822  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
ern  Colorado.  They  comprise  massive  and  schistose,  granular,  and 
porphyritic  metagabbro  and  areas  of  mashed  granitic  intrusives  and 
other  schistose  rocks,  presumably  altered  quartzites.  No  evidence  of 
surface  origin  is  noted  in  the  igneous  rocks.  The  greenstones  ante- 
date the  Algonkian  sediments  to  the  north,  as  shown  by  the  pebbles 
contained  in  the  Algonkian  conglomerate,  and  have  an  older  aspect 
than  the  other  rocks  of  the  neighboring  areas.  They  are  therefore 
assigned  to  the  early  Algonkian.  Attention  is  called  to  their  simi- 
larity to  the  greenstones  of  the  Menominee  and  Marquette  districts 
of  Michigan  and  to  rocks  near  Salida,  Colo. 
Cross,  Howe,  and  Ransome,104  in  1905,  map  and  describe  the 
geology  of  the  Silverton  quadrangle,  in  southwestern  Colorado. 
Schists  of  supposed  Archean  age  form  the  walls  of  the  canyon  of 
Animas  River  from  the  monzonite  stock  below  Silverton  to  a  line  a 
short  distance  beyond  the  southern  boundary  of  the  quadrangle,  at 
Whitehead  Creek,  where  they  come  in  fault  contact  with  Algonkian 
quartzites.  They  are  exposed  in  an  irregular  area  that  extends  from 
the  canyon  eastward,  from  which  the  volcanics  have  been  removed  by 
erosion,  and  in  Cunningham  Gulch  down  nearly  to  Stony  Creek. 
The  rocks  are  all  strongly  foliated  and  also  have  a  banded  appear- 
ance, which  is  often  due  to  the  alternation  of  dark  and  light  con- 
stituents. The  schistosity,  which  seems  to  correspond  with  the  banded 
arrangement  of  mineral  particles,  has  within  this  area  a  northeast- 
southwest  strike  and  a  nearly  vertical  attitude,  except  in  the  lower 
part  of  Animas  Canyon,  where  the  dip  flattens  out  and  the  schistosity 
becomes  almost  horizontal.  This  schist  section,  while  varying  in 
petrographic  detail  from  place  to  place,  possesses  no  clearly  marked 
divisions  which  it  is  practicable  or  desirable  to  recognize  as  carto- 
graphic units.  The  schists  are  cut  by  bosses  of  granite  and  their 
apophyses,  as  well  as  by  numerous  basic  dikes.  These  intrusives  are 
undoubtedly  older  than  the  first  Paleozoic  sediments,  but  may  be 
younger  than  the  great  Algonkian  series  of  the  adjacent  Needle 
Mountains. 
Algonkian  quartzites,  slates,  or  shales  appear  in  Uncompahgre 
Canyon,  with  dips  varying  from  vertical  to  less  than  30°,  and  on  the 
eastern  side  of  Ironton  Park  at  the  mouths  of  Albany  and  Brooklyn 
gulches.  The  base  of  the  Uncompahgre  series  is  apparently  near  its 
southern  border  and  the  top  near  Ouray.  The  structural  relations 
of  the  Algonkian  section  in  the  Uncompahgre  Valley  to  that  of  the 
Needle  Mountains  can  not  now  be  ascertained,  for  between  them 
occur  the  older  schists  or  the  Tertiary  volcanics. 
Cross,  Howe,  Irving,  and  Emmons,105  in  1905,  describe  the  Needle 
Mountains  quadrangle.  Gneisses  and  schists  exhibiting  profound 
metamorphism,  but  shown  by  their  composition  to  be  probably  igne- 
ous, are  unconformably  below  quartzites  of  the  Needle  Mountains 
