130  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Marquette  region. — In  general  remarks  on  the  geology  of  the  Mar- 
quette district  the  succession  is  said  to  be:  (1)  Granitic  group,  (2) 
Dioritic  group,  (3)  Quartzite  group,  (4)  Iron  group,  (5)  Arenaceous 
Slate  group,  (6)  Mica  schist  group,  and  (7)  Serpentine  group;. but 
later  it  is  seen  that  the  Quartzite  group  reposes  upon  the  Iron  group, 
so  that  the  order  of  3  and  4  is  reversed.  The  Silurian  sandstones 
rest  horizontally  on  the  other  rock  formations  and  frequently  contain 
fragments  of  the  underlying  formations.  The  crystalline  granitic 
masses  are  directly  confined  to  the  northern  and  southern  limits  of 
the  Marquette  district.  The  dip  of  the  strata  on  the  south  side  of 
the  trough  is  usually  to  the  north,  and  on  the  north  side  to  the  south, 
so  that  we  may  consider  this  area  as  a  synclinal  caused  by  the  up- 
heaval of  its  northern  and  southern  margins.  The  granitic  and  sedi- 
mentary rock  masses  are  traversed  by  rock  belts  of  a  crystalline  char- 
acter, which  represent  lava  streams  intruded  at  different  periods. 
In  the  Granitic  group  the  granites  are  found  interstratified  with 
the  Huronian  schists  of  the  Dioritic  group.  The  granites  are  usually 
middling  coarse  grained  and  in  the  main  are  massive,  although  dis- 
tinct gneissoid  rocks  have  a  limited  occurrence.  Besides  the  dioritic 
dikes  there  are  in  the  granites  crystalline  nonstratified  masses  resem- 
bling eruptive  dikes.  In  several  instances  granite  dikes  show  a  lam- 
inar arrangement  of  the  mica  scales.  Also  syenites  are  associated 
with  the  ordinary  granites.  The  hornblende  rocks  associated  with 
the  granites  are  distinguished  from  those  occupying  a  higher  position 
in  the  series  by  the  brighter  luster  of  the  hornblende  crystals.  Dio- 
ritic rocks  occur  interstratified  with  the  granites,  which  are  probably 
of  the  same  origin  as  the  volcanic  eruptives.  In  the  Laurentian  rocks 
no  limestones,  layers  of  quartzite,  nor  beds  of  iron  ore  are  found. 
The  granitic  rocks  in  their  present  position  are  actually  the  younger 
rocks,  as  shown  by  the  intrusion  of  large  masses  of  granite  between 
the  stratified  sediments  of  the  Dioritic  group. 
The  Dioritic  group  is  regarded  as  remelted,  completely  metamor- 
phosed Huronian  sediments,  their  more  crystalline  character  being 
due  to  their  closer  proximity  to  the  volcanic  forces.  The  rocks  of  the 
Dioritic  group  include  a  large  succession  of  schistose  beds  of  uniform 
character,  interstratified  with  massive  diorite.  In  the  dioritic  rock 
chlorite  frequently  replaces  the  hornblende  and  often  seems  to  be  a 
product  of  its  decomposition.  The  massive  diorites  are  usually  con- 
formably bedded  with  the  schists,  and  often  insensible  gradations 
from  the  schistose  conditions  to  the  massive  diorite  can  be  seen.  The 
exposures  of  massive  diorite  generally  form  a  nucleus  around  which 
the  inclosing  rock  masses  are  arranged  concentrically  with  a  more 
perfect  schistose  structure.  In  the  Dioritic  group  are  conglomerates. 
One  variety  is  well  exposed  at  Deer  Lake  furnace,  where  the  pebbles 
are  of  a  feldspathic  substance  which  on  fresh  fracture  contrasts  little 
