iKViNG.]  OURAY    DISTRICT,    COLORADO.  53 
The  Dakota  formation,  locally  known  as  the  "  Upper  qnartzite," 
has  a  maximum  thickness  of  about  150  feet.  It  consists  of  two  or 
three  massive  quartzite  or  hard  sandstone  beds  separated  by  thin 
bands  of  black  shale.  It  is  one  of  the  most  persistent  horizons  of  the 
region  and  usually  forms  prominent  cliffs  or  escarpments,  easily 
identified  and  traced.  It  is  in  this  formation  that  the  ore  deposits 
of  the  American  Nettie  and  in  part  of  the  Bachelor  occur.  In  a  few 
places  near  Ouray  the  soft  black  shales  of  the  Mancos  formation  arc 
found  above  the  Dakota.  They  occur  in  small  patches  that  were  left 
after  the  great  erosion  that  preceded  the  period  of  volcanic  activity. 
The  whole  formation  has  a  thickness  of  over  1,000  feet. 
The  only  sedimentary  formation  of  Tertiary  age  known  to  occur  in 
this  region  is  the  Telluride  conglomerate.  It  has  extremely  variable 
characteristics  in  different  parts  of  the  San  Juan  Mountains,  and 
southwest  of  Telluride  has  a  thickness  of  1,200  feet.  Near  Ouray, 
however,  it  is  seldom  more  than  50  feet  thick,  and  is  often  altogether 
absent.  It  consists  of  materials  derived  from  the  older  sedimentary 
rocks — sandstone  and  limestone  pebbles — embedded  in  a  calcareous 
cement.  The  color  is  usually  a  dull  pink.  It  is  of  economic  interest 
because  an  occurrence  in  Cobbs  Gulch,  Cow  Creek,  has  been  reported 
to  carry  free  gold  in  small  amounts. 
Volcanic  rocks. — The  next  rocks  in  point  of  age  are  those  of  vol- 
canic origin  that  make  up  a  very  great  part  of  the  San  Juan  Moun- 
tains and  consist  of  a  number  of  well-known  eruptive  types.  The 
only  member  of  this  great  series  that  needs  to  be  considered  in  this  con- 
nection is  the  oldest  one,  the  San  Juan  tuff.  It  has  a  maximum  thick- 
ness in  Canyon  Creek  and  the  amphitheater  of  over  2,000  feet,  and 
;is  easily  recognized  by  its  dull  blue  color.  It  has  the  appearance  of 
a  conglomerate  composed  of  angular  fragments  or  coarse  sand  de- 
rived from  lavas  known  as  "  andesites."  Parts  are  soft  and  crumble 
readily  under  the  influence  of  weather;  others  are  well  consoli- 
dated and  form  the  precipitous  cliffs  of  Bear  Creek  and  the  upper 
part  of  Canyon  Creek.  In  many  places  a  well-defined  bedded  struc- 
ture may  be  observed. 
For  a  long  time  after  the  period  of  the  San  Juan  tuff  other  lavas 
were  poured  out,  and  toward  the  close  of  the  eruptions  the  tuff  itself, 
is.Avell  as  the  sedimentary  rocks  beneath  it,  were  invaded  by  certain 
lavas  that  were  unable  to  reach  the  surface  and  forced  their  way 
petween  the  sedimentary  strata  or  along  the  base  of  the  San  Juan. 
These  intrusive  rocks  are  the  porphyries  that  occur  so  prominently 
is  dikes  or  sheets  or  large  irregular  bodies  near  Ouray.  The  sedi- 
nentary  formations,  especially  between  Cascade  Creek  and  Bridal 
Veil  Creek,  were  extensively  altered  as  a  result  of  these  intrusions, 
ind  the  mineralization  of  the  region  probably  occurred  shortly  after 
his  time. 
