THE   CORDILLERAS.  819 
many  places  the  schistose  character  is  very  distinct  and  the  bedding 
clearly  seen,  but  in  most  cases  no  traces  of  bedding  were  seen,  the 
rocks  being  granitoid.  From  the  number  of  exposures  noticed  it  is 
evident  that  the  rocks  underlie  the  entire  district,  although  from  the 
limited  and  isolated  exposures  it  was  not  possible  to  trace  connections 
from  one  place  to  another.  The  oldest  sedimentary  beds  resting 
upon  these  rocks  are  Carboniferous  or  pre-Carboniferous,  showing 
that  they  are  at  least  pre-Carboniferous,  but  it  is  believed  that  they 
are  pre-Silurian.  Along  the  Gunnison  occur  outcrops  of  quartzitic 
layers  with  softer  red  and  gray  gneissic  layers.  On  the  Little  Dolores 
are  mica  schists  and  quartzites  dipping  northeast  at  angles  of  00°  to 
70°.  As  to  the  origin  of  these  rocks  it  is  said:  They  were  once  de- 
posited as  sediments.  Whence  were  their  materials  derived  ?  We 
have  no  data  from  which  we  are  able  even  to  guess  what  was  the 
extent  of  the  Archean  continent,  or  what  its  character  was.  From 
the  fact  that  in  the  Grand  Canyon  of  the  Colorado  similar  rocks  are 
found  below  the  Potsdam,  and  from  the  profundity  of  their  meta- 
morphism,  it  is  believed  that  these  crystalline  rocks  are  Archean. 
SOUTHWESTERN     COLORADO,     INCLUDING     THE     SAN     JUAN,     NEEDLE,     AND 
LA   PLATA    MOUNTAINS. 
Endlich,97  in  1876,  describes  the  Quartzite  (now  known  as  Xeedle) 
Mountains.  Near  the  northern  border  and  toward  the  middle, 
quartzites  and  schists  predominate,  while  granite  appears  toward  the 
east  and  south.  An  attempt  is  made  to  prove  a  gradation  between 
quartzites,  granites,  and  schists.  The  Devonian  strata  were  deposited 
on  the  granitic  strata  conformably. 
Endlich,81  in  1878,  gives  a  general  discussion  of  the  formations  of 
Colorado.  In  preference  to  the  word  Azoic  the  word  Prozoic  is  used. 
Belonging  to  this  group,  in  southern  Colorado,  is  an  extensive  series 
comprising  gneisses,  granites,  various  schists,  and  diorites.  Of  these 
the  first  named  appear  to  be  the  oldest,  as  may  be  inferred  from  the 
relations  to  the  granites  more  particularly.  The  schists  arc  in  subor- 
dinate quantity.  It  often  is  a  matter  of  difficulty  to  discriminate  be- 
tween the  Prozoic  and  the  next  group  of  metamorphic  rocks.  This 
latter  is  the  most  varied  and  enormous  in  its  development.  In  several 
instances  localities  may  be  observed  where  the  transition  from  un- 
doubted sedimentary  into  metamorphic  beds  is  evident.  A  large 
part  of  the  granites  in  southern  Colorado  is  regarded  as  metamor- 
phosed Silurian,  Devonian,  and  in  rare  instances  even  Carboniferous 
strata. 
Comstock,98  in  1883,  states  that  in  San  Juan  County  there  are  no 
rocks  of  Archean  age.  The  granitic  and  quartzitic  series  of  Animas 
River  are  regarded  as  metamorphic  and  said  to  be  of  Upper  Silurian 
or  Devonian  age. 
