boutwell.]  ROCK    GYPSUM    AT    NEPHI,   UTAH.  485 
well  as  the  "waste"  rock,  includes  between  22  and  23  per  cent  of 
anhydrite.  The  significance  of  this  fact  lies  in  the  practical  influence 
of  the  anhydrous  material  upon  the  action  of  the  combined  product; 
for,  although  when  gypsum  is  calcined  the  largest  portion  of  water  is 
driven  off,  it  is  generally  regarded  essential  for  its  suitability  for 
plaster  that  some  of  its  water  be  left.  "When  it  is  heated  to  100- 
120°  it  gives  up  three-fourths  of  its  water  very  quickly,  but  it  requires 
a  temperature  of  from  200-250°  to  expel  the  remainder."  "If  gypsum 
has  been  heated  to  a  little  over  200°,  thus  being  deprived  of  all  of  its 
water,  it  becomes  dead  burnt,  and  takes  up  water  very  slowly  and 
without  hardening.  "a 
It  would  seem,  therefore,  that  in  so  far  as  natural  anhydrous  calcium 
sulphate  (anhydrite)  is  like  artificially  dehydrated  gypsum  in  actual 
practice,  the  presence  of  anhydrite  would  at  least  retard  "  setting." 
In  the  manufacture  of  patent  cements,  however,  heating  to  600-700°  F., 
while  mixing  it  with  sulphuric  acid,  borax,  cream  of  tartar,  etc.,  is 
stated  to  give  a  product,  which,  after  setting  very  slowly,  becomes 
exceedingly  firm  and  hard.6 
It  is  highly  probable  that  anhydrite  occurs  in  a  large  per  cent  of 
the  gypsum  deposits,  so  that  further  reliable  analyses  of  other  gypsums 
which  have  proved  to  afford  plaster  of  good  setting  quality  are  much 
to  be  desired. 
The  presence  of  calcium  carbonate  (limestone)  to  the  extent  of  17 
per  cent  in  the  rock,  which  is  sorted  out  for  rejection,  may  be  advan- 
tageous for  special  uses,  for  in  the  preparation  of  certain  hydraulic 
cements  limestone  is  considered  a  valuable  constituent.  The  signifi- 
cance of  the  content  of  potassium,  magnesium,  and  sodium  is  considered 
hereafter  in  connection  with  the  discussion"  of  the  origin  of  these 
deposits. 
Several  other  deposits  of  value  are  known  in  Salt  Creek  Valley 
east  of  the  main  body,  the  most  important  of  which  is  on  the  north 
side  of  the  creek,  about  one-half  mile  east  of  the  Nephi  plaster  mill. 
The}^  include  rock  gypsum  of  good  grade  and  impure  transitions/ 
In  many  localities  they  would  be  exploited,  but  being  in  such  close 
proximity  to  the  great  body  at  the  mouth  of  the  valley,  these  are 
left  idle. 
GEOLOGIC  OCCURRENCE. 
The  geologic  structure  of  this  region  is  exceedingly  complicated 
and  only  its  broadest  characteristics  can  now  be  stated.  The  Nebo 
massif  to  the  north,  composed  of  Paleozoic  rocks,  appears  to  have  been 
elevated  above  the  tract  of  Jurassic  and  Tertiary  rocks  to  the  south 
and  east  by  faulting  along  a  northeast-southwest  zone.     The  tract  on 
a  Thorp,  T.  E.,  Dictionary  of  Applied  Chemistry,  vol.  1,  p.  406. 
6  Wilder,  F.  A.,  Iowa  Geological  Survey,  vol.  12,  p.  157. 
