478  PRE-CAMBR1AN    GEOLOGY   OF    NORTH   AMERICA. 
been  the  orogenic  movements  that  the  once  rounded  grains,  and  even 
the  pebbles  and  bowlders,  if  they  ever  existed,  have  been  wholly 
destroyed. 
Both  Logan  and  Ells  have  subdivided  the  Grenville  series  into 
formations,  but  their  successions  are  not  the  same,  and  in  Ells's  final 
map  of  the  area  no  division  is  attempted.  While  there  is  no  doubt 
that  several  formations  exist,  the  separation  of  them  is  exceedingly 
difficult  on  account  of  the  complicated  structure  and  the  profound 
metamorphism.  It  follows  that  no  accurate  estimate  of  the  thickness 
of  the  series  has  yet  been  made. 
Anorthosite,  intrusive  into  the  Grenville  series  and  the  Ottawa 
gneiss,  covers  numerous  areas,  some  of  them  thousands  of  square  miles 
in  extent.  The  anorthosite  has  been  so  profoundly  modified  by 
dynamic  action  as  now  to  be  regularly  laminated  over  extensive  areas. 
At  many  places  there  are  gradations  between  the  massive  and  lami- 
nated varieties.  Various  basic  and  acidic  rocks  are  intruded  along  or 
across  the  lamination  of  the  sedimentary  series. 
Resting  unconformably  upon  the  upturned  edges  of  the  deeply 
eroded  pre-Cambrian  rocks  of  the  region  are  the  little-disturbed  Cam- 
brian rocks. 
The  relations  which  obtain  between  the  Ottawa  gneiss  and  the 
Grenville  series  were  long  described  as  those  of  apparent  gradation. 
The  downward  gradation  was  explained  by  Logan  to  be  that  of  pro- 
gressive metamorphism,  the  Ottawa  gneiss  as  well  as  the  Grenville 
series  being  regarded  as  sedimentary.  The  relations  have  been  ex- 
plained by  Lawson  by  the  subcrustal  fusion  of  the  Grenville  series, 
thus  producing  the  granite  gneiss.  Adams  and  Barlow,  principally 
from  their  work  on  the  portion  of  the  protaxis  lying  north  of  Lake 
Ontario  in  the  Hastings  district,  conclude  that  the  granite  gneisses  are 
intrusions  with  the  form  of  batholiths,  around  which  the  sedimentary 
rocks  sweep  in  a  series  of  curves  and  between  which  they  settle  down, 
the  batholiths  having  arched  up  the  overlying  strata  and  risen 
through  them  by  the  process  of  overhead  stoping. 
In  the  Hastings  district  Miller  and  Knight  find  the  "  Grenville  " 
limestone  tying  upon  basic  igneous  rocks  forming  the  basal  series  of 
the  district,  similar  to  the  Keewatin  series  in  the  Lake  Superior 
region.  They  note  a  resemblance  of  the  limestones  and  associated 
igneous  rocks  to  the  iron  formations  and  greenstones  of  the  Keewatin 
of  the  Lake  Superior  region.  They  also  find  a  later  sedimentary 
series  (principally  Hastings),  likewise  containing  limestone,  uncon- 
formably upon  the  Grenville  series.  The  question  is  naturally  raised 
whether  in  the  original  Laurentian  district  similar  series  are  present. 
Nomenclature  and  correlation. — Originally  all  of  the  rocks  of  the 
original  Laurentian  district,  including  both  the  fundamental  gneiss 
and  the  Grenville  series,  were  referred  by  Logan  to  the  "  Lower  Lau- 
rentian "  and  the  intrusive  anorthosite  to  the  "  Upper  Laurentian," 
