944  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225. 
tral  part  of  the  basin,  must  have  a  thickness  of  several  thousand  feet. 
At  the  point  where  it  is  crossed  by  the  trail  from  Manning's  ranch  to 
San  Carlos  its  thickness  is  about  1,200  feet.  It  holds  about  the  same 
thickness  in  the  small  canyon  at  the  extreme  upper  or  eastern  end  of 
the  field,  but  in  passing  to  the  southern  sjde  of  the  basin  the  lime- 
stone is  seen  to  become  thinner  and .  thinner  until  finally  it  disappears 
for  a  short  distance  just  south  of  Mount  Lou,  a  little  knob  in  the 
extreme  end  of  the  basin.  Whether  this  disappearance  of.  the  lime- 
stone is  due  to  faulting  or  nondeposition  was  not  determined.  All 
along  the  southern  side  of  the  field  the  limestone  is  thin  and  greatly 
disturbed,  and -it  seems  probable  that  its  absence  in  the  eastern  end  of 
the  basin  may  be  explained  by  more  severe  crushing  than  usual, 
leaving  the  coal-bearing  rocks  in  direct  contact  with  the  middle  Cam- 
brian quartzite. 
About  4  miles  west  of  Manning's  ranch  the  small  band  of  limestone 
shown  in  the  immediate  front  of  the  ridge  turns  to  the  south  and 
disappears  beneath  a  more  recent  lava  flow  that  forms  a  high  mesa. 
At  their  point  of  disappearance  these  rocks  strike  approximately 
toward  the  outcrop  of  limestone  on  Ash  Creek,  and  for  that  reason 
it  has  been  supposed  that  they  are  connected,  although  concealed  by 
recent  flows  of  lava. 
In  the  basin  of  Ash  Creek  the  limestone  shows  in  a  semicircular 
line  of  outcrop  just  east  of  Saddle  Mountain.  The  region  to  the 
south  was  not  closely  examined,  but  from  Mr.  Mellor's  observations 
it  seems  probable  that  it  was  forced  up  by  a  laccolithic  intrusion  of 
volcanic  rocks,  which  now  show  as  a  dome-shaped  mass  directly  back 
of  the  limestone.  If  this  is  truly  a  laccolith,  the  limestone  beds  on  its 
eastern  side  have  been  eroded,  and  since  then  the  whole  has  been 
deeply  buried  beneath  extensive  flows  of  lava.  In.  this  semicircular 
line  of  outcrop  the  limestone  dips  away  from  the  center  at  an  angle  of 
about  30°,  and  to  the  southwest  it  seems  to  form  a  shallow  synclinal 
trough,  which  extends  southwestward  from  Saddle  Mountain  and 
passes  beneath  the  high  mesa  lying  north  of  Aravaypa  Creek. 
The  limestone  comes  again  to  the  surface  west  of  this  shallow  syn- 
cline  in  a  low,  anticlinal  ridge,  which  terminates  toward  the  northwest 
in  a  fault  that  cuts  out  the  limestone  southwest  of  Saddle  Mountain 
and  south  of  the  road  from  Dudleyville  to  the  camp  of  the  Saddle 
Mountain  Mining  Company.  It  seems  possible,  however,  that  this 
fault  may  extend  to  the  northwest  beyond  the  wagon  road,  and  it  may 
have  been  responsible  for  the  course  of  Ash  Creek  below  this  point. 
Fossils  were  collected  from  the  limestone  southwest  of  Saddle  Moun- 
tain; in  the  Ash  Creek  box  canyon;  at  the  point  marked  "B"  on  the 
sketch  map,  about  1  mile  west  of  Manning's  ranch;  on  the  same  line 
of  outcrop  about  4  miles  farther  east;  and  at  the  head  of  Hawk  Can- 
