60  PRE-CAMBRIAN   ROCKS    OF   NORTH    AMERICA.  [bull. 86. 
oid  and  thick-bedded  gneisses,  which  pass  upward  into  thin -bedded 
gneisses  and  highly  crystalline  micaceous  and  hornblendic  schists. 
The  Huronian  rocks  are  more  variable  in  character.  The  lowest  beds 
are,  for  the  most  part,  hard  green  rocks,  with  little  traces  of  stratifica- 
tion, but  hold  some  well  stratified  micaceous  and  chloritic  schists  and 
also  imperfect  gneiss.  On  these  rest  a  great  thickness  of  massive  beds 
characterized  by  the  predominance  of  conglomerate,  but  including 
quartzites  and  dioritic  rocks.  Above  these  is  an  extensive  series  of 
schistose  and  slaty  beds  generally  more  or  less  nacreous  and  chloritic 
or  talcose,  but  often  hornblendic  and  micaceous.  They  inclose  also  con- 
glomerates, quartzites,  and  diorite  beds.  It  is  believed  two  movements 
have  conspired  to  form  the  present  features  of  the  region,  both  being 
post-Huronian.  The  first  of  these  is  connected  with  the  post-Huronian 
granite  eruptions ;  the  second  and  more  important  is  believed  to  have 
taken  place  later,  and  to  it  is  supposed  to  be  due  the  parallelism  in  the 
folding  of  the  Laurentian  and  Huronian  rocks.  At  Eat  portage  the 
junction  of  the  Laurentian  and  Huronian  is  so  sharply  defined  that  the 
hand  can  be  laid  upon  it.  This  sharp  contact  is  believed  to  be  due  to 
faulting.  Adjacent  to  the  granite  the  Huronian  slate  series  is  meta- 
morphosed, and  the  occasional  gneissic  aspect  of  the  Huronian  is  attrib- 
uted to  the  granitic  intrusions.  The  large  Y-shaped  granite  mass  in  the 
northwest  angle,  in  contact  with  the  altered  sedimentary  rocks,  assumes 
a  more  basic  character  and  a  darker  aspect,  becoming  blackish  gneissic 
diorite  and  gray  syenitic  diorite.  The  conglomerate  beds  are  of  immense 
thickness  and  could  perhaps  be  best  described  as  slate  conglomerates. 
The  pebbles  generally  resemble  the  matrix,  and  best  appear  upon  a 
weathered  surface;  for  on  a  freshly  broken  surface  no  clear  distinction 
appears  between  the  fragments  and  the  inclosing  materials,  and  the 
rock  differs  from  the  more  compact  altered  schists  and  slates  only  in  its 
rougher  surface  of  fracture  and  a  somewhat  spotted  character.  The 
greenstone  conglomerates  of  Bigsby  resemble  roughly  fractured  pieces 
of  diorite  in  a  dioritic  paste.  The  quartzites  show  a  tendency  to  run 
into  conglomerates,  and  certain  of  the  conglomerates  have  an  aspect  of 
a  volcanic  breccia.  There  is  an  entire  absence  of  any  granitic  or  gneissic 
beds  in  the  conglomerates  and  breccias,  and  in  this  respect  these  Huron- 
ian rocks  differ  from  the  typical  area.  It  is  suggested  that  this  fact 
may  mean  that  the  formation  of  the  whole  Huronian  series  took  place 
subsequent  to  that  of  the  typical  Huronian,  and  therefore  are  perhaps 
more  nearly  equivalent  to  those  of  the  Quebec  group.  The  granitoid 
gneisses  and  intrusive  granites  are  universally  cut  by  veins  of  red 
orthoclase  feldspar  associated  with  quartz ;  and  basic  diorite  dikes  cut 
both  the  granitic  and  altered  Laurentian  rocks. 
Bell,23  in  1875,  describes  on  the  north  shore  of  lake  Superior  the 
Laurentian,  Huronian,  and  Upper  Copper-bearing  rocks.  The  Huron- 
ian occupies  a  large  extent  of  country,  alternating  with  bands  of  Lau- 
rentian, on  both  the  north  and  south  shores  of  the  lake.    North  of  lake 
