emmons.]    COPPER  IN  RED  BEDS  OF  COLORADO  PLATEAU.        229 
It  certainly  is  very  singular,  even  if  only  a  coincidence,  that  such 
peculiar  occurrences  of  ore,  possessing  certain  common  characteristics, 
should  have  been  found  in  a  region  so  difficult  of  access  and  so  little 
explored  as  is  the  Plateau  province,  apparently  at  the  same  geological 
horizon,  over  an  area  which,  taking  the  Grand  Canyon  as  its  southern 
base,  is  300  miles  long  by  200  miles  wide. 
We  now  come  to  the  region  of  the  Grand  Canyon  itself.  The 
Grand  Canyon,  the  last  of  the  great  trenches  cat  by  the  corrasion  of 
the  Colorado  River  in  the  practically  horizontal  beds  of  the  Plateau 
province,  lies  in  the  great  plateau  of  northern  Arizona.  Here  the  river, 
bending  at  a  sharp  right  angle,  changes  from  its  general  southern  to 
a  westerly  course,  and  follows  this  general  course,  with  many  wind- 
ings, through  a  distance  in  a  straight  line  of  about  180  miles. 
Throughout  the  greater  part  of  this  extent  the  harder  limestones  of 
the  upper  part  of  the  Carboniferous,  known  as  the  "Aubrey  forma- 
tion," form  the  undulating  and  generally  forest-covered  surfaces  of 
the  Plateau,  which  has  an  average  elevation  of  about  7,000  feet. 
From  whatever  direction  one  approaches  the  canyon  on  this  plateau 
one  gets  no  premonition  of  its  existence  until  within  a  fewT  feet  of  its 
rim,  when  one  looks  down  over  a  vertical  escarpment  of  2,500  feet  into 
the  maze  of  tributary  canyons  cut  through  the  next  bench  or  shelf, 
generally  formed  by  the  lower  Carboniferous  or  Red  Wall  limestones, 
with  whose  general  appearance  all  are  familiar  from  Holmes's  admir- 
able drawings. 
It  would  take  too  much  time  and  involve  too  much  repetition  of 
what  has  already  been  written  to  go  at  any  length  into  the  geological 
structure  of  this  region,  and  I  will  only  mention  a  few  facts  that  bear 
upon  the  subject  under  consideration. 
The  canyon  is  now  reached  by  rail  over  a  60-mile  branch  of  the 
Atchison,  Topeka  and  Santa  Fe  Railroad,  which  leaves  the  main  line 
at  Williams  station,  about  40  miles  west  of  Flagstaff,  the  former 
starting  point  for  the  canyon.  Between  and  to  the  north  of  these 
two  stations  the  grand  mass  of  San  Francisco  Mountain  and  in 
many  outliers  represent  a  recent  basaltic  eruption  on  the  surface  of 
the  plateau,  while  en  route  to  the  canyon  one  passes  the  little  hill 
known  as  "  Red  Butte,"  a  monadnock  of  Permian  beds,  apparently 
the  only  portion  of  this  covering  of  the  Carboniferous  that  has 
escaped  erosion  within  a  radius  of  many  miles.  The  plateau  at  the 
southern  edge  of  the  canyon  is  called  the  Coconino  Plateau:  thai 
opposite  to  it,  on  the  northern  rim,  is  the  Kaibab  Plateau,  which 
extends  north  nearly  to  the  Utah  line,  a  distance  of  about  00  miles. 
The  Carboniferous  beds,  which  are  well  exposed  along  the  immense 
extent  of  canyon  walls,  consist  in  round  numbers,  first,  of  about  500 
feet  of  upper  Carboniferous  limestone  underlain  by  an  equal  amount 
of  cross-bedded  white  sandstone,  which  together  characteristically 
