830  PRE-CAMBRTAN    GEOLOGY   OF    NORTH    AMERICA. 
morphic  granite''  ("M  G").  The  study  of  these  formations  by 
Cross  and  Howe  has  shown  that  the  Hayden  distinction  has  no  mean- 
ing.    The  rocks  are  divisible  into — 
1.  The  Archean  gneisses  and  schists. 
2.  A  group  of  greenstones,  with  derived  gneisses  and  schists,  and 
intimately  associated  with  very  subordinate  quartzites. 
3.  A  group  of  conglomerates,  quartzites,  and  shales  or  slates,  called 
the  Needle  Mountains  group. 
4.  Intrusive  and  still  massive  granite  and  gabbro  bodies  of  large 
size,  with  many  smaller  dikes. 
Archean  gneisses  and  schists. — The  oldest  rocks  of  the  area  are  the 
gneisses  and  schists,  most  prominently  displayed  in  the  Animas  Can- 
yon. These  are  mainly  granitic  or  dioritic  gneisses  and  amphibolitic 
schists.  They  are  exposed  in  a  thickness  of  many  thousand  feet  in 
the  canyon  section,  the  structure  being  complex  and  defying  accurate 
estimate  of  the  thickness.  Some  of  the  gneisses  show  evidence  of  a 
derivation  by  crushing  and  recrystallization  from  igneous  rocks, 
granites,  or  diorites.  No  demonstrable  sedimentary  rocks  have  been 
found  in  the  complex. 
The  dark  amphibolitic  schists  occurring  about  the  mouth  of  Cas- 
cade Creek  have  been  intruded  by  a  biotite  granite  in  an  extremely 
intricate  manner.  Over  several  square  miles  the  schists  are  either 
split  in  thin  strips  and  layers,  a  few  feet  or  perhaps  only  a  few  inches 
thick,  or  the  brecciation  has  been  irregular,  the  intruded  granite 
being  often  in  equal  or  even  subordinate  amount.  In  some  parts  the 
granite  is  nearly  free  from  inclusions  and  becomes  massive. 
The  gneisses  and  schists  are  interrupted  in  the  heart  of  the  moun- 
tains by  granite  or  the  Needle  Mountains  group  of  sediments,  but 
reappear  in  the  canyon  below  Silverton  and  extend  easterly  under 
the  volcanic  rocks  an  unknown  distance. 
Irving  greenstone. — This  formation  is  exposed  in  only  one  area  in 
the  southeastern  part  of  the  mountains,  in  the  valley  of  Vallecito 
Creek.  Petrographically  the  rocks  consist  of  hornblendic  metadia- 
base,  massive  in  texture,  but  grading  into  typical  greenstone  gneisses 
and  schists  almost  indistinguishable  from  the  Quinnesec  schist  of  the 
Menominee  district  on  Lake  Superior.  The  rocks  are  intricately 
folded  and  the  thickness  of  their  visible  section  can  not  be  determined, 
but  it  is  probably  2,000  or  3,000  feet  at  least. 
Typical  quartzite  is  locally  found  with  these  greenstone  schists,  but 
whether  as  a  contemporaneous  deposit  or  as  an  infolded  fragment 
from  a  separate  formation  could  not  be  satisfactorily  determined. 
The  relations  of  the  Irving  greenstone  to  the  gneisses  and  schists 
of  the  Animas  Canyon  section  can  not  be  ascertained,  as  they  are 
separated  by  a  huge  granitic  mass.  Contact  relations  and  an  abun- 
dance of  greenstone  pebbles  and  bowlders  in  the  basal  conglomerate 
