204  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
Grand,  the  desert  plain  is  broken  by  low  hogbacks.  Along  the  crest 
of  one  of  these  the  deposits  under  discussion  are  found. 
General  geology. — The  geology  of  this  region  has  never  been 
studied  in  detail,  and  is  known  only  broadly  by  correlation  with  that 
of  regions  which  have  been  studied  to  the  north  and  in  western 
Colorado.  Such  hasty  observations  as  the  writer  was  able  to  make 
en  route,  supplemented  by  suggestions  of  geologists  who  have  worked 
in  neighboring  areas,  serve  to  establish  the  general  geologic  position 
of  the  formations  in  this  region.0 
The  route  from  Cisco  to  Richardson  passes  down  from  younger  to 
older  geologic  formations,  as  the  general  dip  is  gently  northward 
from  the  intrusive  core  of  the  La  Sal  Mountains  and  beneath  the 
Green  River  basin.  Leaving  the  Cretaceous  shales,  which  form  the 
main  surface  of  the  high-lying  Green  River  basin,  one  descends 
southward  through  variegated  bedded  Cretaceous  sandstones,  includ- 
ing Dakota  Cretaceous,  into  a  thick  formation  of  red  beds,  mainly  sand- 
stones. These  comprises  both  topographically  and  lithologically, 
three  parts — (1)  cliff-making,  well-bedded,  brownish-red  sandstone, 
approximately  1,000  feet  thick,  with  persistent  cross-bedded  members, 
underlain  by  (2)  bench-making,  shaly,  dark-brown  sandstone,  sev- 
eral hundred  feet  thick,  with  alternately  more  and  less  massive 
resistant  members,  and  at  the  bottom  (3)  cliff-making,  thin-bedded, 
brownish-red  sandstone,  with  well-defined  persistent  stratification. 
A  cliff-making,  light  pinkish-purple,  shaly  sandstone,  Avhich  includes 
coarse  cross-bedded  sandstones  and  conglomerates,  with  well-rounded 
granitic  and  porphyritic  pebbles,  underlies  this  series,  with  improved 
stratigraphic  relationship,  and  outcrops  on  the  banks  of  the  river 
and  adjoining  plains  on  the  east.  This  formation  appeared  roughly 
conformable  with  the  overlying  sandstones,  but  this  point  was  not 
specially  studied  and  observations  were  insufficient  to  warrant  any 
positive  statement  in  regard  to  it. 
Peale  observed  that  on  Dome  Plateau  (which  overlooks  the  Rich- 
ardson amphitheater  from  the  west)  the  lower  portion  of  the  Jurassic 
formation  covers  a  considerable  area  south  of  the  hogback  that  bor- 
ders the  Grand  River  Valley  at  this  place.6  He  further  notes,  and 
shows  in  a  section,  that  in  the  Adcinity  of  the  junction  of  Grand 
and  Dolores  rivers  the  Triassic  forms  the  greater  part  of  the  surface, 
but  that  the  Grand  cuts  through  both  Jurassic  and  Triassic  and 
exposes  the  Carboniferous  in  the  canyon  bottoms.     The  general  map 
"  The  writer  gladly  acknowledges  valuable  suggestions  as  to  the  probable  age  of  the  for- 
mations in  question  from  Dr.  Whitman  Cross,  of  this  Survey,  who  has  studied  equivalent 
formations  in  detail  in  Colorado.  Mr.  Cross  is  now  engaged  in  the  preparation  of  a  bul- 
letin for  the  Geological  Society  of  America  on  the  Red  Beds  in  southwestern  Colorado, 
in  which  he  will  discuss  their  probable  correlation  with  formations  to  the  west. 
6  Hayden,  F.  V.,  Tenth  Ann.  Kept.  U.  S.  Geog.  and  Geol.  Surv.  Terr.,  1878,  p.  179. 
