456  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Lanark  and  Renfrew  counties — in  other  words,  of  groups  I,  II,  and 
III ;  that  these  last  have  also  been  shown  to  be  a  low  portion  of  the 
gneiss  and  limestone  series — that  is,  of  groups  IV,  V,  and  VI;  and 
that  these  have  always  been  looked  upon  as  typical  Laurentian.  The 
conclusion  is  consequently  reached  that  the  Hastings  series  is  not,  as  it 
has  been  considered  to  be,  the  most  recent,  but  rather  the  oldest  por- 
tion of  this  great  system  of  rocks  investigated.  It  is  also  clear  that 
this  great  crystalline  gneiss  and  limestone  series  rests  upon  a  still 
older  gneiss  series,  in  which  no  crystalline  limestones  have  yet  been 
discovered.  This  series  is  the  one  referred  to  as  division  A,  where 
limestones  have  been  mentioned,  but  incorrectly.  This  occupies  many 
hundreds  of  square  miles  between  St.  Lawrence  and  Ottawa  rivers, 
and  is  the  rock  which  forms  the  backbone  of  eastern  Ontario  and  the 
nucleus  around  which  have  been  deposited  all  succeeding  formations. 
This,  then,  is  undoubtedly  Archean  and  Lower  Laurentian,  and  conse- 
quently the  crystalline  limestones  and  gneisses  constitute  a  series 
which  would  come  in  beneath  Logan's  Upper  Laurentian  or  Labra- 
dorite  series.  Whether  this  latter  exists  as  a  distinct  formation  is 
doubtful.  In  each  instance  in  which  the  crystalline  limestones  have 
been  found  in  the  interior  of  the  gneiss  country,  these  have  been 
proved  to  occur  in  the  superficial  condition  of  shallow  troughs,  and 
not  as  bands  inter  stratified  in  the  gneiss  itself. 
The  lower,  noncalcareous  Laurentian  is  a  great  series  of  crystalline 
rocks,  not  only  highly  metamorphosed,  but  most  intricately  contorted. 
In  the  entire  area  studied  the  gneiss  and  syenite  are  by  far  the  most 
abundant  rocks,  while  gneisses  with  interstratified  crystalline  lime- 
stones occupy  but  a  comparatively  limited  area,  and  this  only  toward 
the  margins  of  the  former.  The  relative  volumes  of  the  two  distinct 
sets  of  rocks — that  is,  the  gneisses  with  the  crystalline  limestones — 
bear  about  the  same  relations  to  the  volume  of  gneiss  and  syenite  that 
the  comparatively  narrow  belt  of  Silurian  does  in  this  section  of 
country  to  both  of  these  together. 
There  is  thus  in  these  old  crystalline  rocks  a  great  noncalcareous 
division  and  a  smaller  calcareous  one.  The  first  of  these  may  be 
further  subdivided  into  a  stratified  and  unstratified  portion,  of  which 
the  latter  is  undoubtedly  the  lower  and  older.  As  shown  by  the 
map,  north  and  northwest  of  the  line,  at  the  base  of  the  gneiss  and 
limestone  series,  are  numerous  and  repeated  troughs  of  the  lower 
member  of  this  division,  which  separate  out  over  the  great  funda- 
mental gneiss  system  in  a  most  irregular  manner,  and  it  is  these  that 
have  given  rise  to  the  supposition  that  the  older  gneiss  and  syenite 
are  interstratified  with  the  crystalline  limestone.  The  three  great  sub- 
divisions in  eastern  Ontario  are,  then,  first,  a  great  gneissic  and  sye- 
nitic  series  without  limestone;  second,  a  thinner  gneissic  series  with 
labradorites  and  limestones;  and,  third,  Lower  Silurian  (Potsdam  to 
