vanh.sk  i  DISCISSIONS    OF    PRINCIPLES.  489 
• 
Can  not  be  more  accurately  located  are  called  post  Tertiary.  In  like 
manner  the  granite-gneisses  which  are  recognized  as  eruptives  of  later 
age  than  sedimentaries  should  be  assigned  if  possible  to  the  age  to  which 
they  belong,  and  if  not,  their  limitations  expressed.  This  restriction 
of  the  term  Laurentian  will  undoubtedly  eliminate  from  it  large  areas 
of  rocks  which  have  heretofore  been  recognized  as  Laurentian,  but  to 
include  both  the  original  granite-gneiss  and  later  granite-gneisses  under 
a  single  time  term  is  but  to  deliberately  continue  a  confused  classifica- 
tion after  facts  have  been  discovered  which  render  it  unnecessary.  All 
were  first  referred  to  the  Laurentian  only  because  a  discrimination  as 
to  age  had  not  been  made  between  them. 
It  appears  to  me  that  the  best  use  which  can  now  be  made  of  the 
old  term  Laurentian  is  to  restrict  it  to  the  Archean  granite-gneisses, 
including  in  it  no  rocks  which  traverse  Algonkian  sedimentaries.  This 
use  of  the  term  Laurentian  is  different  from  the  original  proposal  of 
Logan.  The  original  Laurentian  is  largely  and,  with  the  exception  of 
the  great  lower  "orthoclase"  gneiss,  perhaps  wholly  a  series  of  detrital 
origin,  being  composed  of  limestones,  quartzites,  and  regularly  lami- 
nated gneisses  which  almost  certainly  are  altered  clastic  rocks. 
Dawson  (Sir  William)  in  a  late  paper  insists  that  the  bedded  sedimen- 
tary character  of  this  and  adjacent  series  can  not  be  doubted.  It 
was  natural  in  the  days  in  which  lamination  of  any  kind  was  by 
most  geologists  regarded  as  proof  of  sedimentation,  that  the  term  Lau- 
rentian should  be  carried  over  to  the  underlying  thoroughly  crystalline 
granitoid  gneisses.  The  known  area  of  this  latter  class  has  now  become 
so  great  that  the  elastics  of  the  type  series  and  other  elastics  here 
placed  are  insignificant  in  comparison.  The  Hastings  series,  after- 
wards connected  by  Vennor  with  the  original  Laurentian,  was  first 
referred  by  him  to  the  Iluroniau,  and  there  is  little  doubt  that  this 
series  would,  if  newly  found  in  the  great  expanse  of  northern  Canada, 
be  thus  placed.  This  is  still  more  emphatically  true  of  the  so-called 
Laurentian  elastics  of  lake  Mpissiug,  which  were  referred  to  the  Huro- 
nian  by  Selwyn.  That  two  groups  of  rocks  were  included  in  the  Lau- 
rentian of  Logan,  that  author  early  recognized  by  his  subdivisions: 
Lower  Laurentian,  including  the  basal  granitoid  formation;  and  Upper 
Laurentian,  including  all  the  limestones,  etc.  The  later  work  of  the 
Canadian  survey  has  emphasized  the  reality  of  this  division.  In  recent 
years  the  practice  of  the  Canadian  Survey  has  been  to  include  all  or 
nearly  all  of  the  more  regularly  laminated  rocks  which  may  be  posi- 
tively asserted  to  be  of  clastic  origin,  as  well  as  many  of  which  this 
assertion  can  not  be  made  to  the  Huronian.  The  difference  between 
the  Laurentian  of  lake  Superior  and  that  of  Logan's  original  area  was 
early  noted  by  Macfarlane,  Brooks,  and  Rominger.  The  last  two 
went  so  far  as  to  suggest  that  about  the  lake  Superior  region  it  is  prob- 
able that  the  Laurentian  of  eastern  Canada  is  not  represented.  That 
possibly  the  fragmental  rocks  of  the  Laurentian  of  the  East  may  be 
