476  PRE-CAMBRIAN   ROCKS   OF   NORTH   AMERICA.  [bull.  86/ 
is  a  series  of  elastics  12,000  feet  thick,  and  above  this  a  great  uncon- 
formity ;  it  includes  much  of  the  great  area  of  northern  Canada  known 
as  Laurentian,  between  which  and  the  Cambrian  in  various  districts  are 
clastic  series. 
In  all  of  these  regions  in  which  the  basal  complex  is  vastly  older 
than  the  Cambrian,  it  consists  of  a  most  intricate  mixture  of  nearly 
massive  rocks,  among  which  granite  and  granite-gneiss  are  predominant;  , 
of  gneissic  and  schistose  rocks,  all  of  which  are  completely  crystalline, 
and  so  folded  and  contorted  that  nowhere  has  any  certain  structure 
ever  been  made  out  over  considerable  areas.  The  granites  and  basic 
eruptives  may  occupy  considerable  areas;  the  gneisses  maybe  regu- 
larly laminated  and  grade  into  the  granites;  the  crystalline  schists  may 
occupy  the  outer  zones  of  an  area ;  they  may  all  be  confusedly  inter- 
mingled, schists,  gneisses,  and  granites  alternately  predominating! 
sometimes  the  schistose  rocks  appear  in  dike-like  forms  in  the  granites; 
at  other  times  the  massives  are  in  dike-like  forms  in  the  schists;  at 
still  other  times  the  alternations  of  granite,  gneiss,  and  schists  are 
quite  uniform  and  persistent  for  considerable  areas.  The  granites 
usually  show  a  rough  lamination,  which  may  not  appear  in  hand  speci- 
mens, but  which  is  evident  in  large  masses. 
The  minerals  in  the  rocks  generally  show  evidence  of  dynamic  action ; 
they  do  not  have  the  clear  cut,  definite  relations  characteristic  of  the 
later  plu tonic  rocks.  In  the  chief  mineral  constituents  of  the  rocks 
there  is  essential  uniformity  in  all  of  the  areas,  although  certain  less 
common  minerals  may  be  found  in  one  area  which  have  not  been 
discovered  in  another.  Orthoclase  and  acid  plagioclase  feldspar, 
quartz,  hornblende,  muscovite,  and  biotite  are  the  standard  minerals. 
To  describe  accurately  the  appearance  of  the  rocks  of  the  basal  com- 
plex is  exceedingly  difficult,  but  any  one  who  examines  a  series  of 
specimens  from  the  various  areas  will  perceive  the  truth  of  the  state- 
ment made  as  to  the  essential  likeness  of  the  rocks  from  different 
regions.  A  suite  from  any  one  of  the  regions  which  has  been  person-  { 
ally  examined  by  me,  if  unlabeled,  could  by  no  possibility  be  asserted  x 
not  to  come  from  any  other. 
The  unparalleled  intricacy  of  the  structure  of  this  complex,  the  gen- 
eral laminated  arrangement  of  its  parts,  and  the  broken  and  distorted 
forms  of  the  constituent  minerals  are  evidence  of  repeated  dynamic 
movements  of  the  most  powerful  character.  Further,  the  basal  com 
plex  is  not  only  recognized  by  its  positive  but  by  its  negative  charac- 
ters. Nowhere  in  it  is  a  persistent  thick  formation  of  quartz-schist 
(although  vein-quartz  is  abundant),  of  limestone  or  marble,  of  a  graph- 
itic schist,  or  of  a  conglomerate.  If  sandstones  and  limestones  or  other 
sedimentary  materials  have  been  a  part  of  this  system  the  profound 
and  varied  mutations  through  vast  lapses  of  time  have  wholly  obliter- 
ated all  evidence  of  their  presence. 
Besides  the  areas  mentioned  in  which  these  most  ancient  rocks  occur, 
