vanhise.]  DISCUSSIONS    OF    PRINCIPLES.  495 
to  determine  at  wliat  i»]<ni<*  the  elastics  end  and  the  crystallines  begin. 
It*  it  has  proved  difficult  to  separate  the  unfolded  elastics  from  under- 
lying crystallines  liow  much  nunc  difficult  must  it  of  necessity  be  to 
separate  series  that  have  together  been  subjected  to  intense  and  per- 
haps repeated  dynamic  actions. 
The  Algonkian  has  been  defined  as  including  all  recognizable  pre-Oam- 
brian  elastics  and  their  equivalent  crystallines.  In  the  consideration  of 
the  character,  origin,  and  delimitation  of  the  Archean  the  lower  limit  of 
the  Algonkian  has  been  given.  Its  basal  plane  is  the  lowest  of  the 
recognizable  clastic  rocks.  It  has  been  seen  that  there  are  great  dif- 
ferences in  the  ease  of  recognition  of  the  basal  Algonkian  plane  indif- 
ferent regions.  In  the  Uinta  mountains,  in  the  Grand  canyon  region 
of  Arizona,  in  portions  of  the  lake  Superior  region,  in  the  original 
Huronian  region,  and  elsewhere,  between  the  Algonkian  and  the 
Archean,  there  are  great  unconformities,  above  which  are  the  readily 
recognizable  clastic  rocks,  and  below  which  are  the  thoroughly  crystal- 
line basal  complexes.  Even  in  many  regions  in  which  there  have  been 
repeated  foldings  since  Archean  time,  and  in  regions  obscured  by  erup- 
tive activity,  it  is  perfectly  clear  that  a  large  part  of  the  rocks  are 
clastic  and  belong  with  the  Algonkian,  while  other  parts  have  all  the 
characteristics  of  the  iundamental  complex.  Occupying  an  interme- 
diate position  are  occasionally  found  areas  of  rocks  which  can  not  cer- 
tainly be  placed  with  the  Algonkian  or  Archean,  but  this  difficulty  is 
not  peculiar  to  this  separation  any  more  than  to  other  general  recog- 
nized planes,  such  as  that  separating  the  Cambrian  and  Silurian  in 
folded  districts.  Many  of  the  members  of  the  Canadian  Geological 
Survey  have  described  the  Huronian  and  the  Laurentian  as  conform- 
able, with  gradations  between  the  two.  This  apparent  accordance  and 
gradation  is  in  many  cases  due  tc  the  fact  that  placed  with  the  Huro- 
nian are  many  rocks  which  would  under  the  use  of  the  terms  here 
proposed  be  regarded  as  Archean.  In  other  cases  there  are  apparent 
conformities  and  gradations  between  undoubted  elastics  and  the  under- 
lying rocks  having  all  the  characteristics  of  the  fundamental  complex. 
The  significance  of  these  gradations  is  discussed  in  another  place, 
where  it  was  seen  that  they  are  not  inconsistent  with  genuine  struc- 
tural breaks. 
It  has  been  stated  that  the  reasons  for  placing  the  base  of  the  Cam- 
brian at  the  Oienellus  fauna  are  considered  by  Walcott  in  this  series  of 
correlation  papers,  and  that  his  results  are  here  accepted.  It  is,  how- 
ever, to  the  point  to  consider  whether  this  horizon  answers  equally  well 
for  the  upper  limit  of  the  Algonkian.  Evidently  all  the  arguments 
brought  forward  by  Walcott  for  placing  this  fauna  as  the  base  of  the 
Cambrian  apply  as  well  for  considering  the  horizon  below  this  as  upper- 
most Algonkian;  for  the  widespread  character,  both  European  and 
American,  of  the  Olenellns  fauna  makes  it  a  particularly  easy  one  to 
identity  and  therefore  valuable  for  the  purposes  of  discrimination.     In 
