490  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull.  225. 
[f  water  from  Great  Salt  Lake  be  evaporated  to  the  points  where 
various  salts  crystallize  out,  the  results  of  such  fractional  crystalliza- 
tion, while  following  approximately  the  results  obtained  by  evaporat- 
ing" sea  water,  will  present  certain  interesting  differences.  These 
differences  are  due  in  part  to  the  difference  in  composition  between 
Great  Salt  Lake  water  and  sea  water,  and  in  part  to  the  fact  that  at 
Great  Salt  Lake  certain  critical  temperatures  are  reached  naturally  in 
the  course  of  the  year. 
Salt  ponds. — The  harvesting  ponds  of  the  Inland  Crystal  Salt  Com- 
pany are  each  about  800  feet  wide  and  1,000  feet  long.  A  number  of 
them  are  set  side  by  side  next  to  a  larger  "stock"  or  "settling"  pond. 
About  May  1  the  pumping  season  commences.  The  stock  pond  is 
pumped  full  of  water  from  the  lake,  and  this  brine  is  allowed  to  stand 
there  until  a  reddish  precipitate  of  iron  oxide,  etc.,  has  settled.  It  is 
then  allowed  to  flow  into  the  harvesting  ponds,  which  are  usually  kept 
with  about  6  inches  depth  of  water  in  them.  In  a  normal  season  the 
evaporation  of  water  in  these  harvesting  ponds  will  amount  to  about 
2  inches  a  day,  which  must  be  replaced  from  the  stock  pond,  while 
this  is  in  turn  kept  full  by  pumping  from  the  lake.  The  brine  thus 
treated  would  not  give  a  product  containing  much  over  96  per  cent 
NaCl  if  it  were  not  for  the  fact  that  the  mother  liquor  (containing 
part  of  the  magnesium  and  sodium  sulphates  and  practically  all  of  the 
magnesium  and  lime  chlorides)  is  not  allowed  to  evaporate,  but  is 
returned  at  intervals  to  the  lake.  A  further  purification  is  effected  by 
the  fact  that  the  sodium  sulphate,  which  floats  when  formed,  is  driven 
by  the  wind  into  the  leeward  ponds  and  against  the  sides  of  the  ponds. 
The  material  from  such  ponds  is  therefore  marketed  for  cattle  and 
silver-mill  use,  while  that  from  the  windward  ponds  is  a  very  pure 
salt,  and  is  shipped  to  the  mill  for  further  treatment. 
Until  about  September  1  the  ponds  are  kept  full  of  water,  after 
which  time  they  are  allowed  to  evaporate  completely,  with  the  excep- 
tion, as  before  noted,  of  the  lighter  brines  containing  the  bulk  of  the 
impurities.  In  an  average  season  about  3  inches  of  salt  will  be 
obtained,  though  6-inch  crops  have  been  reported  in  exceptionally 
favorable  years. 
In  harvesting  the  salt,  after  the  contents  of  the  ponds  have  entirely 
evaporated,  the  salt  is  shoveled  up  into  wheelbarrows  and  heaped  into 
large  stacks.  The  removal  of  any  adhering  mother  liquors  is  effected 
by  the  rain,  and  the  salt  is  then  ready  for  the  refining  mill  or  for 
market  if  it  is  to  be  sold  crude. 
Salt  milling. — The  mill  of  the  Inland  Crystal  Salt  Company  is  con- 
structed in  two  duplicate  sections,  each  containing  one  drier  and  six 
sets  of  rolls,  fans,  shaking  sieves,  etc.  Each  of  these  sections  is  oper- 
ated as  follows: 
The  crude  salt  arrives  at  the  mill  in  standard-size  cars  over  a  switch 
