404  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
ranges  from  800  to  1,500  feet  above  sea  level.  The  topography  is  that 
of  an  elevated  plateau  which  has  been  deeply  dissected  by  the  short 
rapid  streams  flowing  into  Ohio  River.  Two  watersheds,  lying 
roughly  parallel,  divide  the  quadrangle  from  north  to  south  into 
three  secondary  drainage  basins  of  approximately  equal  size. 
GENERAL  GEOLOGY. 
Stratigraphy. — The  rocks  of  this  area  are  all  of  Carboniferous  age 
and  they  have  been  divided  into  three  formations — Conemaugh,  Mo 
nongahela,  and  Dunkard.  In  the  northern  part  of  the  quadrangle 
the  lowest  member  of  the  group,  the  Conemaugh,  is  most  widely  ex 
posed,  its  lowest  recognizable  member  being  the  Ames  limestone 
The  overlying  Monongahela  formation  caps  only  the  highest  hills 
Toward  the  south  the  Conemaugh  gradually  decreases  in  area,  while 
the  Monongahela  shows  a  corresponding  increase  in  the  extent  of  it: 
outcrop.  A  little  south  of  the  middle  of  the  quadrangle  the  Cone 
maugh  disappears  with  the  exception  of  two  small  areas.  At  th< 
same  place  the  characteristic  limestone  and  coal  beds  of  the  lowe] 
Dunkard  make  their  first  appearance  in  the  tops  of  the  hills.  Thi 
formation  in  turn  gradually  widens  in  extent  southward,  supersed 
ing  the  Monongahela,  as  the  latter  passes  beneath  the  surface  with  i 
gentle  dip  toward  the  center  of  the  basin. 
Geologic  structure. — Taken  as  a  whole  the  rocks  of  this  section  di] 
slightly,  but  by  no  means  constantly,  to  the  southeast.     From  th 
northwestern  to  the  southeastern  corner  of  the  quadrangle  carefu 
leveling  shows  a  drop  of  280  feet  in  the  Pittsburg  coal,  yet  along  th  | 
line  joining  these  points  there  are  differences  in  the  elevation  of  thi 
coal  of  as  much  as  300  feet  in  3  miles.     The  structure  responsible  fo  | 
this  variation  in  dip  is  a  broad,  shallow,  irregular,  synclinal  trougi 
which  passes  through  the  center  of  the  quadrangle  from  south  tt| 
north.     Its  axis  intersects  the  southern  border  of  the  territory  abou : 
5  miles  from  the  southwest  corner,  which  is  about  1  mile  east  of  Wes  | 
Middletown.     The  axis  trends  a  little  east  of  north,  passing  east  o 
Burgettstown  west  of  Bavington,  and  thence  with  a  more  easterl 
direction  into  the  southeast  corner  of  the  Beaver  quadrangle,  whic 
lies  directly  north  of  the  Burgettstown  quadrangle. 
This  syncline  is  in  reality  a  chain  of  oval-shaped  basins  of  differl 
ent  sizes  and  depths,  linked  together  by  low  structural  divides.  Th  I 
center  of  the  one  farthest  to  the  south  is  near  Wilsons  Mill,  froit 
which  locality  it  has  received  its  name.  At  this  point  the  PittsburiJ 
coal  is  at  an  elevation  of  only  720  feet  above  sea  level.  The  centeJ 
of  the  next  basin  to  the  north  lies  beneath  Raccoon  Creek  at  a  poir  I 
about  3  miles  south  of  Burgettstown  and  H  miles  east  of  the  villac  | 
of  Cross  Creek,  from  which  place  it  derives  the  name  of  the  Croi: 
