386  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1905. 
amounts  of  asbestos  (15  to  50  per  cent  of  the  whole)  for  pipe  coverings  and  boiler  lagging 
A-  the  sulphate,  it  is  known  as  Epsom  salts, large  amounts  of  which  are  consumed  in  war) 
sizing  or  weighting  in  cotton  mills  and  smaller  amounts  in  medicine.  The  chloride  is  use< 
as  a  cement  and  the  hydrate  in  sugar  making. 
The  low  price  of  magnesite  about  $6  per  ton  landed  at  New  York  City — makes  its  min 
ing  on  the  Pacific  coasl  profitable  only  for  the  local  demands  unless  manufactured  int< 
more  valuable  forms.  The  Rose  Brick  Company,  a  subsidiary  company  of  the  America! 
Magnesite  Company,  has  recently  established  a  factory  at  Oakland  with  the  intention  o 
making  brick  for  shipment  to  the  steel  mills  of  the  East,  and  other  products  will  be  manu- 
factured by  allied  companies.  The  magnesite  intended  for  brick  will  be  burned  in  long 
sloping,  brick-lined  steel  cylinders,  somewhat  similar  to  those  used  for  pyritic  roasting 
Crude  oil  will  be  used  for  fuel. 
DESCRIPTION   OF   DEPOSITS. 
GENERAL    STATEMENT. 
The  California  magnesite  deposits,  so  far  as  known,  all  occur  as  veins  in  serpentine  or  simi- 
lar magnesian  rocks.  By  far  the  larger  part  are  in  the  Coast  Range  in  die  serpentines  that 
stretch  from  southern  California  into  Oregon.  These  serpentines  are  probably  Lower  Cre  ta- 
re, >us  in  age  "  and  cover  large  areas,  Becker  &  estimating  that  between  dear  Lake  and  New 
Idria,  a  distance  of  about  200  miles,  there  are  over  1 ,000  square  miles  of  serpent  inc.  Through 
a  large  part  of  this  area  magnesite  veins  of  varying  sizes  are  found.  Those  large  enough  to 
be  more  or  less  workable  are  reported  from  many  places  in  Mendocino,  Xapa,  Placer, Sonoma 
Alameda.  Stanislaus,  Santa  Clara,  Fresno,  Tulare,  Kern,  and  Riverside  counties.  Of  these 
tin-  deposits  in  Placer,  Tulare,  and  Fresno  counties  are  in  the  foothills  of  the  Sierras. 
The  serpentine-  of  the  Coasl  Range  are  ordinarily  given  or  bluish,  greatly  broken  and 
faulted,  a  solid  block  a  loot  in  diameter  being  a  rarity  in  many  localities.  They  are  derived 
from  olivine-pyroxene  rock-,  in  which  the  amounts  of  the  minerals  have  varying  ratios  at 
differenl  localities.  At  Porterville,  the  only  place  along  the  Sierra  Nevada  ai  which  these* 
pent  ine-  were  seen,  t  hey  seem,  from  a  preliminary  examinal  ion,  to  be  derived  from  an  olivine- 
pyroxene  rock.  Instead  of  the  usual  green  color,  the  serpentine  is  here  brown,  and  as  com- 
pared with  the  others  described  is  remarkably  solid. 
Magnesite  is  formed  from  the  breaking  dow  n  of  t  he  serpentine,  a  hydrous  magnesium  sili- 
cate. The  magnesia  is  dissoh  ed  by  percolating  waters  carrying  carbon  dioxide,  changed  to 
the  carbonate,  magnesite,  and  precipitated  in  cracks  and  crevices  as  veins.  The  silica  is 
carried  away  by  the  water  and  is  often  deposited  in  other  veins  as  opal  or  quartz. 
The  magnesite  frequently  stands  out  prominently  from  the  surrounding  serpentine,  irom 
the  fact  that  it  seems  to  weal  her  less  readily  than  the  serpent  ine  and  also  that  the  existence 
of  a  magnesite  vein  generally  indicates  that  the  surrounding  serpentine  is  considerably 
decomposed  and  so  erodes  rather  easily.  The  boldness  of  outcrop  makes  the  veins  readily 
noticeable,  so  that  their  occurrence  is  generally  known. 
In  the  larger  veins  there  is  often  a  central  portion  of  comparal  ively  pure  magnesite,  while 
on  one  or  both  sides  there  may  be  many  inclusions  of  serpentine.  This  mixed  condition  of  the 
magnesite  and  serpentine  in  the  vein-  is  common  among  the  large  veins  seen  along  the  Coast 
Range.  Small  inclusion-  of  serpentine  are  apt  to  extend  well  into  the  vein.  As  the  side  is 
neared  the  inclusions  form  a  gradually  larger  proportion  of  the  mass  until  the  magnesite 
appears  only  as  a  great  number  of  small  veins  in  the  broken  serpentine.  Or  if  the  main  mass 
i-  approached  from  the  side,  a-  along  a  tunnel,  a  stockwork  of  small  veins  of  magnesite  first 
appear-,  grow  ing  thicker  toward  the  large  vein,  until  the  larger  pari  of  the  mass  is  magnesite 
and  the  pieces  of  the  serpentine  are  so  separated  as  to  become  inclusions  in  the  magnesite. 
Near  I  he  veins  the  serpent  ine  ha-  alnio-t  always  lost  its  normal  color  and  is  badly  rotted  and 
-  I  airbanks,  Harold  W  ..  Description  of  the  San  Luis  quadrangle:  Geologic  Atlas  U.  S.,  folio  ]()],  U.  S. 
Geol.  Survej  .  L904,  p.  6. 
'  Becker,  Geo.  I'..  Geology  of  the  quicksilver  deposits  of  t'ic  Pacific  slope :Mon.  U.  S.  Geol.  Survey, 
vol.  13,  1888,  p.  103. 
