246  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull.  225. 
indicates  that  the  age  is  Cretaceous,  but  the  exact  horizon  can  not  be 
determined  from  the  material  in  hand." 
Although  the  evidence  regarding  the  age  of  these  beds  is  not 
entirely  conclusive,  there  seems  to  be  a  general  agreement  that  they 
belong  to  the  Cretaceous  system  and  presumably  were  deposited  in 
the  later  stages  of  that  period.  The  contact  between  the  Cretaceous 
rocks  and  the  underlying  Carboniferous  limestone  is  usually  marked 
by  a  thin  bed  of  conglomerate  composed  principally  of  fragments  of 
chert  derived  from  the  underlying  limestone  and  cemented  by  oxide 
of  iron. 
On  the  northern  side  of  the  Deer  Creek  Basin  the  sandstone  and 
shale  of  the  coal -bearing  formation  have  a  thickness  of  -100  to  500  feet. 
They  are  well  exposed  in  the  Lower  held,  the  material  consisting  for 
the  most  part  of  coarse,  greenish-buff  sandstones,  which  are  well 
exposed  and  form  conspicuous  features  of  the  topography.  In  the 
Upper  field  the  measures  seem  to  be  composed  more  largely  of  shale, 
and  they  are  generally  reduced  to  a  comparatively  level  surface. 
Walcotta  states  that  the  thickness  of  strata  exposed  below  the  lower 
main  workable  coal  bed  has  a  thickness  of  2,100  feet,  but  probably 
this  thickness  is  due  to  reduplication  of  the  measures  through  minor 
folding,  and  it  is  doubtful  if  the  correct  thickness  can  be  determined 
at  any  point  in  the  Upper  held. 
On  the  south  side  of  the  basin  the  coal-bearing  rocks  appear  to  be 
thinner,  but  it  is  difficult  to  obtain  an  accurate  measure  since  the}7 
stand  nearly  vertical  and  run  for  long  distances  up  the  slope  of  the 
mountain.  Still  farther  west,  where  they  are  cut  by  Ash  Creek,  the 
rocks  themselves  are  not  exposed,  but  the  interval  in  which  they  are 
due  has  a  thickness  of  not  over  220  feet.  Southwest  of  Saddle  Moun- 
tain they  are  still  further  reduced,  having  a  thickness  of  5  to  20  feet 
between  the  Carboniferous  limestone  and  the  overlying  andesite. 
Whether  this  variation  in  thickness  is  due  to  different  amounts  of 
sediment  being  deposited  on  the  two  sides  of  the  basin,  or  whether  it 
is  due  to  nondeposition  on  the  western  side  is  uncertain,  but  it  seems 
probable  that  the  sediments  were  laid  down  in  inclosed  basins,  and 
consequently  that  the  amount  of  material  may  have  varied  greatly  in 
different  parts  of  the  area. 
After  the  deposition  of  the  coal- bearing  sandstone  and  shale  the 
region  was  the  scene  of  great  volcanic  activity,  and  large  masses  of 
andesite  were  poured  out.  There  is  no  crater  visible  from  which 
this  material  was  ejected,  and  it  seems  probable  that  it  came  out 
through  extensive  fissures  which  were  opened  in  the  lower  part  of  the 
basin  near  Gila  River.  The  change  from  clastic  sediments  to  igneous 
rocks  is  not  abrupt  nor  does  it  appear  to  have  taken  place  contempo- 
raneously throughout  the  basin.     The  sediments  are  interbedded  with 
d»H?n^CreneiC  Coal  Field'  White  Mountain  Indian  Reservation,  Arizona:  Senate  Ex.  Doc.  No.  20, 
48tn  Cong.,  2d  sess.,  p.  6. 
