AREA   NORTH   AND    NORTHEAST   OF   LAKE    HURON.  415 
change  occurring  within  5  or  6  feet.  Here  the  solid  granite  gneiss  is 
first  broken;  then  in  passing  upward  the  angular  fragments  have 
moved  somewhat;  in  passing  still  farther  upward  they  become  round- 
ish and  are  mingled  with  extraneous  material,  until  a  bowlder  con- 
glomerate is  reached  which  is  in  every  respect  like  that  before  de- 
scribed. This  relation  is  not  uncommon  when  an  encroaching  shore 
line  overrides  a  rock  formation. 
It  is  concluded  that  between  the  lowest  members  of  the  Original 
Huronian  series  and  the  granite-gneiss-schist  Basement  Complex 
which  Logan  has  called  Laurentian  there  is  the  clearest  evidence  of  a 
very  great  unconformity ;  also,  that  the  Laurentian  series,  instead  of 
being  a  simple  one,  consists  of  rocks  of  many  different  kinds  and  has 
a  most  complex  history. 
Bell,31  in  1891,  reports  on  the  Sudbury  mining  district.  The  rocks 
are  divided  into  three  groups,  in  ascending  order:  (1)  A  gneiss  and 
hornblende  granite  series — Laurentian.  (2)  A  series  comprising 
quartzites,  massive  graywackes,  often  holding  rounded  and  angular 
fragments;  slaty  graywackes,  with  and  without  included  fragments; 
drab  and  dark-gray  argillites  and  clay  slates;  dioritic,  hornblendic, 
sericitic,  felsitic,  micaceous,  and  other  schists;  and  occasionally  dol- 
omites, together  with  large  included  masses  or  areas  of  pyritiferous 
greenstones.  This  group  constitutes  the  ordinary  Huronian  of  the 
district.  (3)  A  division  consisting  of  a  thick  band  of  dark-colored 
siliceous  volcanic  breccia  and  black  slate  (generally  coarse),  overlain 
by  drab  and  dark-gray  argillaceous  and  nearly  black,  gritty  sand- 
stones and  shaly  bands.  The  breccia  is  underlain  in  places  by  quartz- 
ite  conglomerate.  (4)  In  addition  to  these,  dikes  of  diabase  and 
gabbro  cut  through  all  the  foregoing,  and  are,  therefore,  newer  than 
any  of  them,  although  they  may  not  belong  to  a  later  geological 
period. 
Flanking  the  Huronian  rocks  on  the  southeast  is  gneiss,  and  on  the 
northwest  a  mixture  of  gneiss  and  hornblende  granite.  The  first  of 
these  rocks  is  of  the  characteristic  Laurentian  type,  but  the  horn- 
blende granite  and  quartz  syenite  on  the  northwest  are  not  always 
characteristic  of  the  Laurentian.  These  rocks,  however,  pass  into  the 
gneiss  in  such  a  way,  and  are  so  mingled  with  them  on  both  a  large 
and  a  small  scale,  that  it  was  impossible  to  separate  them.  Within 
the  Huronian  trough,  and  parallel  with  it,  is  also  a  tongue  of  gneiss 
and  hornblende  granite  2  or  3  miles  wide  and  39  miles  long. 
The  Huronian  division  forms  a  part  of  the  great  Huronian  belt  ex- 
tending from  Lake  Superior  and  Lake  Huron  nearly  to  Lake  Mittas- 
sini.  The  bedding  of  the  Huronian  is  usually  nearly  vertical,  or 
stands  at  high  angles.  Occasionally  the  rocks  have  been  sheared  by 
pressure.  Graywacke  conglomerate,  in  places  full  of  rounded  pebbles 
of  gray  quartz  syenite,  is  found  on  the  Blue  River  branch  of  Spanish 
