THE   CORDILLERAS.  773 
can  be  made  of  them.  Still,  on  geological  evidence,  these  beds  are 
considered  Silurian. 
Walcott,19  in  1883,  describes  a  great  series  of  unconformable  sedi- 
ments below  the  Tonto,  which  are  divided  into  two  groups,  the  Chuar 
and  the  Grand  Canyon,  between  which  there  is  an  unconformity  by 
erosion.  The  lower  or  Grand  Canyon  group  is  made  up  of  an  im- 
mense mass  of  sandstones  and  interbedded  greenstones,  and  the  Chuar 
group  is  a  series  of  sandy  and  clay  shales.  The  Archean  at  the  base 
of  the  Grand  Canyon  group  consists  of  thin-bedded  quartzites  broken 
by  intrusive  veins  of  a  flesh-colored  granite,  the  layers  of  quartzite 
standing  nearly  vertical.  The  Grand  Canyon  and  Chuar  groups  un- 
conformably  deposited  over  the  underlying  Archean  are  referred  to 
the  Lower  Cambrian  and  placed  as  the  stratigraphic  equivalent  of 
the  Keweenawan  group  of  Lake  Superior.  In  the  Grand  Canyon 
series  are  found  a  few  obscure  fossils.  The  Chuar  and  Grand  Can- 
yon series  are  both  wholly  unmetamorphosed  and  but  slightly  dis- 
turbed. 
Walcott,20  in  1880,  states  that  the  Tonto  sandstone  of  the  Grand 
Canyon  district  is  Upper  Cambrian  or  Potsdam.  Then  below  occurs 
a  great  unconformity  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,  middle  and  lower 
Cambrian  times  being  in  the  Grand  Canyon  district  a  period  of 
erosion.  The  Chuar  formation,  or  upper  G,000  feet  of  limestones  and 
argillaceous  shales,  lithologically  resembles  the  Trenton  limestone 
and  Utica  shales  of  the  New  York  section.  There  is  no  evidence  of 
the  great  age  of  these  strata  in  their  physical  aspect.  The  lower 
G,000  feet  of  Grand  Canyon  formation  are  sandstones  with  inter- 
bedded lava  flows  toward  the  upper  portions.  Ripple  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  seg- 
ment of  a  trilobite  belonging  to  a  genus  allied  to  the  genera  Olenellus, 
Olenoides,  and  Paradoxides.  There  is  also  an  obscure  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  thai  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 
