832  PRE-CAMBRTAN    GEOLOGY    OF    NORTH    AMERICA. 
section  to  the  south.  Other  minor  batholiths  of  granite  occur  cutting 
the  gneisses  and  schists  in  Animas  Valley.  One  of  these  occurs  at 
the  lower  end  of  the  canyon,  near  Rockwood,  and  extends  beneath  the 
Cambrian  quartzite.  A  gabbro  mass  in  gneiss  occurs  just  north  of 
Ignacio  Lakes,  and  is  covered  by  the  sediments  on  its  western  side. 
The  Needle  Mountains  quartzites  and  slates  are  not  penetrated 
by  any  large  granite  masses,  but  the  Eolus  batholith  appears  to  have 
come  up  along  the  southern  fault  border  of  the  quartzites  and  sends 
some  small  arms  into  them.  It  is  evident,  moreover,  that  the  granite 
must  be  later  than  the  Needle  Mountains  sediments,  because  it  bears 
no  evidence  of  crushing  or  other  metamorphism  at  the  time  of  the 
complex  folding  and  shear  faulting  to  which  the  quartzites  and  slates 
were  subject.  The  development  of  chiastolite  in  some  of  the  slate 
bands  may  be  plausibly  attributed  to  the  metamorphosing  action  of 
the  granitic  mass. 
It  would  appear  that  the  massive  granites  of  the  Needle  Mountains 
may  belong  to  the  same  epoch  of  intrusion  as  the  Pikes  Peak  granite, 
which  cuts  the  quartzites  of  the  Front  Range  and  includes  many 
fragments  of  the  same. 
UNCOMPAHGRE    CAN Y<  >  X .° 
In  the  course  of  mapping  the  Silverton  and  Ouray  quadrangles 
opportunity  was  given  for  the  study  of  the  section  of  quartzites  and 
slates  or  shales  referred  to  the  Algonkian  by  Van  Hise  (1889)  and 
Emmons  (1890).  This  section  extends  for  several  miles  along  the 
Uncompahgre  Canyon,  being  overlain  by  volcanic  tuffs  except  near 
Ouray,  where  a  fault  brings  the  upturned  lowest  Paleozoic  strata, 
there  of  Devonian  age,  against  the  quartzites.  The  structure  of  these 
beds  is  that  of  an  elongated  dome,  the  apex  being  near  the  southern 
end  of  the  exposures  in  the  Silverton  quadrangle.  Neither  base  nor 
top  of  the  section  is  visible.  Some  8,000  feet  of  alternating  quartzites 
and  minor  slates  are  exposed,  and  the  formation  is  seemingly  identical 
with  that  forming  the  compressed  synclinorium  in  the  Needle 
Mountains. 
RICO   MOUNTAINS." 
At  the  heart  of  the  Rico  Mountains  uplift,  which  is  due  partly  to  a 
quaquaversal  fold  and  partly  to  laccolithic  intrusions  of  porphyry, 
there  are  small  faulted  plugs  of  massive  quartzite  and  of  dense  schist, 
which  have  been  referred  by  Cross  to  the  Uncompahgre  formation. 
LA  PLATA  MOUNTAINS." 
The  metamorphic  area  represented  by  the  Hayden-map  in  the 
heart  of  the  La  Plata  Mountains  is  of  Paleozoic  and  Mesozoic  sedi- 
ments.   There  are  no  pre-Paleozoic  rocks  exposed  there. 
a  By  Whitman  Cross. 
