264         CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,  L906,   PART    l. 
It  is  reported  that  the  gypsum  lias  been  used  locally  for  making  piaster 
for  a  riumber  of  years,  although  no  workings  were  seen  by  the  writer. 
A  partial  analysis  by  W.  T.  Schaller,  of  the  United  States  Geological 
Survey,  of  a  sample  taken  from  the  gypsum  bed  at  the  surface  at  this 
point  is  as  follows: 
Analysis  of  gypsumfrom  west  of  San  Miguel  copper  mine. 
Calcium  oxide  (CaO) 34.^4 
Sulphur  trioxide  (SO,)  46.61 
Water 1 8 .89 
[nsoluble  residue .  18 
Loss os 
This  analysis  shows  thai  the  gypsum  at  this  point  is  practically 
pure.  It  contains  about  2.5  per  cent  of  calcium  oxide,  more  than  can 
be  in  combination  with  the  sulphur  trioxide  present.  It  is  likel\  that 
this  excess  is  in  combination  with  CO,,  which  has  been  included  as 
water  and  loss. 
At  the  head  of  a  tributary  to  Rio  Salado,  about  3  miles  southeast  of 
Ojo  del  Espiritu  Santo,  at  an  elevation  between  7,500  and  8,000  feet, 
the  gypsum  bed  reaches  a  maximum  thickness  of  about  LOO  feet, 
Near  the  mountains  the  bed  is  conformable  with  the  underlying  rocks, 
which  dip  westward  at  an  angle  of  70  .  Within  a  short  distance, 
however,  across  the  dip,  the  gypsum  bed  becomes  very  flat,  con- 
forming with  the  dip  of  the  overlying  Dakota(?)  formation.  The 
relations  here  are  not  plain,  hut  it  is  believed  that  the  dip  of  the  under- 
lying Mesozoic  strata  decreases  westward  an  equal  amount.  As  a 
consequence  of  this  flattening  away  from  the  mountains  and  the  pres- 
ence of  a  valley  heading  near  this  locality,  a  large  area  of  gypsum 
i-  exposed  and  can  be  mined  to  advantage  by  open  cut.  It  should  be 
accessible  by  a  wagon  road  built  at  slight  expense  up  the  valley  of  ;i 
tributary  of  Rio  Salado.  In  appearance  the  gypsum  is  identical  with 
that  analyzed  from  near  San  Miguel  mine.  In  the  section  it  will  be 
noticed  that  the  limestone  bed,  which  was  50  feet  thick  at  Senprita, 
is  absent  here. 
Section  of  rocks  exposed  near  Ojo  del  Espiritu  Santo. 
Feet. 
A  In  mating  Bhales  and  sandstones  i  Dakota?) •"><><> 
Massive,  white  gypsum I  no 
Shales  and  argillaceous  sandstone "  250 
Red  sandstone  (Jurassic-Triassic) 200 
Pre-Carboniferous  I  pre-Cambrian?)  granite 
The  gypsum  bed  outcrops  continuously  for  several  miles  in  a  /one 
extending  northward  from  the  locality  just  described.  It  dips  west- 
ward at  an  angle  of  50°  and  extends  to  a  point    I  miles  farther  south. 
a  The  exposure  in  the  upper  part  was  too  poor  t<>  enable  the  writer  t<>  affirm  <>r  deny  ""■  presence  <>f 
the  yellow  saml  below  the  gypsum. 
