NOTES    ON    GLASS    SANDS.  381 
Except  near  joint  planes,  where  it  is  somewhat  iron  stained  and  harder. 
The  sand  is  white  to  yellowish,  water-worn,  and  partly  subangnlar; 
100  per  cent  of  the  grains  pass  a  20-mesh  sieve,  23  per  cent  pass  a  40- 
mcvsh,  23  per  cent  pass  60-mesh,  and  11  per  cent  pass  80-mesh.  There- 
fore the  sand,  although  uneven,  is  mostly  of  medium  grain.  The  beds 
post  suitably  located  for  working  are  in  the  south  half  of  sec.  32,  T. 
^3  N.,  R.  17  W.,  where  the  rock  outcrops  within  one-fourth  mile  of 
the  Chicago,  Rock  Island  and  Pacific  Railway. 
Nebraska.® — The  most  accessible  and  prominent  exposures  of  Dakota 
sandstone  in  this  State  are  near  Ponca,  Jackson,  Homer,  Tekamah, 
Ashland  (at  mouth  of  Salt  Creek),  Lincoln,  and  Beatrice,  and  in  the 
southern  part  of  Jefferson  County.  The  sand  is  variably  indurated. 
A.t  places  it  contains  siliceous  cement  and  is  a  quartzite.  There  is 
much  less  calcium  carbonate  in  the  stone  than  is  usually  supposed. 
It  may  be  said  that  ferric  oxide  is  the  prevailing  matrix.  Generally 
the  sandstone  is  friable  and  easily  crushed.  It  is  thin  bedded  where 
interst ratified  with  shales  or  clay  and  mostly  massive  in  the  upper 
and  lower  parts  of  the  formation.  At  some  places  massive  ledges 
2Tade  laterally  into  arenaceous  clays  within  a  short  distance.  The 
color  ranges  from  gray  to  the  yellows  and  browns,  varying  with  the 
amount  of  iron  stain  present.  The  iron  oxide  covers  or  coats  the 
grains,  but  rarely  occurs  as  an  impurity  in  the  quartz.  The  sands 
tnay  be  freed  of  iron  by  washing.  The  chemical  analysis  of  a  badly 
stained  sample  taken  at  Robbers'  Cave,  near  Lincoln  (see  p.  382), 
shows  that  the  sand,  if  washed,  might  be  used  for  green  or  dark 
glass. 
When  sieved,  most  of  this  sand  was  caught  on  meshes  30,  40,  and 
50,  so  that  it  is  of  remarkably  even  grain.  It  is  composed  largely  of 
quartz,  except  when  clay  and  mica  are  found.  These  are  present 
where  the  sand  rock  grades  horizontally  into  clay.  In  general  the 
friable  sand  rock  is  clean,  except  for  the  iron. 
a  Communicated  by  Dr.  Geo.  E.  Condra,  Dept.  of  Geol.,  Univ.  of  Nebraska,  Lincoln,  Nebr. 
Bull.  315—07 25 
