HAHNS    PEAK    GOLD    FIELD,    COLORADO. 
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up  largely  of  sediments  of  Mesozoic  and  later  periods.     Tin  se  sedimentary 
lilted,  folded,  and  broken  along  the  immediate  flanks  of  the  older 
mountain  mass  and  extend  away  to  the  west  in  gentler  folds  until 
they  dip  beneath  the  mantle  of  less  consolidated  Tertiary  sedi- 
ments. 
Halms  Peak  and  the  Elkhead  Mountains  are  uplifts  of  recent 
intrusives  which  break  through  the  sedimentary  rocks  of  the 
eastern  portion  of  the  Colorado  Plateau.  The  region  of  these  later 
uplifts  is  one  of  marked  diversity  of  features.  The  rugged  peaks 
and  ridges  of  igneous  formation  are  interspersed  with  patches  of 
the  plateau  topography  and  structure  (see  PI.  II). 
The  rock  formations  about  Hahns  Peak  are  similar  to  those 
found  along  the  eastern  base  of  the  Rocky  Mountains. 
ARCHEAN. 
The  oldest  are  the  Archean  gneisses,  schists,  and  granitic  rocks 
already  referred  to.  The  rocks  of  Farwell  Mountain  belong  to  this  ^ 
general  series.  They  consist  of  gneisses,  schists,  diorites,  coarse  P 
red  granite,  pegmatites,  and  other  rocks.  They  are  largely,  if  not  T 
wholly,  igneous,  although  some  of  the  schists  may  represent  an-  ^ 
cient  and  much  altered  sediments.  Their  general  distribution 
only  is  indicated  on  the  map. 
RECENT  IGNEOUS  ROCKS. 
More  recent  igneous  rocks  of  at  least  two  distinct  types,  rhyolitic 
and  basaltic,  are  present.  One  of  these,  locally  known  as  the 
"white  porphyry,"  forms  the  core  of  Hahns  Peak.  This  rock  is 
light  colored,  usually  more  or  less  coarsely  porphyritic,  showing 
much  quartz  in  distinct  rounded  masses,  and  often  phenocrysts  of 
orthoclase  feldspar.  In  the  outlying  dikes  these  feldspar  crystals 
are  often  large  and  very  perfectly  formed.  The  groundmass  varies 
in  color  from  white  to  pink  and  gray.  Thin  sections  have  been 
prepared  from  specimens  of  this  rock  collected  from  the  summit 
of  the  peak,  from  the  wall  rock  of  the  Southern  Cross  tunnel,  and 
from  the  smaller  knobs  south  of  the  main  peak.  These  slides  were 
examined  by  Mr.  Albert  Johannsen,  who  gives  the  following  petro- 
graphic  description.  Under  the  microscope  the  slides  show  phe- 
nocrysts somewhat  less  in  amount  than  the  groundmass.  The 
phenocrysts  consist  of  quartz  and  orthoclase  about  equal  in 
amount,  plagioclase  in  less  amount  than  either  of  the  foregoing, 
and  a  still  smaller  quantity  of  biotite  mica.  The  groundmass  is 
apparently  a  devitrified  glass  of  feldspar-quartz  aggregate  with 
numerous  biotite  shreds.  Both  phenocrysts  and  groundmass  are 
considerably  altered.  The  rock  is  classified  as  a  rhyolite-porphyry 
containing  considerable  plagioclase. 
The  mass  composing  Hahns  Peak  seems  to  have  been  thrust  up 
from  below,  in  places  lifting  and  arching  the  Cretaceous  rocks  and 
in  other  places  cutting  through  the  formations  (see  fig.  1).  The 
main  summit  has  the  topographic  outline  of  a  volcanic  peak,  but 
this  summit  is  only  a  small  part  of  a  huge  mass  of  rhyolitic  mate- 
rial of  fairly  uniform  composition.  The  irregular  surface  of  this 
intrusive  mass  is  exposed  on  several  minor  peaks  on  various  sides 
of  Hahns  Peak,  and  many  dikes  of  the  same  material  radiate 
into  the  surrounding  sediments,   These  dikes  cut  through  instead  of  arching  the  sediments. 
=■     3- 
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