480  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
the  southeast  side  of  this  possible  fracture  zone  includes  a  considerable 
thickness  of  olive  and  gray  calcareous  shales,  buff  limestone,  black 
paper  shales  with  massive  interbedded  members,  gray  sandstone,  red- 
dish conglomerate,  and  a  heavy  series  of  grayish-brown  sandstone. 
The  general  eastward  dip  is  frequently  lost  in  excessively  folded  and 
tilted  structures.  The  geologists  of  the  Wheeler  Survey  were  of  the 
opinion  that  an  unconformity  between  the  Tertiary  and  Paleozoic 
rocks  appears  in  this  valley. 
The  area  in  the  immediate  vicinity  of  the  main  gypsum  deposit  lies 
on  the  southeast  side  of  the  possible  zone  of  fracture,  and  the  beds  are 
extremely  deformed.  On  the  north  side  of  the  mouth  of  Salt  Creek 
Valley  gray  and  black  shales  stand  vertical  and  strike  northeast,  far- 
ther east  they  swing  into  east  and  southeast  strikes  and  dip  northward, 
thus  forming  the  nose  of  an  anticline  which  plunges  steeply  north- 
ward. The  gypsum  bed  lies  on  the  south  side  in  a  series  of  buff 
limestones  and  gray  and  buff  calcareous  shales.  The  strike  of  this 
series  is  northwest-southeast,  and  the  general  dip  is  vertical  or  steeply 
to  the  southwest,  except  that  there  are  local  variations  adjacent  to 
fissures  or  to  closely  appressed  folds.  The  gypsum  body  stands  nearly 
vertical,  dipping  locally  85°  southwest,  between  shale  walls,  and  strikes 
southeastward.  Owing  to  the  snow  which  covered  the  ground  at  the 
time  of  visit,  neither  the  upper  nor  lower  contact,  nor  either  limit 
along  the  strike  were  observed,  so  the  precise  nature  of  the  occurrence 
can  not  be  positively  stated.  It  appears,  however,  to  be  in  the  form  of  a 
bed  deposited  contemporaneously  with  inclosing  sediments,  and  it  is 
probably  either  a  thick  lens  which  pinches  rapidly  along  the  strike  or 
a  bed  which  has  been  terminated  in  either  direction  along  the  strike 
by  faulting. 
Although  the  stud}^  of  the  region  was  too  hasty  to  obtain  sufficient 
data  for  a  complete  explanation  of  the  origin  of  the  deposit,  general 
observations  and  facts  brought  out  by  the  anatyses  afford  accordant 
evidence  for  an  opinion.  The  bedded  structure  of  the  gypsum,  the 
agreement  of  this  bedding  with  that  of  the  country  rock,  the  inter- 
bedding  of  the  wall  rock  and  g}^psum,  and  the  occurrence,  in  the  same 
series  in  this  neighborhood,  of  rock  salt,  common  salt,  and  gypsum  of 
various  degrees  of  impurity,  tend  to  indicate  a  sedimentary  origin; 
and  the  analyses,  showing  the  presence  of  about  17  per  cent  calcium 
carbonate,  one-half  of  1  per  cent  of  magnesian  carbonate,  and  minor 
amounts  of  potassium  and  sodium  salts,  afford  additional  basis  for  the 
belief  that  this  gypsum  is  the  product  of  deposition  from  inland 
water  bodies,  such  as  lakes,  lagoons,  or  bayous. 
The  geologic  date  of  this  deposition  can  not  be  settled  until  further 
paleontological  evidence  has  been  obtained.  The  rocks  of  the  region 
are  indicated  on  the  geological  map  of  the  Wheeler  Survey  as  Jurassic, 
and  in  the  report  it  is  stated  that  "the  Jurassic  rocks  are  everywhere 
