7Y4  PRE- CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
border;  and  as  the  stratigraphic  evidence  favors  this  view,  it  is 
thought  that  it  can  not  be  considered  of  Cambrian  age. 
Walcott,21  in  1880,  refers  the  section  laid  bare  in  the  Grand  Can- 
yon of  the  Colorado  to  the  Keweenaw  group.  This  section  presents 
one  of  the  best  opportunities  known  to  the  author  for  the  discovery 
of  a  -pre- Olenellus  fauna. 
Walcott,22  in  1890,  gives  the  Algonkian  section  of  the  Grand  Can- 
yon as  follows:  Chuar  (shales  and  limestones),  5,120  feet:  Grand 
Canyon  (sandstones  with  lava  flows  in  upper  part),  6,830  feet; 
Vishnu  (bedded  quartzites  and  schists),  1,000  feet. 
Walcott,23  in  1894,  gives  the  results  of  his  study  of  the  Algonkian 
rocks  of  the  Grand  Canyon  of  the  Colorado.  The  following  classifica- 
tion of  the  rocks  is  adopted : 
Cambrian :  Tonto. 
Unconformity. 
Algonkian  :  Grand  Canyon \ 
Great  unconformity. 
Algonkian  ( ?)  Vishnu. 
The  Vishnu  at  the  one  point  examined,  due  south  of  Vishnu's 
Temple,  consists  of  micaceous  schists  and  quartzites,  cut  by  dikes  and 
veins  of  granite.  The  Unkar  terrane,  6,830  feet  thick,  consists  of 
limestones,  sandstones,  conglomerates,  and  intrusive  and  extrusive 
basic  rocks  of  various  kinds.  The  basal  conglomerate  is  formed 
largely  of  pebbles  derived  from  the  upturned  edges  of  the  pre-Unkar 
strata.  The  Chuar  terrane,  5,120  feet  thick,  consists  mainly  of  shales 
of  various  kinds,  but  contains  285  feet  of  limestone.  Resting  uncon- 
formably  upon  the  Grand  Canyon  series  is  the  Tonto  Cambrian. 
Before  the  deposition  of  the  latter  the  Grand  Canyon  series  was 
planed  to  a  base-level  and  all  the  strata  of  the  series  were  truncated. 
The  entire  Grand  Canyon  series  is  placed  in  the  Algonkian  period 
or  Proterozoic  era.  Various  possible  correlations  of  the  Grand  Can- 
yon with  other  series  may  be  made,  but  it  is  evident  that  until  char- 
acteristic fossils  are  found  in  the  various  terranes  now  referred  to 
the  Algonkian  it  will  be  impossible  to  make  any  correlations  that  will 
be  more  than  tentative  suggestions. 
Van  Hise,24  in  1907,  observed  the  pre-Cambrian  rocks  of  the  Grand 
Canyon  of  the  Colorado.  The  Unkar  series  consists  of  conglomerates, 
cherty  limestone,  shales,  slates,  and  indurated  sandstones  approaching 
quartzites.  The  lower  part  of  the  series,  that  below  Red  Canyon 
Creek,  consists  from  the  base  upward  of  a  persistent  conglomerate 
bed,  in  which  greenish  cherty  limestone  30  to  50  feet  thick  predomi- 
nates; above  this  a  very  considerable  thickness  of  rock  in  which 
dark-colored  cherty  limestone  50  to  75  feet  thick  predominates;  and 
above  this  sandstones  and  shales.  The  pebbles  of  the  conglomerate 
comprise  the  greatest  variety  of  granites,  gneisses,  and  schists,  and 
