246  FEE-CAMBRIAN    ROCKS    <)V    NORTH    AMERICA.  I  hull.  86, 
we  lire  to  place  in  the  Cambrian  all  rocks  which  con  tain  well  recognized 
fossils  or  show  the  evidence  of  life.  Similar  reasoning  would  place  in  the 
Cambrian  a  large  part  of  the  rocks  which  have  heretofore  been  referred 
to  the  Animikie  and  the  Huronian  in  the  lake  Superior  region.  This 
question  is  more  fully  discussed  in  another  place.  We  know  positively 
only  that  the  gold-bearing  series  is  pre-Oarboniferous,  as  the  Carbonifer- 
ous rocks  are  the  oldest  found  in  contact  with  the  former.  Between 
these  two  is  a  great  unconformity.  How  much  time  this  break  repre- 
sents we  have  at  present  no  means  of  judging. 
Dawson  is  uncertain  whether  the  more  crystalline  mica-schists  and 
gneisses,  which  have  a  somewhat  widespread  occurrence  in  .southern 
Nova  Scotia,  are  to  be  regarded  as  older  than  the  gold-bearing  slates. 
Hind,  in  1870,  maintained  that  a  set  of  older  schists  and  gneisses  lie 
unconformably  below  the  slates  on  the  Windsor  and  Halifax  railway. 
Not  only  this,  but  that  this  set  of  schists  and  gneisses  is  unconformably 
above  another  series  which  he  referred  to  the  Laurentian.  As  no  sub- 
sequent observer  has  mentioned  the  first  of  these  unconformities,  and 
as  it  occurs  at  so  readily  accessible  a  place,  it  may  well  be  considered 
doubtful  whether  the  facts  were  rightly  interpreted.  As  suggested  b$ 
Faribault,  it  does  not  appear  at  all  improbable  that  the  gneisses  asso* 
ciated  with  the  gold-bearing  slates  are  due  to  dynamic  and  contact 
metamorphism  of  the  intruding  granite.  The  coarsely  crystalline  rocks 
which  Hind  referred  to  the  Laurentian  are  certainly  the  series  which 
Dawson  and  other  observers  regard  as  later  granitic  eruptions,  and  the 
unconformity  mentioned  is  perhaps  an  eruptive  unconformity. 
In  northern  Nova  Scotia  and  in  cape  Breton,  among  the  rocks  referred 
to  the  pre-Cambrian  there  is  a  great  variety  of  rocks  both  eruptive  and 
sedimentary.  The  sedimentaries  are  frequently  much  more  metamor- 
phosed than  are  the  gold-bearing  series  of  Nova  Scotia.  With  com- 
mendable caution  Eobb,  in  1876,  only  says  of  this  older  series  that  it 
is  pre-Carboniferous.  Later,  Fletcher,  on  lithological  grounds,  corre- 
lated it  with  the  Laurentian  of  Canada,  and,  like  the  original  area  oJ 
the  Laurentian,  the  Ottawa  rocks,  he  divided  it  into  an  Upper  anc* 
Lower  Laurentian,  the  lower  consisting  of  various  massive  and  lamina 
ted  rocks  all  presumably  of  igneous  origin  whatever  their  age,  and  tin 
upper  series,  including  all  limestones,  sandstones,  slates,  conglomer 
ates — that  is,  all  which  are  certainly  clastic  in  character.  Associatec 
with  this  upper  series  are  eruptives  of  the  same  character  as  those  foun( 
in  the  Lower  Laurentian. 
These  two  parts  of  the  Laurentian  were  first  described  as  in  appaii 
ent  conformity,  but  in  1881  Fletcher  describes  them  as  probably  uncon 
forinable.  The  detailed  evidence  for  this  conclusion  it  has  not  beei 
possible  fully  to  give  in  the  foregoing  summary,  but  a  close  scrutiny  o 
the  evidence  seems  to  point  more  strongly  toward  the  later  intrusiy 
character  of  the  so-called  Lower  Laurenttftn.  The  lines  of  contact  be- 
tween  the  Lower  and  Upper  Laurentian  are  very  irregular,  and  in  get 
