ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY.  747 
the  Sierra  Diablo.  The  Franklin  Mountains  are  a  narrow  range 
which  extends  northward  from  El  Paso,  and  is  composed  chiefly  of 
westward-dipping  Paleozoic  strata.  These  consist  of  300  feet  of 
Cambrian  sandstone  and  about  5,000  feet  of  limestone  containing 
lower  and  upper  Ordovician,  Silurian,  and  upper  Carboniferous  fos- 
sils. The  Devonian  and  lower  Carboniferous  are  unrepresented  by 
sediments.  Coarse  red  granite,  of  probable  post-Paleozoic  age,  ex- 
tends along  the  eastern  base  of  the  mountains,  and  crosses  the  range 
in  one  locality.  The  pre-Cambrian  ( ?)  rocks  occur  along  the  central 
eastern  slope  and  also  constitute  the  culminating  point  of  the  range. 
They  consist  of  quartzite  and  rhyolite  porphyry,  which  outcrop  in 
several  disconnected  areas,  the  most  complete  section  occurring  in  the 
central  part  of  the  mountains  10  miles  north  of  El  Paso.  The  lower 
portion  is  composed  of  about  1,800  feet  of  fine-textured,  thoroughly 
indurated  quartzite,  which  has  been  intruded  by  a  few  dikes  and  sills 
of  diabase.  The  quartzite  is  succeeded  by  rhyolitic  conglomerate, 
ranging  from  0  to  400  feet  in  thickness,  which  contains  pebbles  of  the 
underlying  quartzite.  Thin  sheets  of  rhyolite  porphyry  locally  occur. 
intercalated  in  the  conglomerate,  which  is  overlain  by  a  mass  of 
rhyolite  porphyry  about  1,500  feet  thick.  The  porphyry  is  a  massive 
red  rock  consisting  of  phenocrysts  of  quartz  and  feldspar  in  a  dense 
groundmass,  and  apparently  lies  parallel  to  the  underlying  strata. 
The  entire  series  dips  westward  at  an  angle  of  about  20°,  conforming 
with  the  general  structure  of  the  Franklin  Mountains.  At  the  top 
of  the  section  the  basal  bed  of  Paleozoic  sandstone  contains  pebbles 
of  the  underlying  rhyolite  porphyry  and  rests  unconformably  upon 
it.  The  sandstone  contains  brachiopod  shells,  determined  by  Walcott 
as  either  upper  or  middle  Cambrian.  The  underlying  rocks,  there- 
fore, are  probably  pre-Cambrian. 
South  of  the  Sierra  Diablo,  between  Eagle  Flat  and  Van  Horn 
stations  on  the  Texas  and  Pacific  Railway,  about  100  miles  southeast 
of  the  Franklin  Mountains,  there  is  a  more  or  less  metamorphosed 
complex,  consisting  of  red  sandstones,  cherty  limestone,  breccia,  and 
a  few  igneous  intrusions;  and  an  area  of  schistose  rocks  is  exposed 
south  of  the  railroad  adjacent  to  Allanioore.  The  less  metamor- 
phosed rocks  which,  are  exposed  north  of  the  railroad  have  been  tilted, 
folded,  aud  faulted,  the  main  structural  trend  being  east  and  west. 
The  outcrops  for  the  most  part  are  low  lying,  and  are  separated  by 
flat,  wash-filled  areas,  so  that  a  complete  section  can  not  be  measured. 
Apparently  the  oldest  rock  in  the  area  north  of  the  railroad  i<  a 
homogeneous,  fine- textured,  bright  red  sandstone,  which  is  at  least 
500  feet  thick.  Associated  with  the  sandstone  are  several  hundred 
feet  of  massive  magnesian  limestone  seamed  with  crumpled  bands  of 
chert,  and  an   indurated,   coarse   breccia-conglomerate   composed   of 
