472  PRE-CAMBRIAN   ROCKS    OF    NORTH   AMERICA. 
[BULL.   80. 
Iii  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  de- 
gree of  metamorphism .  Workin  g  under  these  principles,  as  granites  and 
granite-gneisses  are  so  abundant  in  the  original  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,  limestones,  or  dark,  fine  grained  schists 
were  referred  to  the  Huronian,  and  this  reference  was  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-Cam- 
brian  rocks  to  the  Huronian  and  Laurentian  were  the  series  now  known 
as  Keweenawan  and  Animikie.  These  were  recognized  as  resting 
unconformably  upon  the  so-called  Huronian  of  lake  Superior,  while  the 
Keweenawan  was  seen  to  be  of  a  wholly  different  lithological  charac- 
ter 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  Laurentian 
series.  In  their  later  work  of  a  very  much  less  detailed  character  over 
vast  areas  the  terms  were  applied  somewhat  indiscriminately,  and  in 
such  a  way  as  to  imply  that  below  the  Upper  Copper-Bearing  rocks 
there  are  only  two  systems,  one  of  which  is  equivalent  to  the  original 
Huronian,  and  the  other  of  which  is  equivalent  to  the  original  Lauren- 
tian. 
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  of  any  names  for  designating  the  ancient  rocks. 
If  Logan  and  Murray  departed  in  their  later  work  from  strict  scien- 
tific methods  in  their  use,  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 
as  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  charac- 
teristic of  the  series  and  here  referred  all  the  laminated  rock,  including 
even  coarse  grained  laminated  gneisses;  others  took  the  volcanics 
associated  with  the  Huronian  to  be  its  characteristic  feature  and  so 
called  various  pre-Cambrian  volcanic  series  Huronian ;  others  regarded 
metalliferous  rocks  as  the  important  feature  of  the  Huronian. 
