THE   CORDILLERAS.  823 
group  (Algonkian),  and  are  assigned  to  the  Archean.  These  rocks 
follow  the  canyon  of  the  Animas  from  a  point  to  the  west  in  the 
Engineer  Mountain  quadrangle,  in  a  band  3  or  more  miles  wide, 
northward  to  the  Algonkian  quartzites  near  Elk  Park,  with  an  arm 
extending  northeastward  from  the  mouth  of  Ruby  Creek  to  Balsam 
Lake.  They  are  interrupted  at  several  places  by  intrusive  bodies  of 
granite.  North  of  the  zone  of  Algonkian  rocks  the  schists  again 
appear  at  the  mouth  of  Whitehead  Creek  and  extend  continuously 
along  the  northern  border  of  the  quadrangle  from  near  Molas  Lake 
to  Beartown.  These  exposures  continue  for  a  few  miles  northward 
in  the  Silverton  quadrangle  until  covered  by  sedimentary  or  volcanic 
rocks. 
Provisionally  assigned  to  the  Algonkian  are  the  Irving  greenstones, 
consisting  of  a  complicated  series  of  schists,  greenstones,  and  subor- 
dinate quartzites,  a  portion  of  which  is  prominently  exposed  in  the 
southeastern  part  of  the  quadrangle  and  of  which  Irving  Peak  is  com- 
posed. In  some  respects  similar  to  the  Archean  schists  of  Animas 
Canyon,  the  Irving  greenstone  must,  nevertheless,  be  distinguished 
from  them  on  account  of  the  lesser  degree  of  metamorphism,  the  dis- 
tinctive character  of  certain  of  the  more  massive  members  of  the 
series,  and  the  presence  of  sedimentary  rocks.  The  actual  base  of  the 
Irving  has  nowhere  been  seen,  for  on  the  west  it  has  been  found  only 
in  contact  with  the  Eolus  granite,  while  to  the  east  it  appears  to  be 
in  many  cases  in  fault  or  shear  contact  with  the  Vallecito  conglomer- 
ate or  the  Uncompahgre  quartzite. 
To  the  Algonkian  are  assigned  the  Uncompahgre  quartzite  and 
the  Vallecito  conglomerate,  together  forming  the  Needle  Mountains 
group,  with  a  thickness  of  not  less  than  7,000  feet.  The  Vallecito 
conglomerate,  exposed  at  Vallecito  Creek  and  Pine  River,  contains 
pebbles  derived  from  the  Irving  greenstone.  To  the  east  of  the 
quadrangle  the  conglomerate  grades  up  into  conglomeratic  quartzite 
similar  to  that  at  the  base  of  the  Uncompahgre  quartzite.  The  Un- 
compahgre quartzite  crosses  the  quadrangle  in  its  northern  portion. 
The  lowest  member  of  the  Paleozoic  is  quartzite,  probably  of 
Upper  Cambrian  age,  resting  unconformably  upon  the  rocks  here 
described. 
Cross,  Howe,  and  Irving,106  in  1907,  map  and  describe  the  geology 
of  the  Ouray  quadrangle,  in  southwestern  Colorado.  Southward 
from  Ouray,  on  Uncompahgre  River,  are  quartzites  and  slates  of  the 
Uncompahgre  formation  of  the  Needle  Mountains  group,  assigned 
to  the  Algonkian.  They  are  limited  on  the  north  by  a  fault  which 
brings  them  into  contact  with  Paleozoic  sediments.  On  the  easl  they 
are  covered  by  the  San  Juan  tuff  and  on  the  west  by  the  oldesl  Paleo- 
zoic sediments.  The  formation  is  exposed  farther  south,  in  the  Sil- 
verton quadrangle,  for  nearly  2  miles. 
