488  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
mentaryj  structural  methods  can  uot  apply.  Its  subdivisions  must  be 
made  upon  purely  lithological  grounds.  If  a  part  of  it  is  demonstrably 
eruptive  the  relative  ages  of  these  parts  may  often  be  ascertained  and 
all  are  necessarily  newer  than  the  part  not  recognized  as  eruptive,  else 
these  could  not  be  shown  to  have  this  origin.  Many  attempts  to  apply 
strati  graphical  methods  to  parts  of  the  Archean  have  been  made,  but 
they  have  not  thus  far  been  successful.  Such  attempts  have  been 
based  upon  the  belief  that  foliation  represents  sedimentation,  but  even 
working  upon  this  erroneous  basis  it  has  been  stated  that  the  struc- 
tures are  so  complicated  that  little  progress  has  been  made.  So  far  as 
attempts  to  apply  stratigraphical  methods  to  this  fundamental  com- 
plex are  concerned  the  conclusions  of  Whitney  and  Wads  worth  are 
very  largely  true.  If  their  review  of  the  "  Azoic  rocks"  had  been  confined 
to  this  basement  system,  which  is  perhaps  truly  Azoic,  the  conclusion 
as  to  its  indivisibility  on  a  structural  basis  would  have  plausibility. 
The  only  division  generally  applicable  to  the  Archean  warranted  by 
present  knowledge  is  its  separation  into  (1)  fine  grained  mica-schists, 
feldspathic  mica- schists  (technically  gneisses),  hornblende- schists,  horn 
blende-gneisses,  etc;  and  (2)  the  granites  and  granitoid  gneisses  with 
their  associates.  The  first  class  is  generally  dark  colored,  the  second 
light  colored.  The  lithological  affinities  of  the  second  are  with  the  ig- 
neous rocks.  As  already  indicated,  the  change  from  a  granite- gneiss 
area  to  a  schistose  area  is  not  infrequently  a  transition.  In  passing 
from  the  schists  to  the  granites  often  veins  or  dikes  of  granite  are  first 
found,  or  interlaminations  of  granite- gneiss  with  the  schist.  After  a 
time  the  schistose  rocks  and  granite-gneiss  are  about  equally  impor- 
tant. Proceeding  onward  the  granite- gneisses  become  predominant. 
These  relations  are  precisely  those  already  described  and  interpreted 
by  one  school  to  mean  that  the  granite- gneisses  are  extraneous  intru- 
sives,  by  another  that  they  are  the  aqueo-igneous  fused  sedimentary 
beds  in  situ,  and  by  a  third  that  they  are  selectively  metamorphosed 
beds.  To  designate  the  gneissoid  granite  part  of  the  Archean  the  term 
Laurentian  has  always  been  employed  and  is  now  generally  restricted, 
the  finely  laminated  crystalline  schists  being  commonly  referred  to  the 
Huroiiian.  Laurentian  under  this  usage  is  made  to  include  the  impli- 
cated granite-gneiss  of  the  basal  complex,  somewhat  regularly  lami- 
nated  granite-gneiss,  and  also  many  areas  of  nearly  massive  granite,  a 
part  of  which  latter  may  be  and  probably  is  of  later  age  than  the  former. 
The  first  is  clearly  Laurentian.  It  is  equally  clear  that  the  granite- 
gneisses  known  to  be  of  later  age  than  Algonkian  elastics,  like  those 
described  by  Lawson  northwest  of  lake  Superior,  should  be  excluded 
from  the  Laurentian.  To  place  these  rocks  here  is  to  introduce  a  new 
principle  in  geology;  i.  e.,  it  is  giving  rocks  of  recognized  eruptive  ori- 
gin a  separate  term  when  they  should  be  given  the  name  of  their  con- 
temporaneous elastics.  The  eruptives  which  are  contemporaneous  with 
the  Tertiary  rocks  are  so  named.     Rocks  which  cut  Tertiary  rocks  and 
