60  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
"  Lower  Laurentian "  unconformably  underlying  the  Huronian 
doubtless  was  the  reason  for  placing  these  as  equivalents. 
In  the  later  work  of  Logan  and  Murray  the  names  "  Huronian  "  and 
"  Laurentian  "  were  applied  to  regions  far  distant  from  the  original 
areas,  the  guiding  principles  for  so  doing  being  wholly  lithological 
likeness  and  degree  of  metamorphism.  Working  under  these  prin- 
ciples, as  granites  and  granite  gneisses  are  so  abundant  in  the  orig- 
inal Laurentian,  and  are  nearly  absent  in  the  original  Huronian,  it 
became  customary  with  these  authors  to  refer  to  granitoid  areas  as 
"  Laurentian,"  while  sedimentary  series  containing  quartzites,  lime- 
stones, or  dark,  fine-grained  schists  were  referred  to  the  "  Huronian," 
and  this  reference  wras  frequently  made  when  the  series  as  a  whole 
was  very  much  more  crystalline  than  the  original  Huronian.  The 
only  exception  to  a  reference  of  all  pre-Cambrian  rocks  to  the  Huron- 
ian and  Laurentian  were  the  rocks  now  known  as  Keweenawan  series 
and  Animikie  group.  These  were  recognized  as  resting  uncon- 
formably upon  the  so-called  "  Huronian  "  of  Lake  Superior,  while 
the  Keweenawan  was  seen  to  be  of  a  wholly  different  lithological 
character  from  the  Lake  Huron  rocks.  These  series  were  called  the 
"  Upper  Copper-bearing  series,"  the  original  Huronian  often  being 
called  the  "  Lower  Copper-bearing  series." 
We  find  these  two  geologists,  Logan  and  Murray,  starting  with 
scientific  principles,  laboriously  studying  year  after  year  the  detailed 
occurrences  of  the  rocks  in  the  midst  of  a  forest-covered  wilderness, 
until  their  inductions  built  up  the  "  Original  Huronian  "  and  "  Origi- 
nal Laurentian  "  series.  In  their  later  work,  of  a  very  much  less  de- 
tailed character,  over  vast  areas  the  terms  were  applied  somewhat 
indiscriminately  and  in  such  a  way  as  to  imply  that  below  the 
"Upper  Copper-bearing'1  rocks  there  are  only  two  series,  one  of 
which  is  equivalent  to  the  original  Huronian  and  the  other  of  which 
is  equivalent  to  the  original  Laurentian. 
These  terms,  Huronian  and  Laurentian,  were  gradually  adopted 
by  geologists  working  on  the  United  States  side  of  the  boundary,  so 
that  in  recent  years,  with  the  exception  of  Archean,  they  have  been 
the  most  widely  used  names  for  designating  the  ancient  rocks. 
If  Logan  and  Murray  in  their  later  work  departed  from  strict 
scientific  methods  in  the  use  of  these  terms,  this  departure  was  as 
nothing  compared  with  the  extremes  to  which  later  geologists  of 
America  have  gone.  By  many  geologists,  coarse-grained  granites 
and  granite  gneisses  were  designated  "  Laurentian  "  without  reference 
to  evidence  as  to  whether  they  were  intrusive  rocks  of  far  later  age. 
In  applying  the  term  "  Huronian  "  the  methods  followed  were  even 
worse.  Sometimes  authors  took  a  green  color  to  be  a  characteristic 
feature  of  the  Huronian  and  here  referred  all  the  green  schists ;  others 
took  a  laminated  structure  to  be  characteristic  of  the  series  and  here 
