320  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.      '      [bull.  86. 
than  the  true  quartzites  forming  the  main  bulk  of  the  mountains  still 
farther  north.  Taking  into  consideration,  therefore,  the  observed  con- 
formity of  the  underlying  metamorphics  with  the  overlying  sediment- 
aries ;  taking  into  consideration,  furthermore,  the  analogous  character 
of  stratigraphical  relations,  the  conclusion  must  be  reached  that  those 
sedimentary  beds,  which  existed  below  the  Devonian,  furnished  the 
material  for  the  metamorphic  masses. 
Comstock,65  in  1883,  states  that  in  San  Juan  county  there  are  no 
rocks  which  are  of  Archean  age.  The  granitic  and  quartzitic  series  of 
the  Animas  river  are  regarded  as  metamorphic  and  said  to  be  of  Upper 
Silurian  or  Devonian  age. 
Comstock,66  in  1887,  describes  the  metamorphic  series  in  southwest- 
ern Colorado  as  probably  Silurian  or  Devonian.  This  series  is  sus- 
ceptible of  division  into  an  u£per  or  granitic  division  and  an  underlying 
quartzitic  formation.  The  quartzitic  group  is  exposed  in  the  Animas 
canyon  below  Silverton,  forming  a  line  of  jagged  peaks  to  the  eastward, 
the  Needle  mountains.  Whenever  the  quartzite  is  well  uncovered  the 
more  recent  granites  are  usually  traceable  along  the  flanks  of  the  belt. 
The  geological  map  brings  out  no  apparent  system,  in  the  metamorphic 
rocks. 
Lakes,67  in  1889,  describes  on  the  Mears  road,  south  of  Ouray,  as 
succeeding  the  Carboniferous  limestone,  a  thickness  of  13,000  feet  of  dis- 
tinctly stratified  and  hard  vitreous  quartzites,  slates  and  schists.  Part 
of  these  may  belong  to  the  Silurian  and  Cambrian,  but  as  these  com- 
bined rarely  attain  in  Colorado  a  thickness  of  1,000  feet,  so  great  a 
body  is  extraordinary  and  suggests  that  the  lower  j>art  of  it  may,  as  in 
Canada,  belong  to  the  Huronian  or  Laurentian,  upper  divisions  of  the 
Archean  not  elsewhere  represented  in  Colorado.  The  dip  of  the  quartz- 
ite is  about  75°  to  the  north.  The  uplifted  crests  have  been  deeply 
eroded  and  in  the  hollows  so  formed  rest  the  massive  volcanic  breccias. 
Emmons,  (S.  F.),45  in  1890,  states  that  on  the  north  slope  of  the  San  Juan 
mountains,  near  Ouray,  is  over  10,000  feet  of  closely  folded  quartzites, 
conglomerates,  and  slates  of  the  pre-Cambrian  age,  and  it  is  believed 
that  the  quartzite  peaks  in  the  southern  portion  of  this  region  are 
probably  composed  of  the  same  series  of  rocks.  These  are  referred  to 
the  Algonkian. 
Van  Hise,9  in  1889,  made  observations  along  the  Animas,  the  rail- 
road being  followed  from  below  Needleton  to  Silverton,  a  distance 
of  about  17  or  18  miles.  As  mapped  by  Endlich  on  Sheet  xv  of  the 
atlas  of  Colorado,  this  course  is  situated,  with  the  exception  of  5  or  6 
miles,  in  the  quartzite  area.  Quartzites  occur  for  a  little  more  than  2 
miles  in  the  vicinity  of  Elk  park  in  the  middle  of  the  area  mapped  as 
quartzite.  The  granitic  area  was  found  to  be  a  most  intricate  complex 
of  massive  granite,  coarse  and  fine,  white  and  black  banded  gneiss,  and 
black  hornblende-schist  or  gneiss  in  dike-like  forms.  The  strikes  and 
dips  vary  greatly,  although  for  the  most  part  they  are  high,  running 
