804  PRE-CAMBRTAN    GEOLOGY    OF    NORTH    AMERICA. 
rocks,  granites,  and  syenites  are  placed  among  the  eruptives.  This 
is  done  in  deference  to  commonly  received  opinion;  but  as  there  is  no 
locality  in  which  these  rocks  do  not  pass  imperceptibly  into  gneiss, 
the  conclusion  is  reached  that  the  preponderance  of  evidence  is  in 
favor  of  their  metamorphic  origin. 
In  speaking  of  Colorado  metamorphic  rocks  in  general,  Stevenson 
says  that  the  prevailing  rock  is  a  micaceous  schist  passing  into  gneiss 
and  containing  much  granite,  which  in  some  localities  entirely  re- 
places the  others.  Not  infrequently  the  mica  schist  is  displaced 
gradually  by  hornblende  schist,  which  becomes  a  hornblende-  gneiss, 
containing  masses  or  strings  of  syenite,  as  the  other  form  contains 
ordinary  granite.  Slates  are  -almost  wanting,  and  thick  strata  of 
quartzite  belonging  to  this  series  were  observed  at  only  two  or  three 
localities.  Serpentine  and  limestone  seem  to  be  absent  altogether. 
It  is  impossible  in  the  present  state  of  our  knowledge  to  come  to 
any  definite  conclusion  respecting  the  relations  of  these  rocks.  Hay- 
den,  in  one  of  his  reports,  has  referred  them  with  doubt  to  the  Lau- 
rentian.  To  determine  this  matter  careful  investigation  at  the  north 
is  still  needed. 
King,30  in  1878,  states  that  in  the  Colorado  Range  are  two  series 
which  are  probably  unconformable.  The  upper  group  is  distinctly 
bedded,  has  a  variable  amount  of  mica,  and  is  correlated  with  the 
upper  horizons  of  the  Medicine  Bow  and  the  higher  members  of  the 
Park  Range,  Red  Creek  in  the  Uinta,  the  Wasatch,  and  Salt  Lake 
islands,  and  the  exposures  in  the  Humboldt  Mountains,  Franklin 
Buttes,  and  Kinsle}^  district.  In  the  Clear  Creek  region  the  series 
is  not  less  than  25,000  feet  thick. 
Emmons  (S.  F.),cg  in  1890,  states  that  Cross  has  discovered  in  the 
hills  east  of  the  Arkansas  River,  at  Salida,  a  thickness  of  about 
10,000  feet  of  slates  and  schists  entirely  distinct  from  the  Archean 
and  probably  unconformable  with  it.  These  are  referred  to  the 
Algonkian. 
Lakes,07  in  1890,  made  observations  upon  the  district  of  South 
Boulder,  Coal,  and  Ralston  creeks.  In  the  South  Boulder  and  Coal 
creeks  area  were  found  between  the  Trias  and  the  heavily  bedded 
gneisses  a  series  of  quartzites,  schists,  and  conglomerates,  the  clastic 
character  of  which  is  unmistakable.  The  series  has  been  subjected 
to  intense  dynamic  action,  so  that  the  pebbles  of  the  conglomerate 
are  elongated,  and  if  it  were  not  for  the  bands  of  this  material  it 
would  be  difficult  to  show  that  the  series  was  an  original  clastic  one, 
as  the  finer  grained  rocks  are  completely  crystalline  quartz  schists 
and  mica  schists.  The  dip  of  the  series  is  at  a  high  angle  away  from 
the  main  mass  of  the  mountains.  Its  higher  members  are  quartzite 
and  pass  down  into  mica  schists  and  quartz  schists,  which  are  inter- 
stratified  with  beds  of  conglomerate.    In  passing  downward  the  mica 
