148  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,  1905. 
I 
biotite,  rare  hornblende,  and  bits  of  magnetite.  The  feldspars  are  orthoclase  and  albitq 
commonly  present  in  the  minutely  intergrown  form  known  as  microperthite.  A  small  are* 
of  monzonite-porphyry  of  post-Comanche  age  occurs  near  the  Rio  Grande. 
Two  prominent  sets  of  joints,  striking  in  general  north-south  and  east-west,  traverse  tfl 
rocks  of  the  Franklin  Mountains,  and  the  distribution  of  the  strata  reveals  several  large 
faults  in  both  of  these  directions.  The  main  faulting  is  approximately  parallel  with  the, 
north-south  trend  of  the  range,  and  as  the  strata  dip  toward  the  west  the  internal  struc- 
ture shows  a  series  of  tilted  blocks.  Whether  the  whole  range  is  a  large  block  limited  M 
the  east  by  a  fault,  although  suggested  by  the  topography,  has  not  been  determined. 
The  fact  of  recent  movements  is  proved  by  the  presence  of  minor  faults  in  a  sand  bank  in 
the  northern  part  of  El  Paso.  These  displacements  follow  the  same  trend  as  the  principal 
faults  in  the  mountains. 
Occurrence  of  the  ore. — Prospecting  has  revealed  the  presence  of  tin  ore  in  a  small  area  in 
granite  at  the  eastern  base  of  the  Franklin  Mountains  about  12  miles  due  north  of  El  Paso, 
but  not  enough  development  has  been  done  to  make  possible  generalizations  of  much 
value  concerning  its  occurrence. 
Reference  to  the  map  brings  out  the  fact  that  the  prospect  lies  adjacent  to  one  of  the! 
main  north-south  faults.  Here  the  displacement  amounts  approximately  to  3,000  feet, 
shown  by  the  fact  that  the  Silurian  limestone  which  outcrops  on  the  crest  of  the  range 
occurs  on  low  hills  east  of  the  prospect.  The  exact  position  of  the  fault  plane  is  obscured 
by  the  granite,  which  is  intrusive  along  it.  Other  near-by  faults  are  also  indicated  on  thi 
map,  and  there  is  evidence  of  minor  displacements.  The  joint  planes  present  in  the  rocks 
throughout  the  range  are  here  characteristically  developed,  and  the  tin  occurs  as  cassit- 
erite,  associated  with  quartz  veins  along  the  east-west  joints  in  the  granite.  The  joints 
along  which  the  veins  occur  strike  transversely  to  the  trend  of  the  range,  and  the  veins  dip 
perpendicularly  or  at  high  angles  either  northward  or  southward.  The  greatest  distance 
that  any  of  these  veins  has  been  traced  along  the  surface  is  about  1,000  feet,  and  one  has 
been  followed  downward  to  a  depth  of  about  50  feet.  The  quartz  is  massive,  white,  and 
of  variable  thickness,  varying  from  a  fraction  of  an  inch  to  2  feet,  irregularly  thickening 
and  pinching  out.  Locally  the  walls  of  the  vein  are  plane  and  the  contact  with  the  coun^ 
try  rock  is  smooth.  At  other  places  the  veins  grade  off  into  the  granite.  The  mineralized 
zone  at  the  greatest  width  measured  is  not  over  6  feet  thick  and  ranges  from  this  to  0.  Sec- 
tions across  the  zone  show  irregular  veins  of  quartz  in  the  granite,  which  is  minutely  seamed 
with  joint  cracks.  Cassiterite  occurs  in  the  vein  quartz  and  more  abundantly  immediately 
contiguous  to  it.  In  the  vein  quartz  the  oxide  of  tin  occurs,  both  in  bunches  and  irregu- 
larly disseminated,  intergrown  with  the  quartz.  More  concentrated  deposits  of  cassiterite 
occur  intimately  associated  with  the  quartz  and  feldspar  of  the  granite  adjacent  to  the 
veins.  Mineralization  apparently  occurs  only  contiguous  to  the  veins  and  fades  out  a  feva 
inches  from  them.  The  cassiterite  is  present  in  crystals  less  than  a  tenth  of  an  inch  in 
size,  both  twinned  and  in  simple  tetragonal  forms,  and  it  also  occurs  massive.  Specimens 
of  nearly  pure  cassiterite  weighing  several  pounds  are  reported  to  have  been  obtained  from 
this  locality,  but  the  area  has  been  well  picked  over  and  at  present,  without  further  work, 
good  specimens  are  difficult  to  find.  The  present  stage  of  development,  therefore,  is  unfa- 
vorable for  detailed  study.  Minerals  usually  associated  with  tin  apparently  do  not  occur 
abundantly  here.  Weed  reports  the  occurrence  of  wolframite  and  of  a  single  grain  of  tour- 
maline. Fluorspar  is  sparingly  present,  associated  with  the  cassiterite,  which  occasionally 
includes  it.  Some  pyrite  is  present,  and  limonite  occurs  superficially  through  the  veins 
and  locally  forms  a  selvage  between  them  and  the  granite.  No  dikes  have  been  observed 
in  the  granite  in  the  vicinity  of  the  tin  prospect,  although  a  specimen  in  the  office  of  .Judge 
C.  R.  Morehead,  one  of  the  owners  of  the  property,  suggests  that  pegmatite  may  be  pres- 
ent. On  the  crest  of  the  granite  ridge  2\  miles  southeast  of  the  main  tin  prospect  there  is 
a  vein  of  common  garnet,  with  no  associated  vein  minerals,  parallel  to  the  east-west  joints 
in  a  coarse-textured  phase  of  the  granite.  The  vein  varies  from  an  inch  to  a  foot  in  width 
and  is  at  least  200  feet  long. 
