98  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.8S. 
(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  part  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  upheaval  of  its  northern  and  southern 
margins.  The  granitic  and  sedimentary  rock  masses  are  traversed  by 
rock  belts  of  a  crystalline  character,  which  represent  lava  streams 
intruded  at  different  periods  subsequently  to  the  rocks  cut. 
In  the  Granitic  group  the  granites  are  found  inter  strati  fled  with  the 
Huronian  schists  of  the  Dioritic  group.  The  granites  are  usually  mid- 
dling coarse  grained  and  in  the  main  are  massive,  although  distinct 
gneissoid  rocks  have  a  limited  occurrence.  Besides  the  dioritic  dikes 
there  are  in  the  granites  crystalline  non-stratified  masses  resembling 
eruptive  dikes.  In  several  instances  granite  dikes  show  a  laminar 
arrangement  of  the  mica  scales.  Also  syenites  are  associated  with  the 
ordinary  granites.  The  hornblende  rocks  associated  with  the  granites 
are  distinguished  fron;  those  occupying  a  higher  position  in  the  series 
by  the  brighter  luster  of  the  hornblende  crystals.  Dioritic  rocks  occur 
inter  stratified  with  the  granites,  which  are  probably  of  the  same  origin 
as  the  volcanic  eruptives.  In  the  Lauren ti an  rocks  no  limestones,  lay- 
ers 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  Dio» 
ritic  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  conn 
formably  bedded  with  the  schists  and  often  insensible  gradations  from 
the  schistose  conditions  to  the  massive  diorite  can  be  seen.  The  expos- 
ures of  massive  diorite  generally  form  a  nucleus  around  which  the 
inclosing  rock  masses  are  arranged  concentrically  with  a  more  perfect 
svltistose  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  with 
the  surrounding  schistose  mass.  Also  extensive  conglomeratic  masses 
are  found  full  of  granite  pebbles  of  large  size  in  Sec.  2,  T.  48  N.,  E.  2G 
W.,  and  in  Sec.  29,  T.  48  K,  R.  25  W.  In  opposition  to  Brooks  it  is 
maintained  that  there  is  but  one  iron- group  formation. 
