368  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86 
from  which  the  .conclusion  is  formed  that  the  Xaugus  head  is  older  thai 
the  Huronian.  In  fact,  the  Huronian  granite  pierces  every  rock  in  tin 
region  save  the  Naugus  head  series  and  the  newer  uncrystallines,  s( 
that  it  can  not  be  doubted  .that  it  overlies  this  ancient  terrane.  TM 
latter  must  be  regarded  as  the  lowest,  and  hence  the  oldest  member  o: 
the  succession. 
The  Huronian  is  principally  composed  of  granite,  petrosilex,  diorite 
hornblendic  gneiss,  and  limestone,  this  being  the  order  of  age  from  tq 
base  upward.  The  petrosilex  is  the  most  characteristic  rock.  In  tin 
Huronian  distinctly  bedded  rocks  are  the  exception.  Although  manf 
apparently  structureless  rocks  are  probably  really  stratified,  it  is  un 
doubtedly  true  that  a  large  part,  perhaps  the  greater  part,  of  the  forma 
tion  has  been  more  or  less  fluid,  and  extravasation  maybe  set  down  a? 
a  characteristic  structural  feature.  Besides  cutting  various  rocks,  in 
the  granite  at  many  points  at  the  contact  with  the  mica-slates  aw 
found  angular  fragments  of  the  latter.  The  induration  of  the  slate  and 
conglomerate  at  points  where  they  adjoin  the  granite,  with  the  frequenl 
development  of  amygdaloid  characters,  are  facts  which  tell  strongly  ir. 
favor  of  the  igneous  character  of  the  granite.  The  petrosilex  is  fre- 
quently cut  by  the  granite,  but  the  reverse  is  never  the  case.  There  is 
an  apparent  transition  between  the  granite  and  the  petrosilex  or  por- 
phyry. The  two  are  regarded  as  conformable,  both  having  been  meta- 
morphosed from  the  same  set  of  sediments,  the  more  crystalline  chara< 
ter  of  the  granite  being  due  to  its  greater  depth.  With  the  diorite  i 
included  all  the  basic  rocks  which  have  been  fluid.  These  blend  with 
and  grade  into  the  stratified  hornblende-gneisses. 
The  Montalban  series  in  the  area  covered  is  the  most  important 
the  three  systems.  It  comprises  the  ascending  conformable  succession, 
granite,  gneiss,  mica-slate,  argillite.  and  limestone,  although  subordi- 
nate breaks  of  no  great  importance  occur.  The  granite  acts  as  an 
exotic  to  a  large  degree,  although  it  is  believed  to  belong  with  and 
come  from  the  endogenous  metamorphic  granite  at  the  base  of  the 
series.  The  exotic  granite  has  sometimes  cut  rocks  as  new  as  the  Car- 
boniferous. Granite  grades  into  gneiss,  the  gneiss  into  mica-slate,  and 
the  mica-slate  into  argillite,  in  which  are  found  the  conglomerate 
bands.  In  these  conglomerates  in  many  cases  the  pebbles  have  been 
flattened,  bent,  and  even  drawn  out  into  lenticular  layers,  developing 
a  schistose  structure.     One  of  the  best  localities  is  that  at  Bellingham. 
The  Shawmut  group  is  a  fragmental  series  resting  un  conform  ably 
upon  the  Huronian  terranes.  The  chief  constituents  of  the  Shawmut 
group  are  breccia  and  amygdaloids,  the  relations  of  which  are  some- 
what uncertain.  The  petrosilex  breccia  of  the  Shawmut  group  are 
oftentimes  very  like  the  petrosilex  of  the  Huronian.  This  group  is  per- 
haps equivalent  to  the  copper-bearing  rocks  of  lake  Superior  which 
they  resemble  lithologically.  T^he  Paleozoic  formations  form  diflere 
troughs  which  rest  unconformably  upon  the  older  series. 
