TIN,  QUICKSILVER,  PLATINUM,  ETC. 
TIN  IN  THE  FRANKLIN  MOUNTAINS,  TEXAS. 
By  G.  R  Richardson. 
Introduction.-  Since  the  publication  of  Mr.  W.  II.  Weed's  report  od  the  El  Paso  tin 
depositsa  the  geology  of  the  Franklin  Range,  in  which  the  ore  occurs,  has  been  further 
studied,  and  some  additional  prospecting  has  been  done.  In  view  of  the  scarcity  of  tin  in 
the  United  States,  ii  seems  desirable  to  outline  the  conditions  as  now  known  in  the  western 
Texas  tin  Held. 
Topography.  The  Franklin  Mountains  are  the  southern  extremity  of  the  long,  narrow 
range,  known  locally  by  different  names,  thai  extends  southward  from  the  Rocky  Moun- 
tains  cast  of  the  Rio  Grande  as  far  as  El  Paso.  The  Franklin  Mountains  are  23  miles  in 
length,  1")  of  which  are  in  Texas,  average  3  miles  in  width,  and  rise  3,000  feel  above  the 
adjacent  lowlands,  culminating  in  North  Franklin  Peak,  whose  elevation  is  7,!  '<>  feel 
The  western  face  is  relatively  little  eroded  and  constitutes  a  dip  slope.  The  eastern  side, 
on  the  contrary,  is  much  dissected  and  exposes  cross  sections  of  the  rocks.  The  range  is 
practically  hare  of  vegetation,  its  lower  slopes  only  being  occupied  by  desert  growth* 
among  which  greasewood,  lechuguilla,  sotol,  and  prickly  pear  are  prominent.  The  rocks, 
therefore,  are  plainly  exposed,  except  where  covered  by  local  accumulations  of  debris. 
General  geology.  -The  accompanying  geologic  map  (fig.  7)  outlines  the  distribution  of 
the  different  formations  in  the  Franklin  Mountains.  The  rocks  have  keen  studied  in  detail 
only  in  the  eastern  part  of  the  range  in  the  area  lying  within  the  El  Paso  quadranglJ 
wesl  of  which  only  general  conditions  are  shown.  For  the  present  purpose  it  will  not  he 
necessary  to  do  more  than  out  line  the  broader  features  of  the  general  geology,  since  details 
will  he  published  in  the  forthcoming  El  Paso  folio. 
Tl Idesl   lock-,  in  the  Franklin  Mountans  are  of  pre-Cambrian  age  and  include  two 
distinct  formations.  The  lower  one  consists  of  about  1,800  feel  of  light  and  dark  (jiiartz- 
ites  and  subordinate  slates,  which  have  been  cut  by  a  few  thin  diabase  dikes.  These  rocks 
are  succeeded  by  a  bed  of  rhyolitic  conglomerate,  which  attains  in  places  a  thickness  of  -400 
feet,  above  which  is  a  mass  of  porphyritic  red  rhyolite  over  1,000  feet  thick.  About  300 
feet  of  indurated,  fine-textured  sandstones  carrying  Upper  Cambrian  fossils  overlie  the! 
rhyolite  and  contain  rounded  pebbles  of  the  latter  in  the  basal  bed.  'Die  sandstone  is 
succeeded  by  a  considerable  thickness,  amounting  to  5,000  feet  or  more,  of  massive  grai 
limestone,  which  lithologically  is  difficult  to  subdivide  but  paleontologically  is  separable 
into  four  main  parts.  These  are  referred  to  the  Lower  and  Upper  Ordovician,  Silurian, 
and  Upper  Carboniferous,  the  approximate  thickness  of  limestone  assigned  to  each  division 
being,  respectively,  1,000.  (00,  1.000,  and  3,000+  feet.  The  Devonian  and  Mississippial 
are  not  represented  by  sediments.  All  of  these  rocks  from  the  pre-Cambrian  to  the  Car- 
boniferous are  structurally  conformable  and  dip  westward  at   angles  varying  in  different 
a  Bull.  r.  s.  Geol.  Survey  No.  178, 1901. 
I  hi 
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