THE   CORDILLERAS.  771 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
The  present  state  of  knowledge  of  the  pre-Cambrian  of  this  region 
is  covered  in  the  preceding  summary  by  Lindgren  and  Graton. 
SECTION  3.     ARIZONA  AND  ADJACENT  PARTS  OF  NEVADA. 
SUMMARY    OF    LITERATURE. 
Powell,14  in  1874,  states  that  below  the  Carboniferous  are  sedi- 
mentary rocks  provisionally  called  Devonian  and  Silurian.  Still 
underlying  these  is  an  extensive  series  of  inetamorphic  crystalline 
schists,  in  some  places  yet  showing  faint  traces  of  the  original  stratifi- 
cation, but  usually  these  are  so  degraded  that  the  total  thickness  of 
the  beds  Avas  not  determined.  In  places  they  constitute  about  a  thou- 
sand 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  intrusive,  hence  of  igneous  origin.  In  some  places  the  evidence  is 
complete.  An  extensive  period  of  erosion  separates  these  schists  and 
granite  from  the  overling  Silurian  and  Devonian  rocks. 
In  the  Grand  Canyon  are  the  records  of  an  extensive  period  of 
deposition  in  the  schists,  followed  by  plication,  erosion,  Assuring,  and 
eruption.  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,15  in  1875,  further  describes  the  Grand  Canyon  group.  Un- 
conformable 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,  unconformably,  a  thousand  feet  of  crystalline  schists  with  dikes 
of  greenstone  and  beds  of  granite.  This  lower  series  i-  composed 
chiefly  of  metamorphosed  sandstones  and  shales,  which  have  been 
folded  so  many  times,  squeezed  and  heated,  that  their  original  struc- 
ture as  sandstones  and  shales  is  greatly  obscured  or  entirely  destroyed. 
so  that  they  are  inetamorphic  crystalline  schists.  After  these  beds 
were  deposited,  folded,  and  deeply  eroded  they  were  fractured,  and 
through  the  fissures  came  floods  of  molten  granite,  which  now  stands 
in  dikes  or  lies  in  beds. 
Gilbert,10  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  pegma- 
tite, but  is  probably  inetamorphic. 
