THE   CORDILLERAS.  831 
of  the  Needle  Mountains  group  establish  the  relative  age  of  these 
two  formations. 
Needle  Mountains  group. — The  sedimentary  rocks  observed  by 
Endlich,  and  much  later  by  Van  Hise,  form  a  very  important  ele- 
ment of  the  Needle  Mountains  and  have  received  the  name  Needle 
Mountains  group.  They  occur  in  a  highly  compressed  and  faulted 
synclinorium  forming  a  wide  crescentic  zone  through  the  northern 
and  eastern  part  of  the  mountain  group.  Many  rugged  peaks  ex- 
ceeding 13,000  feet  in, height  occur  within  this  zone.  While  the 
prominent  quartzites  gave  rise  to  the  name  used  by  the  Hayden  geo- 
logists for  these  mountains  and  to  the  distinction  of  the  large  part 
of  the  area  on  the  map  as"  metamorphic  Paleozoic,"  the  sediments 
really  occupy  but  about  half  of  the  area  thus  designated,  the  re- 
mainder being  chiefly  a  very  massive  granite. 
The  Animas  Canyon  section  of  the  Needle  Mountains  group,  some 
miles  long,  which  was  studied  by  Van  Hise  in  1889,  exhibits  only 
quartzites  and  slates  or  shales,  the  outer  boundaries  being  very  steeply 
inclined  faults,  in  general  parallel  to  the  complex  folds  and  faults 
of  the  geosyncline.  Quartzite  in  very  massive  hard  banks,  often  con- 
glomeratic, but  with  very  small  pebbles,  is  the  predominant  rock. 
Dark  shale,  now  variably  metamorphosed  to  slate,  constitutes  the 
minor  part  of  the  section. 
This  complex  of  shales  and  quartzites  has  been  called  the  Un- 
compahgre  formation,  since  it  is  apparently  equivalent  with  the 
lithologically  identical  succession  of  pre-Cambrian  beds  shown  in  the 
Uncompahgre  Canyon,  on  the  northern  side  of  the  San  Juan  Moun- 
tains, some  miles  away. 
The  lower  formation  of  the  Needle  Mountains  group  is  exposed  in 
the  Vallecito  Valley,  in  the  southeastern  part  of  the  Needle  Moun- 
tains quadrangle,  and  extending  a  few  miles  east  into  the  valley  of 
Pine  River.  It  is  preeminently  a  conglomerate  of  coarse  texture,  as 
a  rule,  and  has  been  named  the  Vallecito  conglomerate.  Its  base  is  seen 
resting  unconformably  on  the  Irving  greenstone,  and  the  lower  part 
of  the  conglomerate  is  largety  made  up  of  greenstone  pebble-.  Up- 
ward, quartzite  and  various  schists  are  more  and  more  abundant 
among  the  pebbles.  Small  pebbles  of  jasper  and  hematite  or  mag- 
netite are  also  abundant  in  the  upper  part  of  the  Vallecito  conglom- 
erate, testifying  to  a  former  iron-bearing  formation  which  seems 
probably  to  have  been  associated  with  the  Irving  greenstones.  The 
thickness  of  the  Vallecito  conglomerate  has  not  been  measured,  hut 
it  is  2,000  feet  or  more. 
Intrusive  granites  end  <>tln  r  rocks. — The  high  central  peaks  of  the 
Needle  Mountains,  including  Pigeon  Peak  and  Mount  Eolus,  are 
wholly  of  a  massive  granite  belonging  to  a  huge  batholith  exposed 
over  many   square    miles   and   disappearing   beneath    the    Paleozoic 
