312  PRE-CAMBRIAN   ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
Currant  creek,  at  the  west  side  of  South  park,  near  mount  Lincoln,  at 
Idaho  springs,  on  the  east  side  of  South  park,  massive  granite  or  syen- 
ite is  seen  grading  into  gneiss  and  mica-schist.  At  Chicago  creek 
coarsely  crystalline  granite  is  sharply  separated  from  the  adjacent 
gneiss,  the  junction  being  as  sharp  as  between  a  trap  dike  and  adjoin- 
ing rock.  Some  of  the  porphyritic  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  it  is  said 
the  prevailing  rock  is  a  micaceous  schist  passing  into  gneiss,  and  con- 
taining much  granite,  which  in  some  localities  entirely  replaces  the 
others.  Not  unfrequently  the  mica-schist  is  displaced  gradually  by  horn- 
blende-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  re- 
specting the  relations  of  these  rocks.  Hayden,  in  one  of  his  reports, 
has  referred  them  with  doubt  to  the  Laurentian.  To  determine  this 
matter  careful  investigation  at  the  north  is  still  needed. 
King,7  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  hori- 
zons of  the  Medicine  bow  and  the  higher  members  of  the  Park  range, 
Eed  creek  in  the  Uinta,  the  Wasatch  and  Salt  lake  islands,  and  the 
exposures  in  the  Humboldt  mountains,  Franklin  buttes,  and  Kinsley 
district.  In  the  Clear  creek  region  the  series  is  not  less  than  25,000 
feet  thick. 
Emmons,  (S.  F.),45  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,46  in  1890,  made  observations  upon  the  district  of  South  Boul- 
der, Coal  and  Ealston  creeks.  In  the  South  Boulder  and  Coal  creek 
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  dy- 
namic 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 
