to  Cross,  is  as 
follows:  The  earliest  rocks  were  andesites  containing  some  orthoclase.. 
Then  came  a  series  of  allied  phonolitic  rocks,  rich  in  alkalies  and; 
moderately   rich   in   silica,   together   with   some   andesites.     Among  jj 
them    are   trachytic   phonolite,   nepheline-syenite,    syenite-porphyry,  j| 
phonolite,    mica-andesite,    and    pyroxene-andesite.      Phonolite    was 
erupted  at  several  periods.     The  nepheline-syenite  he  considered  as 
probably  younger  than  the  trachytic  phonolite.     At  the  close  were 
intruded  a  small  number  of  narrow  dikes  of  basic  rocks,  the  so-called 
basalts,  which  contrast  very  markedly  with  the  phonolite. 
MODIFICATION    OF    EARLIER    RESULTS. 
In  the  course  of  the  present  investigation  the  geology  of  the  dis 
trict  has  been  entirely  remapped  upon  the  carefully  revised  topo  I 
graphic  base.     The  granites,  gneisses,  and  schists  have  been  different 
tiated  and  outlined  in  greater  detail  than  was  practicable  in  theearliei 
investigation.     The  oldest  rocks  in  the  district  are  muscovite-  ail 
fibrolite-schists.     These  are  closely  associated  with  the  fine-grain m 
granitic  gneisses  such  as  underlie  most  of  the  town  of  Cripple  Creel 
Tli is  gneiss,  in  the  earlier  report,  was  mapped  partly  as  schist  am 
partly  as  granite,     Both  gneiss  and  schist  are  cut  by  a  reddish  granit  i 
which  occupies  a  considerable  area  extending  from  Anaconda  west- 
ward  beyond  the  limits  of  the  area   studied.     This  granite  is  weM 
exposed  along  Cripple  Creek  in  the  vicinity  of  Mound. 
.V  second  type  of  granite  distinguished  and  mapped  is  the  coarsel ' 
porphyritic  rock  referred  to  by  Cross  as  the  Pikes  Peak  type  of  grai 
ite,  This  rock  occupies  over  half  of  the  district  and  is  the  prevailin  ! 
type  along  the  northern,  eastern,  and  southern  borders  of  the  area 
The  present  investigation  indicates  some  necessary  modification 
the  earlier  report  in  the  way  of  stronger  emphasis  on  the  intimal 
is  c 
