470  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
tion  of  this  portion.  It  seems  probable  that  the  age  of  the  Hastings 
series  will  be  shown  to  be  Huronian. 
The  Grenville  and  Hastings  series  are  unconformably  overlain  by, 
and  disappear  to  the  south  beneath,  flat-lying  Cambro-Silurian  rocks. 
Adams,44  in  1897,  reports  on  the  geology  of  a  portion  of  the  Lauren- 
tian  area  lying  to  the  north  of  the  island  of  Montreal. 
A  previous  report 40  covers  about  the  southeast  quarter  of  this  area, 
and  in  a  general  way  the  conclusions  reached  for  this  southeastern 
area  are  applied  to  the  larger  area  under  discussion. 
The  Archean  geology  is  summarized  by  the  author  as  follows : 
1.  The  Archean  rocks  in  this  area  are  of  Laurentian  age  a  and  are 
in  part  referable  to  the  Grenville  series  and  in  part  to  the  Funda- 
mental gneiss. 
2.  The  Grenville  series  contains  gneisses,  as  well  as  limestones  and 
quartzites,  which  are  of  aqueous  origin,  having  the  chemical  composi- 
tion and  the  stratigraphical  attitude  of  sedimentary  rocks.  With 
these  are  intimately  associated,  however,  other  gneisses  which  are  of 
igneous  origin. 
3.  The  Fundamental  gneiss  consists  largely,  if  not  exclusively,  of 
igneous  rocks  in  which  a  banding  or  foliation  has  been  induced  by 
movements  caused  by  pressure. 
4.  Both  series  are  penetrated  by  various  igneous  masses,  of  which 
the  most  important  are  great  intrusions  of  anorthosite,  a  rock  of  the 
gabbro  family,  characterized  by  a  great  preponderance  of  plagioclase. 
This  rock  is  in  places  perfectly  massive,  but  generally  exhibits  the 
irregular  structure  which  is  so  often  observed  in  gabbros  and  which  is 
brought  about  by  a  variation  in  the  size  of  the  grain  or  the  relative 
proportion  of  the  constituents  from  place  to  place.  In  addition  to 
this  original  structure,  the  rock  almost  always  shows  a  peculiar  pro- 
toclastic,  cataclastic,  or  granulated  structure,  which  is  especially  well 
seen  in  the  foliated  varieties.  This  differs  from  the  structure  char- 
acteristic of  dynamic  metamorphism  in  the  great  mountainous  dis- 
tricts of  the  world,  having  been  produced  by  movements  in  the  rock 
mass  while  this  was  still  deeply  buried  in  the  crust  of  the  earth  and 
probably  very  hot — perhaps  near  the  melting  point. 
5.  The  same  granulated  structure  is  also  seen  in  all  those  gneisses 
which  have  been  formed  from  massive  igneous  rocks  by  dynamic 
movements. 
6.  The  fine-grained  aqueous  rocks  of  the  Laurentian,  on  the  other 
hand,  have  been  altered  chiefly  by  a  process  of  recrystallization. 
7.  The  "  Upper  Laurentian  "  or  "Anorthosite  group  "  of  Sir  Wil- 
liam Logan  does  not  exist  as  an  independent  geological  series — the 
anorthosite,  which  was  considered  to  be  its  principal  constituent,  being 
a  In  the  sense  of  pre-Cambrian  or  original  Laurentian. 
