230  CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,  1904.        [bull.  260. 
form  the  upper  vertical  member  of  the  canyon  walls.  Below  these 
are  1.200  feet  of  red  sandstones  with  some  thin  shales  and  limestone 
horizons,  which  form  Avails  less  strictly  vertical  and  whose  base  is 
generally  covered  by  talus.  Under  these  are  550  feet  of  blue  lime- 
stones, which  form  a  second  platform  within  the  upper  canyon  walls. 
Powell  gave  the  names  of  Upper  and  Lower  Aubrey  and  Red  Wall 
limestone  to  these  respective  formations,  but  where  and  on  what 
grounds  he  drew  his  lines  of  division  I  am  unable  to  determine. 
The  lower  limestones,  which  form  vertical  cliffs,  generally  colored 
red  on  the  surface,  are  evidently  in  part  at  least  Red  Wall  limestone. 
They  correspond  to  the  lower  Carboniferous  limestones  of  Utah  and 
Colorado,  while  the  upper  beds,  consisting  of  a  great  thickness  of 
prevailingly  siliceous  sediments  capped  by  calcareous  beds,  corre- 
spond also,  in  a  broad  way,  with  the  other  developments  of  the 
Carboniferous  in  those  regions. 
E.  P.  Jennings a  has  recently  published  a  brief  account  of  the 
copper  deposits  of  the  Kaibab  Plateau.  These  are  best  developed 
near  Jacobs  Lake,  30  miles  south  of  the  Utah  line,  where  they  are 
16  feet  thick,  of  unknown  width,  and  more  or  less  continuous  for 
5  miles,  and  he  says  that  outcrops  of  ore  have  been  found  at  various 
points  along  the  plateau  surface  from  Jacobs  Lake  south  to  the 
Grand  Canyon,  a  distance  of  40  miles.  He  describes  the  orje  beds 
as  white  chert  impregnated  with  malachite  and  azurite,  small 
amounts  of  earthy  cuprite,  copper  glance,  and  chalcopyrite  having 
also  been  observed.  The  beds  are  intersected  by  many  small  faults, 
the  ore  near  these  faults  having  been  crushed  and  recemented  by 
silica  and  copper.  He  finds  no  evidence  of  copper  in  the  Paleozoic 
strata  below  these  beds  as  exposed,  either  along  the  fault  which  forms 
the  western  escarpment  of  the  plateau  or  in  the  Avails  of  the  Grand 
Canyon,  but  mentions  a  few  copper-bearing  dikes,  with  the  usual  con- 
tact impregnation  of  copper  ore  and  copper-stained  rock  in  the 
metamorphic  series  at  the  bottom  of  the  canyon,  nearly  a  mile  beloAV 
the  copper-bearing  beds.  He  says  there  are  no  other  known  erupjj 
fives  that  could  have  furnished  the  copper.  To  the  north  and  east, 
however,  are  Triassic  sandstones  sufficiently  copper  bearing  to  be 
mined  with  profit;  hence  he  concludes  that  these  ores  must  have 
been  formed  by  the  leaching  down  of  deposits  in  the  Triassic  strata 
that  originally  covered  them,  by  carbonated  alkaline  waters,  and 
reprecipitation  in  the  Carboniferous  limestone.  This  theory  is,  td 
say  the  least,  ingenious  and  plausible,  but  it  can  hardly  be  finally 
accepted  until  the  region  has  been  systematically  studied. 
I  saw  ores  similar  to  those  described  by  him  that  came  from  a 
mine  on  the  Coconino  Plateau  to  the  Avest  of  the  railroad,  and  hence 
Trans.  Am.  Inst.  Min.  Eng.,  vol.  34,  p.  839. 
