THE   CORDILLERAS.  829 
brian  sediments  rest  unconformably  against  the  crystalline  rocks, 
which  are  therefore  pre- Cambrian,  but  scarcely  to  be  more  closely 
correlated  with  present  evidence. 
SA WATCH    RANGE    AND    ADJACENT    VALLEYS.0 
The  Sawatch  Range  is  composed  mainly  of  granites,  gneisses,  and 
schists.  Near  Salida,  Marshall  Pass,  and  Tincup  Pass  the  schists  are 
hornblendic,  micaceous,  and  chloritic,  believed  by  Cross  to  be  a  series 
of  metamorphosed  Algonkian  lavas,  though  containing  at  Tincup 
Pass  highly  crystalline  marble.  Abundant  quartzites,  interbedded 
with  schistose  acidic  porphyries  and  diorities,  were  observed  by  Van 
Ilise  and  Leith  in  the  Salida  area.  Some  of  the  green  schists 
observed  by  Van  Hise  and  Leith  along  Tincup  Pass  are  thought  to 
be  metamorphosed  ellipsoidal  basalts  very  similar  to  those  character- 
istic of  the  Keewatin  series  of  the  Archean  system  of  the  Lake 
Superior  region. 
The  presence  of  limestone  and  quartzite  in  the  pre-Cambrian  schists 
of  the  Sawatch  and  adjacent  valleys,  making  up,  in  the  case  of  the 
quartzites,  the  major  part  of  the  series,  suggests  the  Algonkian  age 
of  this  part  of  the  series,  but  the  granites  and  the  other  gneisses  and 
schists  may  be  of  the  same  or  different  ages.  Sufficient  work  has 
not  yet  been  done  to  separate  them  lithologically  or  structurally.  The 
best  that  can  be  done  is  to  refer  the  entire  complex  to  the  pre- 
Cambrian. 
GRAND  AND  GUNNISON  VALLEYS.a 
Here  are  considerable  areas  of  granites  and  gneiss  of  pre-Cambrian 
age.  In  the  Gunnison  Valley  Peale  h  reports  the  existence  of  quartz- 
itic  layers  with  the  gneiss.  Quartzites,  taken  to  be  of  pre-Cambrian 
age,  are  abundant  south  of  Iola  on  Gunnison  River. 
NEEDLE    MOUNTAINS    AND    VICINITY. 
The  following  summary  for  Needle  Mountains  and  vicinity  has 
been  furnished  by  Whitman  Cross: 
General  statement. — The  area  of  pre-Cambrian  rocks  occurring  in 
the  Needle  (Quartzite)  Mountains  and  Animas  Canyon  of  south- 
western Colorado  has  been  studied  in  the  Needle  Mountains.  Engi- 
neer Mountain,  Silverton,  and  Durango  quadrangles.  The  extreme 
eastern  part  has  not  been  examined. 
The  Ilayden  map  represents  the  northern  and  central  pail  of  the 
Needle  Mountains  as  composed  of  "metamorphic  Paleozoic" 
("  M  P"),  while  bordering  the  mountains  on  the  easl  and  south  and 
extending  down  the  Animas  Canyon  is  an  irregular  area  of  "  tneta- 
"  By  (lie  authors. 
*Peale,  A.  C,  Geological  reporl  on  the  Grand  River  district;  Ninth  Ann.  Rept.  r.  s 
Geol.  and  Geog.  Survey  Ten-.,  ISTT,  pp.  ::!>    KM,  with  atlas  shoots. 
