ikying.]  OURAY   DISTRICT,    COLORADO.  51 
GEOLOGY,  a 
Introduction. — The  geology  of  the  mining  region  in  the  immediate 
vicinity  of  the  town  of  Ouray  includes  nearly  all  of  the  features 
that  are  characteristic  of  the  San  Juan  Mountain  area.  Sedimentary 
rocks,  representing  various  periods,  from  the  Algonkian  to  the  Ter 
tiary,  are  more  or  less  typically  developed,  and  fourteen  different 
formations  have  been  recognized  from  fossil  evidence  or  on  litho- 
logic  and  stratigraphic  grounds.  A  great  thickness  of  volcanic  rocks, 
andesites,  and  rhyolites,  erupted  during  Tertiary  time,  covers  the  sedi- 
ments, while  still  younger  porphyries  intrude  both  the  sedimentary 
and  volcanic  rocks.  At  several  periods  during  the  history  of  the 
region  faulting  and  folding  occurred,  which  have  affected  all  of  the 
rocks,  some  more  than  others. 
Unlike  the  adjoining  districts  of  Silverton  and  Telluride,  the 
important  mines  are  in  the  areas  of  sedimentary  rocks  and  stratig- 
raphy becomes  an  important  factor  in  the  discussion  of  the  ore 
deposits. 
Sedimentary  formations. — The  oldest  rocks  that  are  found  in  the 
region  are  the  massive  quartzites  and  slates  that  are  exposed  south 
of  Ouray  in  the  Uncompahgre  Canyon,  where  a  thickness  of  fully 
8,000  feet  is  known  to  occur.  The  beds  dip  steeply  to  the  north,  and 
although  neither  the  top  nor  the  bottom  of  the  formation  has  been 
seen,  it  is  believed  that  the  upper  portion  lies  to  the  north.  The 
name  Uncompahgre  has  been  applied  to  this  formation,  which  is 
assumed  to  be  of  Algonkian  age. 
At  Ouray,  a  short  distance  southeast  of  the  Mineral  Farm  mine, 
the  Paleozoic  section  begins  with  a  series  of  calcareous  shales  and 
sandstones,  not  more  than  50  feet  in  thickness,  which  rest  on  the 
upturned  edges  of  the  Uncompahgre  quartzites  and  slates.  Upper 
jDevonian  fossils  have  been  found  preserved  in  corresponding  beds 
:on  the  southern  slopes  of  the  Needle  Mountains,  and  the  formation 
has  been  named  the  Elbert.  A  series  of  Upper  Cambrian  quartzites, 
the  Ignacio  formation,  is  usually  found  directly  beneath  the  Elbert, 
jbut  at  Ouray  this  is  lacking,  although  its  presence  is  indicated  in  Cow 
Creek  by  a  few  fossils  that  have  been  obtained  from  the  bottom  of 
an  old  shaft. 
The  Ouray  formation,  which  succeeds  the  Elbert,  consists  mainly 
)f  light-colored  limestones, thin  bedded  below  and  very  massive  above, 
A  late  Devonian  fauna  characterizes  the  formation,  except  in  the 
ipper  portion,  where  Mississippian  fossils  have  been  found,  but  this 
>art  can  not  be  differentiated  on  lithologic  grounds  from  the  under- 
ying  beds.     These  limestones  occur  in  a  limited  area  south  of  Ouray, 
"  By  Ernest  Howe. 
