176  PRE-CAMBRLA.N    ROCKS    OF    NORTH    AMERICA.  [bull.sr. 
could  not  have  bad  as  surface  rocks;  and  which  .shows,  whether  igne- 
ous or  aqueous,  profound  induced  structures  and  deep  erosion,  and 
therefore  a  system  of  rocks  which  had  an  intricate  history  before  the 
elastics  were  deposited.  In  this  connection  the  physical  break  at  the 
base  of  the  Animikie  and  its  equivalents  is  not  cited,  because  this 
series  occupies  a  higher  position  than  the  series  overlying  the  break 
just  considered. 
As  further  evidence  for  a' great  physical  break  between  the  base- 
ment complex  and  the  lowest  overlying  elastics  are  cited  by  Brooks  and 
Irving  (1)  the  strong  contrast  in  the  litliological  characters  of  the  two, 
the  fundamental  complex  being  thoroughly  crystalline,  while  the  over- 
lying rocks  are  mainly  plainly  detrital,  or  such  as  may  have  been 
derived  from  detritals;  (1)  the  fact  that  the  complex  is  cut  by  very 
numerous  granite  dikes,  which  are  but  rarely  found  in  the  elastics; 
and  (3)  their  general  field  relations,  the  complex  having  most  ob- 
scure structures  and  very  great  variability  in  strike  and  tip,  while 
the  elastics  are  less  intricately  folded,  showing  that  the  older  series 
has  been  subjected  to  orographic  movements  prior  to  the  newer. 
As  before  stated,  Brooks,  Roininger,  and  Irving — the  three  who  have 
done  the  most  detailed  and  continuous  work  on  the  south  shore — had 
at  the  outset  different  opinions  as  to  the  relations  of  the  elastics  to  a 
basement  granite-gneiss,  but  they  all  came  to  the  same  final  conclu- 
sion, that  between  the  two  is  a  great  structural  break.  Pumpelly  and 
Ohamberlin,  who  have  also  done  much  general  work  in  this  region, 
agree  with  this  conclusion.  While  this  is  true,  all  of  these  writers  do 
not  agree  in  every  district  as  to  the  position  at  which  this  plane  is 
found. 
Whether  the  physical  break  which  exists  below  the  clastic  series 
on  the  south  shore  is  paralleled  by  a  widespread  unconformity  on  the 
north  shore  is  less  certain,  although  such  an  unconformity  is  found 
locally.  Lawson,  who  has  done  the  most  work  in  this  region,  maintains 
that  between  the  Keewatin  and  Coutchiching  a  profound  physical 
change  took  place  in  the  conditions  of  deposition,  consequent  upon 
which  are  great  differences  in  litliological  characters  and  a  prevalent 
more  crystalline  condition  in  the  inferior  series.  He  also  describes 
conglomerates  in  two  places  at  the  base  of  the  Keewatin  which  bear 
both  granitic  fragments  and  fragments  from  the  Coutchiching.  From 
these  facts  Lawson  believes  that  there  is  a  real  unconformity  be- 
tween the  two  series,  although  at  the  present  time  they  have  been  so 
squeezed  together  and  secondary  structures  formed  jas  to  be  usually 
in  apparent  conformity,  and  the  separation  in  mapping  is  based  for 
the  most  part  upon  litliological  grounds.  Lawson,  however,  believes 
that  the  •Coutchiching  is  a  bedded  sedimentary  series,  rather  than  a< 
part  of  the  basal  complex,  as  here  defined.  If  this  be  true, this  uncoil 
formity  does  not  bear  on  the  question  under  discussion. 
Pumpelly  and  ►Smyth,  who  have  recently  made  a  rather  extended  trifj 
