van  Hiss.)  THE    CORDILLERAS.  327 
beds  was  not  determined.  In  places  they  constitute  about  a  thousand 
feet  of  the  altitude  of  the  walls.  These  beds  are  traversed  by  dikes  of 
granite,  and  beds  of  granite  are  found  which  are  believed  to  be  intru- 
sive, hence  of  igneous  origin.  In  some  places  the  evidence  is  complete. 
An  extensive  period  of  erosion  separates  these  schists  and  granite  from 
the  overlying  Silurian  and  Devonian  rocks. 
In  the  Grand  canyon  are  the  records  of  an  extensive  period  of  depo- 
sition in  the  schists,  followed  by  plication,  erosion,  Assuring  and  erup- 
tion. Again  we  have  an  invasion  of  the  sea,  which  remains  until 
10,000  feet  of  shales,  sandstones  and  limestones  are  deposited;  and 
this  is  followed  by  a  dry-land  period,  marked  in  some  places  by  at  least 
10,000  feet  of  erosion  and  accompanied  by  plication,  Assuring  and 
eruption. 
Powell,69  in  1875,  further  describes  the  Grand  canyon  group.  Un- 
conformably  below  the  Carboniferous  of  the  Kaibab  plateau  is  a  middle 
series  of  slates,  sandstones  and  limestones  500  feet  thick,  so  inclined  that 
the  total  thickness  of  its  beds  is  10,000  feet.  Below  these  are  uncon- 
formably  a  thousand  feet  of  crystalline  schists  with  dikes  of  greenstone 
and  beds  of  granite.  This  lower  series  is  composed  chiefly  of  meta- 
morphosed sandstones  and  shales,  which  have  been  folded  so  many 
times,  squeezed  and  heated,  that  their  original  structure  as  sandstones 
and  shales  is  greatly  obscured  or  entirely  destroyed,  so  that  they  are 
metamorphic  crystalline  schists.  After  these  beds  were  deposited, 
folded  and  deeply  eroded  they  were  fractured,  and  through  the  fis- 
sures came  floods  of  molten  granite,  which  now  stands  in  dikes  or  lies 
in  beds,  and  the  metamorphosed  sandstones  and  shales,  with  the  beds 
of  granite,  present  evidence  of  erosion  subsequent  to  the  periods  just 
mentioned,  yet  antedating  the  deposition  of  the  nonconformable  sand- 
stones. Here,  then,  we  have  evidences  of  another  and  more  ancient 
period  of  erosion  or  dry  land.  Three  times  has  this  great  region  been 
left  high  and  dry  by  the  ever-shifting  sea;  three  times  have  the  rocks 
been  fractured  and  faulted ;  three  times  have  floods  of  lava  been  poured 
up  through  the  crevices,  and  three  times  have  the  clouds  gathered  over 
the  rocks  and  carved  out  valleys  with  their  storms.  The  first  time  was 
after  the  deposition  of  the  schists ;  the  second  was  after  the  deposition 
of  the  red  sandstones ;  the  third  time  is  the  present  time. 
Gilbert,36  in  1875,  describes- the  axis  of  the  Black  and  Colorado 
mountains  in  northwestern  Arizona  as  consisting  of  granitoid  rocks 
and  highly  crystalline  schists.  In  Bowlder  canyon  of  this  range  upon 
a  nucleus  of  syenite  are  plicated  crystalline  schists.  In  Virgin  canyon 
the  nucleus  is  gneissic,  with  a  general  anticlinal  structure.  In  Black 
canyon  the  nucleus  is  a  homogeneous  rock  resembling  pegmatite,  but 
is  probably  metamorphic. 
In  the  Grand  canyon  of  the  Colorado  theTonto  sandstone  rests  directly 
on  plicated  and  eroded  schists  and  associated  granites,  and  demonstrates 
them  to  be  pre- Silurian.    Following  down  the  river  the  same  relation  is 
