496  PRE-CAMBR1AN    ROCKS    OF    NORTH    AMERICA.  I  bull.  88. 
the  lake  Superior  region  and  in  many  other  localities  above  the  upper 
Algonkian  are  unconformities,  the  first  of  the  Cambrian  being  middle 
or  upper.  In  other  regions,  as  in  Newfoundland,  the  upper  Algonkian 
is  marked  by  an  unconformity,  and  the  formation  immediately  above 
bears  the  Olenellus  fauna.  This  is  the  most  favorable  and  clear  case. 
In  the  Wasatch  and  several  other  ranges  of  Utah  and  Nevada,  in . 
British  Columbia,  and  probably  in  the  southern  Appalachians,  below 
the  Olenellus  fossiliferous  Cambrian  are  conformable  series  of  quartz- 
ites  and  slates  of  great  thicknesses.  Are  these  lowest  Cambrian  or 
uppermost  Algonkian?  May  not  the  Olenellus  fauna  in  the  future  be 
found  to  extend  downward  through  a  greater  or  less  thickness  of  these 
apparently  barren  rocks?  If  in  any  region  the  fauna  be  found  to  ex- 
tend downward  for  a  long  way,  it  is  probable  that  species  and  genera 
characteristic  of  the  Olenellus  horizon  as  now  known  will  drop  out  and 
others  appear  which  are  different.  The  Olenellus  would  thus  grade 
into  a  pre-Olenellus  fauna.  Such  a  gradation  will  doubtless  some- 
where be  found,  while  in  other  regions  the  change  from  an  Olenellus 
fauna  to  one  of  a  pre-Olenellus  type  may  occur  abruptly.  In  either 
case  there  will  finally  appear  a  fauna  which  is  not  the  present  known 
Olenellus  fauna,  but  which  is  as  different  from  it  as  is  the  Cambrian 
from  the  Silurian  (Ordovician).  As  the  term  is  here  used,  such  a  fauna 
is  pre- Cambrian,  and  the  rocks  containing  it  are  Algonkian.  In  the 
following  paragraphs  great  barren  inferior  series  conformably  below 
the  known  Cambrian  are  placed  with  the  Algonkian  on  the  ground  of 
probability.  The  presence  of  an  abundant  lower  Cambrian  life  at  a 
certain  horizon  within  the  conformable  succession,  with  apparent  com- 
plete absence  of  life  in  immense  thicknesses  of  rocks  conformably  below, 
which,  so  far  as  lithological  character  is  concerned,  are  equally  likely  to 
bear  fossils,  throws  the  weight  of  evidence  in  favor  of  the  Algonkian 
age  of  these  rocks.  It  is,  however,  more  than  probable  that  some  part 
of  the  conformable  downward  extensions  of  the  Cambrian  which  are 
here  provisionally  referred  to  the  Algonkian  will  in  the  future  be  found 
to  belong  with  the  post- Algonkian. 
The  newest  Proterozoic  or  Algonkian  rocks  of  different  regions  may 
stand  in  different  positions,  just  as  the  superior  rocks  of  the  Paleozoic 
may  in  any  given  region  be  Cambrian,  Silurian,  Devonian,  or  Carbon- 
iferous. 
DIFFICULTIES   IN  ALGONKIAN   STRATIGRAPHY. 
Since  among  the  pre-Cambrian  elastics,  paleontology  is  not  yet  avail- « 
able  in  correlation,  it  is  exceedingly  difficult  to  make  widespread  sub- 
divisions of  the  Algonkian,  such  as  are  made  in  later  time.  The  diffi- 
culty is  further  increased  by  the  unequal  metamorphism  in  different 
regions  of  series  of  the  same  age.  The  Algonkian  is  in  just  such  a 
position  as  regards  wide  correlation  of  its  constituent  series  as  would 
be  the  Paleozoic  and  Mesozoic  if  their  known  fossil  contents  were  so 
