576  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.          [bull.  260. 
The  portion  of  the  Toyah  basin  within  the  area  described  is  a  broad 
plain  that  rises  gradually  westward.  The  elevation  is  2,500  feel  at 
Pecos  and  about  3,600  feet  at  the  western  border  near  San  Martine. 
Though  in  general  flat,  the  basin  can  be  characterized  as  consisting 
of  broad,  gently  undulating  valleys  separated  by  intervening  low 
divides.  At  the  western  margin  a  number  of  arroyos  emerge  from 
the  hills,  but  few  waterways  maintain  their  channels  completely 
across  the  basin.  Cottonwood  Draw  is  a  conspicuous  example.  This 
is  a  comparatively  large  drainage  way  that  heads  in  the  Delaware 
Mountains,  but  midway  across  the  Toyah  basin  it  becomes  lost  in  the 
flat.  Pecos  Kiver,  which  flows  perennially,  meanders  in  a  broad 
flood  plain,  into  which  it  has  cut  a  shallow  channel.  The  flood  plain 
slopes  gradually  away  from  the  river,  and  locally  the  border  between 
the  flood  plain  and  the  Toyah  basin  is  marked  by  a  low  gravel  bluff. 
STRATIGRAPHY. 
The  rocks  of  the  area  here  described  are  sedimentary  and  belong 
chiefly  to  the  Carboniferous,  Cretaceous,  and  Quaternary  systems. 
The  Sierra  Diablo  is  capped  by  over  1,000  feet  of  massive  gray 
limestone,  including  a  few  thin  layers  of  interbedded  drab  shale. 
These  rocks  contain  numerous  fossils  belonging  to  the  Pennsylvanian 
series  of  the  Carboniferous.  A  coarse  conglomerate  marks  the  base 
of  the  Carboniferous  rocks,  which  overlie  unconformably  a  complex 
group  of  sediments  of  probable  pre-Cambrian  age  that  need  not  be 
described  here.  The  Carboniferous  rocks  of  the  Sierra  Diablo  lie 
almost  flat,  with  a  low  westward  dip.  They  are  delimited  on  the  eastt 
by  a  fault  of  considerable  but  undetermined  throw  that  is  marked 
by  the  scarp  of  the  mountains  toward  Salt  basin. 
Salt  basin  is  strewn  with  unconsolidated  "  wash  "  derived   from 
the  contiguous  highlands.     Adjacent  to  the  periphery  the  materials 
are  coarse  textured  and  are  more  or  less  cemented  by  lime.     Toward]! 
the  center  of  the  basin  the  deposits  are  finer,  and  consist  largely  of 
sand  and  clay.     Low,  marshy  areas  south  of  the  New  Mexico-Texas 
boundary    are    commonly    floored    with    gypsum    containing    traces^ 
of  other  salts.     No  very  deep  wells  have  been  put  down  in  the  Salt" 
basin,  the  deepest  recorded  being  343  feet  at  Wild  Horse.     It  may  bfl 
surmised,  however,  that  these  unconsolidated  deposits  extend  to  a, 
depth  of  1,000  feet  and  more,  as  in  the  Hueco  basin  near  El  Paso. 
The  Pleistocene  deposits  of  Salt  basin  conceal  the  relation  of  the]' 
Pennsylvanian    strata    of    the    Sierra    Diablo    to    the    rocks    of    the 
Guadalupe-Delaware  Mountains.     The  scarp  of  the  latter  marks  aji  [ 
belt  of  disturbance.     At  the  western  limit  of  the  range  an  unsymmet-   f, 
rical  anticline  is  developed,  of  which  the  elevated,  gently  east-sloping  j 
limb  terminates  at  the  scarp.     To  the  west,  low-lying  foothills  expose 
