208  CONTRIBUTIONS    TO   ECONOMIC   GEOLOGY,  1904.        [bull.  260. 
SAN  RAFAEL  DEPOSITS. 
Location. — About  15  miles  southwest  of  Green  River  station  (Rio 
Grande  Western  Railway)  deposits  of  carnotite  have  been  found  in 
several  places.  These  deposits  lie  on  the  western  margin  of  the  Green 
River  basin  in  a  series  of  eastward-dipping  cuestas  (hogbacks)  which 
rise  gradually  to  San  Rafael  Reef.  West  of  this  series  of  ridges  is  a 
vwv^  course,  or  wide,  open  valley,  analogous  in  form  and  probably  in 
origin  to  the  race  courses  around  the  Black  Hills  and  the  Bighorns. 
Rising  steeply  from  this  is  a  high,  precipitous  rock  wall,  over  the 
notched  crest  of  which  the  flat-topped  central  plateau  of  San  Rafael 
Swell  appears.  The  sandstone  which  floors  the  central  area,  and  also 
apparently  that  which  makes  the  encircling  reef,  was  considered  by 
Dutton  to  be  Triassic.a  Thus  the  soft  beds  forming  the  low,  con- 
centric valley,  over  1,000  feet  in  thickness,  with  the  gypsum  beds 
intercalated  in  their  upper  portion,  are  ajso  doubtless  Triassic.  The 
overlying  coarse  sandstones  and  fine  conglomerate  forming  the  crest 
of  the  cuesta  which  incloses  this  inner  valley  may  thus  be  Jurassic; 
next  above  are  slightly  less  resistant,  olive,  maroon,  and  gray  carbon- 
aceous shales  with  interbedded  sandstones,  which  underlie  and 
probably  pass  into  a  sandstone  that  may  be  Dakota  Cretaceous.  The 
Green  River  Cretaceous  which  then  comes  in  underlies  the  main 
Green  River  basin  and  apparently  passes  upAvard  into  the  heavy 
series  which  forms  the  Book  Cliffs. 
The  deposits  are  found  about  a  mile  east  of  the  gorge  cut  by 
San  Rafael  River  at  two  or  possibly  three  horizons  that  embrace  a 
thickness  of  about  100  feet,  and  extend  along  their  strike  for  about  2 
miles.  The  particular  series  in  which  these  ore-bearing  members 
lie  are  coarse  sandstones  and  fine  conglomerates,  which  dip  eastward 
at  angles  ranging  from  10°  to  30°  below  the  variegated  shales  and 
about  200  to  250  feet  above  the  main  red  shale  formation.  They  may 
thus  be  tentatively  considered  to  be  of  Jurassic  age. 
The  values  lie  in  a  light-yellow  mineral  which  in  certain  cases 
appears  to  be  carnotite.  Part  of  this  material  is  crystalline,  part  is 
granular,  and  part  forms  a  thin  coating  of  faint  yellow,  greenish- 
yellow,  and  light-green  color.  The  pay  is  much  disseminated  and 
very  lean;  no  massive  pieces  of  amorphous  carnotite  comparable  to 
the  Colorado  ores  have  been  found. 
Ore  has  been  taken  from  eight  separate  spots  which  are  located  in 
three  general  groups.  In  these  groups  there  are  certain  common  and 
certain  distinguishing  features.  Thus  in  all  of  them  the  pay  occurs 
in  sandstone  or  conglomerate  and  in  intimate  association  with  plant 
remains.     In  the  northern  group,  however,  it  is  in  the  form  of  mas- 
«  Dutton,  C.  E.,  Geology  of  the  High  Plateaus  of  Utah,  p.  19. 
