174  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
BOLIVIAN    REGION. 
Bolivia  is  at  present  the  second  tin-producing  region  of  the  world. 
The  mines  are  in  the  eastern  cordillera  of  the  Andes,  in  a  district  that 
extends  from  near  Lake  Titicaca  eastward  and  southward,  through 
a  distance  of  over  300  miles,  from  above  La  Paz  on  the  north,  about 
1G°  S.  latitude,  to  Chorolque  Mountain  on  the  south,  a  little  south  of 
latitude  21°  S.  This  region  is  just  east  of  the  great  Bolivian  Plateau, 
and  contains  many  of  the  high  peaks  of  the  x\.ndes. 
The  rocks  of  the  region  are  arkoses,  sandstones,  slates,  and  shales 
of  both  lower  and  upper  Silurian  age,  with  intrusions  of  porphyritic 
igneous  rocks.  Trachytes  occur  in  the  southern  portions  of  the 
region.  The  sedimentaries  generally  have  a  north-south  strike  and 
high  easterly  dip.  Although  there  seems  to  be  little  or  no  granite 
in  the  tin  district,  it  occurs  in  the  ranges  to  the  east. 
There  are  four  principal  tin-mining  districts  in  Bolivia — La  Paz 
on  the  north,  just  east  of  the  south  end  of  Lake  Titicaca;  Oruro, 
farther  southeast;  Potosi,  still  farther  on,  and  Chorolque,  a  little 
west  of  south  of  Potosi,  in  the  same  Department,  the  other  districts 
being  all  in  Departments  of  like  names.  Tin  is  also  found  in  the 
Departments  of  Cochabamba  and  Santa  Cruz,  east  of  this  belt,  but 
none  has  been  found  west  of  this  cordillera. 
Bolivia  derives  by  far  the  greater  portion  of  its  tin  from  veins, 
and  is  the  world's  largest  producer  of  lode  tin. 
The  tin  veins  or  lodes  occur  in  both  sedimentan^  and  igneous  rocks, 
and,  dipping  steeply,  generally  run  east  and  west  across  the  strike  of 
the  bedded  rocks,  although  occasional  veins  run  north  and  south,  or 
nearly  so.  The  veins  vary  greatly  in  size  and  richness,  reaching 
widths  of  8  or  9  feet,  and  in  places  are  almost  solid  ore,  often  carry- 
ing from  20  to  50  per  cent  cassiterite,  but  probably  averaging  from 
10  to  12  per  cent.  In  places  the  tin  veins  seem  to  be  replaced  in  depth 
by  iron  pyrites,  carrying  but  small  amounts  of  tin  sulphide  and  oxide, 
while  in  other  places  the  veins  are  still  rich  at  1,000  to  1,200  feet 
below  the  surface. 
The  gangue  is  often  a  clay,  probably  a  decayed  feldspathic  dike, 
though  it  is  sometimes  quartzose  and  exceedingly  hard. 
The  accompanying  minerals  are  tourmaline,  fluorite,  wolframite, 
arsenic,  copper  and  iron  pyrites,  sulphides  of  bismuth  and  antimony, 
sphalerite,  galena,  barite,  silver  chloride,  and  gold,  though  all  of  these 
minerals  may  not  be  found  in  any  one  vein.  Tourmaline  seems  not 
to  be  so  abundant  as  in  most  tin  regions.  Many  of  the  veins  ha^e 
been  worked  for  silver,  particularly  at  Oruro,  and  the  tailings  concen- 
trated for  tin,  of  which  they  frequently  carry  2  per  cent  or  more. 
One  mine  on  Huanuni  Hill,  in  the  Oruro  district,  was  worked  to 
a  depth  of  1,000  feet  by  the  Spaniards.     The  ore  was  carried  out 
