772  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
In  the  Grand  Canyon  of  the  Colorado  the  Tonto  sandstone  rests 
directly  on  plicated  and  eroded  schists  and  associated  granites,  and 
demonstrates  them  to  be  pre-Silurian.  Farther  down  the  river  the 
same  relation  is  seen  in  the  Virgin  Range;  and  in  the  next  ridge  to 
the  west,  through  which  the  river  has  cut  Bowlder  Canyon,  are 
gneisses  so  similar  to  those  of  the  Virgin  Range  that  they  may  safely 
be  classed  with  them.  In  Music  Mountain,  in  the  Black  Hills  near 
Prescott,  and  on  Canyon  Creek,  or,  more  generally,  all  along  the 
southwestern  border  of  the  Plateau  region  in  Arizona,  the  Archean 
schists  and  granites  are  seen  beneath  nonconforming  members  of  the 
fossiliferous  rocks,  usually  the  Tonto  sandstone.  To  the  south  and 
west  of  this  line  stretches  a  great  ocean  of  metamorphic  ridges  in 
which  no  one  has  found  fossils. 
Gilbert,  in  1875,  describes  the  range  region  of  eastern  Arizona 
and  western  New  Mexico,  principally  the  latter.  See  summary  under 
section  2,  New  Mexico,  page  768. 
Marvine,17  in  1875,  states  that  granite  is  found  below  the  Tonto 
sandstone  at  the  mouth  of  Grand  Canyon,  at  Music  Mountain,  and 
in  the  canyon  of  New  River.  At  Truxton  and  on  the  road  southwest- 
ward  granite  occurs  in  the  hills,  often  lava  capped ;  is  found  at  Cross 
Mountain,  near  Fort  Rock ;  at  Aztec  Pass ;  at  Juniper  Mountains ;  be- 
tween Prescott  and  Agua  Fria  Valley,  and  in  the  Black  Hills.  In  the 
Juniper  Mountains  there  are  also  found  highly  metamorphic  rocks, 
as  schists,  slates,  etc.,  often  covering  considerable  areas  and  with 
which  many  of  the  silver  and  gold  bearing  lodes  of  the  country  are 
associated.  At  Camp  Verde  on  the  Verde  River  sedimentary  rocks 
rest  upon  syenites.  The  Tonto  sandstone  rests  upon  the  granite  in 
the  Sierra  Ancha,  in  the  San  Carlos  Valley,  and  in  the  Apache  Moun- 
tains. The  main  mass  of  the  Pinal  Mountains  is  granite,  but  upon 
their  northeast  flanks  is  a  long  area  of  highly  metamorphic  rocks, 
consisting  mostly  of  crystalline  schists,  micaceous,  chloritic,  and  tal- 
cose,  their  erosion  forming  an  intricate  maze  of  small  valleys  sepa- 
rated by  sharp  ridges,  which  present  a  strong  contrast  with  the  more 
massive  features  of  the  mountains.  The  granites  and  schists  of  the 
Pinal  Mountains  extend  along  Pinal  Creek  to  Camp  Pinal. 
Powell,1s  in  1876,  further  describes  as  unqonformably  below  the 
Tonto  sandstone  the  Grand  Canyon  sandstones,  shales,  and  limestones, 
10,000  feet  in  thickness;  and  below  this  the  Grand  Canyon  schists, 
of  undetermined  thickness,  composed  of  hornblendic  and  micaceous 
schists  and  slates,  associated  with  beds  and  dikes  of  granite.  The 
Grand  Canyon  group  rests  unconformably  upon  the  crystalline 
schists.  The  plane  of  demarcation  separating  this  group  from  the 
Tonto  group  is  very  sharp.  Fossils  have  been  found  at  the  base  of 
the  Grand  Canyon  series,  but  they  are  not  well  preserved,  and  little 
