836  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
because  they  are  so  widely  separated  from  any  beds  distinctly  recog- 
nized as  such,  and  the  reference  is  based  entirely  upon  lithological 
evidence.  The  rocks  also  present  many  features  in  contrast  with 
Laurentian  rocks  of  the  Colorado  Front  Range. 
The  Park  Range,  the  third  of  the  great  Archean  uplifts  of  the 
Rocky  Mountains,  is  a  system  of  highly  crystalline  rocks  of  Archean 
age.  The  later  rocks  form  a  very  subordinate  part  of  the  uplift, 
rising  not  more  than  a  few  hundred  feet  above  the  plain,  where  they 
rest  unconformably  on  the  older  series.  The  rocks  of  the  Park  Range 
resemble  more  closely  those  of  the  Colorado  Front  Range  than  they  do 
the  Medicine  Bow,  and  are  referred  to  the  Laurentian.  The  range 
contains  much  structureless  granite  overlain  by  gneisses  and  schists 
similar  to  the  series  of  the  Colorado  Range,  but  carrying  more  horn- 
blende-bearing beds  in  the  upper  members.  On  the  other  hand,  there 
are  not  wanting  rocks  which  are  characteristic  of  the  Medicine  Bow 
series  and  which  were  referred  to  the  Huronian  formation.  The 
range  has  a  monoclinal  structure,  with  the  prevailing  dips  to  the 
west,  while  an  outlying  spur  to  the  east  indicates  the  existence  of  the 
eastern  side  of  the  fold. 
Emmons,35  in  1877,  describes  Rawlings  Peak  as  an  outlying  area  of 
Archean  granite  gneiss  which  shows  distinct  lines  of  bedding,  having 
an  inclination  of  45°  W.,  while  the  overlying  quartzites  and  sand- 
stones dip  10°  E. 
King,36  in  1878,  describes  the  rocks  which  unconformably  overlie 
the  Archean  of  the  Colorado  Range  as  varying  from  the  lowest  Paleo- 
zoic up  to  the  post-Pliocene.  The  Archean  core  of  the  range  is  a  broad 
central  anticline,  the  arch  having  a  flat  summit  and  the  dip  increas- 
ing rapidly  as  the  axis  becomes  distant.  In  this  range  complex  fault- 
ing, metamorphism,  and  crystallization,  combined  with  widespread 
erosion,  took  place  before  the  beginning  of  Cambrian  time.  The 
rocks  comprise  granites  and  granite  gneisses,  above  which,  with  no 
apparent  unconformity,  are  red  granites,  showing  distinct  bedding, 
and  above  these  is  a  great  thickness  of  mica  gneisses,  estimated  to  be 
12,000  to  18,000  feet.  From  the  lowest  exposure  to  the  highest  there 
is  a  gradual  passing  from  the  structureless  granite  to  the  dark  mica 
gneisses.  Among  eruptive  rocks  are  granites,  gabbros,  and  felsite 
porphyries.  The  Clarks  Peak  ridge  is  thought  to  be  another  and  later 
series  of  rocks  than  those  of  the  Colorado  Range. 
In  the  Medicine  Bow  Range,  above  the  hornblendic  and  dioritic 
gneisses  and  schists  are  quartzitic  schists,  argillites,  and  limestones. 
The  gneisses  and  hornblende  schist  are  older  and  underlie,  in  apparent 
conformity,  the  quartzites. 
In  the  Park  Range  the  crystalline  rocks  all  dip  to  the  west,  being 
but  half  of  an  anticline,  the  other  half  having  suffered, a  deep  down- 
