826  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
now  much  metamorphosed,  yet  sharply  distinguishable  from  the 
schists  and  gneisses  which  occur  near  them.  The  complex  folding 
and  metamorphism  which  these  ancient  sediments  have  undergone, 
the  great  orogenic  disturbances,  and  enormous  erosion,  all  of  which 
preceded  the  upper  Cambrian  epoch  of  sedimentation,  seem  to  war- 
rant the  conclusion  that  these  older  clastic  rocks  are  of  Algonkian  age. 
Whether  the  Needle  Mountains  group  of  southern  Colorado  can 
be  surely  correlated  with  the  quartzites,  etc.,  of  the  Front  Range  is 
open  to  question.  It  is  a  natural  suggestion  that  the  former  may  be 
equivalent  to  some  portion  of  the  Grand  Canyon  section  of  Algon- 
kian sediments.  The  quartzites  of  the  Front  Range  doubtless  belong 
to  one  series,  and  their  structural  relations  indicate  that  they  may  re- 
appear either  to  the  north  or  to  the  south  of  Colorado,  within  or  on 
the  flanks  of  the  Rocky  Mountain  ranges. 
Massive  igneous  rocks. — Wherever  any  considerable  area  of  Archean 
gneisses  and  schists  in  Colorado  has  been  closely  examined  it  has 
been  found  to  be  penetrated  by  intrusive  igneous  rocks,  often  in  a 
very  intricate  interlacing  of  different  varieties.  The  rocks  here  re- 
ferred to  are  either  unmetamorphosed  or  but  locally  affected.  They 
do  not  penetrate  the  early  Paleozoic  sediments,  where  the  latter  are 
preserved,  and  hence  must  be  considered  as  pre-Saratogan,  if  not 
strictly  pre-Cambrian.  In  the  Pikes  Peak  quadrangle  the  great 
granitic  masses  have  intrusive  relations  to  the  Algonkian  quartzites, 
and  similar,  though  less  marked,  relations  exist  in  the  Needle  Moun- 
tains, so  that  at  least  a  part  of  these  massive  rocks  must  be  con- 
sidered as  of  Algonkian  or  post-Algonkian  age. 
Among  these  intrusives,  granites  are  by  far  the  most  common,  but 
syenite,  monzonite,  diorite,  and  gabbroic  rocks  are  known  as  occur- 
ring in  large  bodies.  A  batholith  of  gabbro  several  miles  in  diameter 
occurs  in  the  Animas  Valley,  lying  in  the  Engineer  Mountain  quad- 
rangle. 
In  addition  to  the  masses  of  considerable  size  just  referred  to, 
there  are  countless  dikes  of  pegmatite,  aplite,  and  granite  on  the  one 
hand,  and  of  diabase,  pyroxenite,  and  other  basic  rocks  on  the  other, 
with  many  intermediate  kinds,  which  are  not  known  to  cut  the  Pale- 
ozoic sediments.  These  basic  dikes  are  distinct  from  the  more  an- 
cient ones  now  represented  by  amphibolitic  schists  which  are  com- 
mon in  the  Archean  complex  in  intricate  relations  with  schists  and 
gneisses. 
FRONT  RANGE.0 
By  far  the  greater,  portion  of  the  Front  Range  of  Colorado  is  com- 
posed of  the  typical  basement  complex,  or  Archean,  and  intrusive 
granites.    In  the  central  part  of  the  range  gneissoid  granites  are  pre- 
a  By  the  authors. 
