330  PRE-CAMBRIAN   ROCKS    OF    NORTH   AMERICA.  [bull.  86. 
vertical.  The  Grand  canyon  and  Chuar  groups  unconformably  depos- 
ited over  the  underlying  Archean  are  referred  to  the  Lower  Cambrian 
and  placed  as  the  stratigraphical  equivalent  of- the  Keweenawan  group 
of  lake  Superior.  In  the  Grand  canyon  series  are  found  a  few  obscure 
fossils.  The  Chuar  and  Grand  canyon  series  are  both  wholly  unnieta- 
morphosed  and  but  slightly  disturbed. 
Walcott,33  in  1886,  states  that  the  Tonto  sandstone  of  the  Grand 
canyon  district  is  Upper  Cambrian  or  Potsdam.  Then  below  a  great 
unconformity  occurs  by  the  erosion  of  an  entire  cross  section  of  13,000 
feet  of  strata  of  the  Chuar  and  Grand  canyon  series;  below  the  un- 
conformable series  rest  unconformably  on  underlying  highly  inclined 
strata,  which  where  the  section  terminates  belong  to  a  system  of  strata 
between  the  Grand  canyon  series  and  the  Archean.  On  account  of; 
this  great  unconformity  below  the  Tonto  it  is  thought  better  to  classify 
all  the  pre-Tonto  strata  as  pre-Cambrian,  the  middle  and  lower  Cam- 
brian times  being  in  the  Grand  canyon  district  a  period  of  erosion.  The 
Chuar  formation  or  upper  6,000  feet  of  limestones  and  argillaceous 
shales  lithologically  resemble  the  Trenton  limestone  and  Utica  shales 
of  the  New  York  section.  There  is  no  evidence  of  the  great  age  of  j 
these  strata  in  their  physical  aspect.  The  lower  6,000  feet  of  Grand 
canyon  formation  are  sandstones  with  interbedded  lava  flows  toward 
the  upper  portions.  Eipple  marks  and  mud  cracks  abound  in  many  of 
the  layers,  but  not  a  trace  of  a  fossil  was  seen.  Midway  in  the  lower 
portion  of  the  overlying  Chuar  strata  the  presence  of  a  fauna  is  shown 
by  a  minute  Discinoid  or  Patelloid  shell,  a  small  Lingula-like  shell,  a 
species  of  Hyolithes,  and  a  fragment  of  what  appears  to  have  been  the 
pleural  lobe  of  the  segment  of  a  trilobite  belonging  to  a  genus  allied  to 
the  genera  Olenellus,  Olenpides,  or  Paradoxides.  There  is  also  an  ob- 
scure Stromatopora-like  form  that  may  or  may  not  be  organic.  The 
fauna  as  given  above  is  very  unsatisfactory,  but  it  shows  the  presence 
of  a  fauna  that  is  Cambrian  in  character,  as  far  as  we  know,  although 
it  may  be  a  trace  of  a  fauna  preceding  that  of  the  Lower  Cambrian  of 
the  Atlantic  border ;  and  as  the  stratigraphic  evidence  favors  this  view 
it  is  thought  that  it  cannot  be  considered  of  Cambrian  age. 
Walcott,35  in  1889,  refers  the  section  laid  bare  in  the  Grand  canyon 
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,72  in  1890,  gives  the  Algonkian  section  of  the  Grand  can- 
yon as  follows:  Chuar  (shales  and  limestones),  5,120  feet;  Grand  can- 
yon (sandstones  with  lava  flows  in  upper  part),  6,830  feet;  Vishnu 
(bedded  quartzites  and  schists),  1,000-f  feet. 
SUMMARY  OF   RESULTS. 
It  is  evident  from  the  literature  that  in  western  New  Mexico  and  in 
the  major  part  of  Arizona  is  a  fundamental,  thoroughly  crystalline 
